chapter seven -broaching

7/28/2019 Chapter Seven -Broaching

1/110

Chapter seven

Broaching

7.1 Introduction

Broaching is a cutting process using a multitoothed tool (broach) having successive cutting

edges, each protruding to a greater distance than the proceeding one in the direction

perpendicular to the broach length. In contrast to all other cutting processes, there is no feeding

of the broach or the workpiece. The feed is built into the broach itself through the consecutive

protruding of its teeth Fig.7.1.

a) b)

Fig.7.1 a) Schematic illustration of broaching operation; b) broaching with round internal broach.

Therefore, no complex motion of the tool relative to the workpiece is required, where the

tool is movedpast the workpiece with a rectilinear motion. Equally effective results are obtained

if the tool is stationary and the work is moved. The total depth of the material removed in one

stroke is the sum ofrises ofteeth of the broach. It maybe as deep as 6 mm broached in one stroke.

Ifmore depth is to be broached, twobroaches may be used to perform the task.

The cutting speedmotion is accomplishedby the linear travel of the broach past the work

surface. Feed in broaching is unique among machining operations, since is accomplished by the

increased step between successive teeth on the broach. This step is actually the feed per tooth, fz.

The feed pertooth is not a constant forall the teeth. The total material removed in a singlepass

of the broach or the total feed fis the cumulative result ofall the steps in the tool.

1


2/110

Since not all of the broach teeth are engaged simultaneously in cutting but only a part of

them, the term active cumulative feedcan be introduced, defined as the sum ofall the steps only of

the active teeth.

Depth of cut in broaching is defined as the length of the active cutting edge. In internal

broaching, which is the most common type of broaching, the entire length of a single broach tooth

is engaged in cutting and the depth ofcut is actually the tooth circumference.

From the definitions of feed and depth of cut it follows that the total area of cut and

respectively the cutting force in broaching will be substantial.

Broaching is generally used to machine through holes of any cross-sectional shape,

straight and helical slots, external surfaces ofvarious shapes, and external and internal toothed

gears.

Some of the typical examples ofshapesproduced by internal broaching are (Fig.7.2).

Fig.7.2 Various shapes produced by internal broaching operation.

To permit the broaching ofspiral grooves and gun-barrel rifling, a rotational movement

should be added to the broach. Broaching usually produces better accuracy and finish than

drilling, boring, orreaming operations. A tolerance grade of IT6 and a surface roughness Raof

about 0,2 m can be easily achieved by broaching.Broaching must be designed individually for a particular job. They are very expensive to

manufacture ($15000-$30000 per tool). It follows that the broaching can only be justified in a very

largeproduction ofparts.

2


3/110

7.2 Broach configuration

Fig.7.3 illustrates the terminology of a pull-type internal broach for enlarging circular

holes. The cutting teeth on the broach have three regions roughing, semifinishing, and finishing

teeth.

On some round broaches, burnishing teeth are provided forfinishing orsizing. These teeth

have no cutting edge, but are rounded. Theirdiameter is oversized by 25-30 m larger than the

finished hole. Irregular shapes are produced by starting from circular broaching in the

workpiece originally provided with drilled, bored, cored, orreamed hole.

Fig.7.3 Typical broach for internal broaching.

The pull end provides a means ofquickly attaching the broach to the pulling mechanism

(Fig.7.4). The front pilot aligns the broach in the hole before it begins to cut and the rear pilotkeeps the tool square with the finished hole as it leaves the workpiece. It also prevents sagging of

the broach. The follower end is ground to fit in the machine follower rest.

3


4/110

Fig.7.4 Broach pulling mechanism

Regarding the application of broaching force, two types ofbroaching are distinguished:

1. Pull-broaching, as the name implies, involves the broach being pulled through the hole

(Fig.7.5). In this case, the main cutting force is applied to the front of the broach, subjecting the

body to tension.

Fig.7.5 Schematic illustration of pull-broaching

operation.

Most internal broaching is done with pull-

broaches. Because there is no problem of buckling,

pull-broaches can be longer than push-types for the

samebroaching depth.

Pull-broaches can be made to long lengths, but

cost usually limits the length to approximately 2 m.

Broaches longer than 2 m are shellbroaches (Fig.7.6) and they are mounted on an arbor. The

cost is less forreplacing damaged orworn sections than for replacing the entire broach. Shell

broaches are superior to solid broaches in that worn or broken shells can be replaced without

discarding the entirebroach.

Fig.7.5 Shell broach.

Shell broaches can be used on the roughing

semi-finishing sections of a broach tool. The

principal advantage of a shell broach is that worn

sections can be removed and resharpened, or

replaced, at far less cost than a conventional single-

piece tool.

When shells are used for the finishing teeth of long broaches, the teeth of the shell can be

ground to fargreater accuracy than those of a long conventional broach tool and the tool can

continue to be used by replacing the shell.

The disadvantage ofshell broaches is that some accuracy and concentricity are sacrificed.

4


5/110

2. Push-broaching applies the main cutting force to the rear of the broach, thus subjecting the

body to compression (Fig7.6a). A push-broach should be shorter than a pull-broach and its

length does not usually exceed 15 times its diameter to avoid buckling.

a)

b)

Fig.7.6 a) Schematic illustration with working with push broach; b) circular push broaches.

7.3 Cutting Tooth Sections

Broach teeth usually are divided into three separate sections along the length of the tool: the

roughing teeth, semi-finishing teeth, and finishing teeth. The first roughing tooth is

proportionately the smallest tooth on the tool. The subsequent teeth progressively increase in size

up to and including the first finishing tooth. The difference in height between each tooth, or tooth

rise, usually is greater along the roughing section and less along the semi-finishing section. All

finishing teeth are the same size.

Individual teeth (see illustration below) have a land and face intersect to form a cutting

edge. The face is ground with a hookorface angle that is determined by the workpiece material.

For instance, soft steel workpieces usually require greater hook angles; hard or brittle steel,

smaller hookangles.

5


6/110

The Land. The land (L) supports the cutting edge against stresses (Fig.7.7). A slight

clearance orbackoffangle () is ground onto the lands to reduce friction. On roughing and semi-

finishing teeth, the entire land is relieved with a backoffangle.

On finishing teeth, part of the land immediately behind the cutting edge is often left

straight so that repeated sharpening (by grinding the face of the tooth) will not alter the tooth size.

Fig.7.7 Typical surface broaching tool (P=pitch, L=land behind the cutting edge (0,25p), = Back-off angle or clearance

angle, R = Radius of the gullet (0.25P), D = Depth of a tooth (0.4P), RPT = rise per tooth, = Hook angle or rake angle).

Distance between Cutting Teeth. The distance between teeth orpitch is determined by the

length of cut and influenced by the type of workpiece material. A relatively large pitch may be

required forroughing teeth to accommodate a greater chip load. Tooth pitch may be smaller on

semi-finishing and finishing teeth to reduce the overall length of the broach tool. Pitch is

calculated so that, preferably, two ormore teeth cut simultaneously. This prevents the tool from

drifting orchattering.

Sometimes a broach tool will vibrate when a heavy cut is taken, especially when the

cutting load is not evenly distributed. Vibration may also occur when tooth engagement is

irregular. The greatest contributing factors to vibration are poor tooth engagement and

extremely hard workpieces. Such problems must be anticipated by the broach designer.

Tooth Rise. The tooth rise or taper is calculated from one tooth to the next so that the

thickness of the chip does not impose too great a strain on individual teeth. A large tooth rise

increases powerrequirements. When all teeth are simultaneously engaged in the workpiece, too

large a tooth rise could cause an increase in power requirements beyond the rated tonnage of the

6
http://en.wikipedia.org/wiki/File:Broach_tooth_geometry.svg


7/110

machine. If the rise is too small to permit the teeth to bite into the workpiece, a glazed orgalled

finish will result.

Tooth Gullet. The depth of the tooth gullet is related to the tooth rise, pitch, and

workpiece material. The tooth root radius is usually designed so that chips curl tightly within

themselves, occupying as little space as possible.

Chip Load. As each broach tooth enters the workpiece, it cuts a fixed thickness of

material. The fixed chip length and thickness produced by broaching create a chip load that is

determinedby the design of the broach tool and the predetermined feed rate.

This chip load feed rate cannotbe alteredby the machine operator as it can in most other

machining operations. The entire chip produced by a complete pass of each broach tool must be

freely contained within the preceding tooth gullet. The size of the tooth gullet (which determines

tooth spacing) is a function of the chip load and the type ofchips produced. However, the form

that each chip takes depends on the workpiece material and hook. Brittle materials produce

flakes. Ductile ormalleable materials produce spiral chips.

7.4 Broaching Forces

A broaching operation has a large variation in strength during the process and the most

important value is the maximum force required. It is with this value that we can determine the

equipment that will perform the operation, but understanding how and why the force varies during

the machining is an important step to understand it fully.

7.4.1 Calculating the Cutting Forces

During the operation the number of teeth cut simultaneously (n) is "constant" and can be

calculated by the expression:

n = L/ p (7.1)

where:

L = length to be broached and p = pitch of the teeth of thinning. It should be noted that if the value

is not an integer, always rounds up.

7


8/110

The pitch (p) defines the tooth construction, strength, and number of teeth in contact with

the workpiece. There shouldbe at least two teeth in contact with the work piece at any time.

The pitch pis usually defined by the workpiece length using the following equation:

XLRPTp =3 (7.2)

where:

RPT= rise per tooth;

l= length to be cut [mm];

X= chip space number (3-5 for brittle workpiece materials and 6-10 for ductile and soft workpiece

material).

The maximum force in the operation ofbroaching can be calculated by expression:

Fmax=A.. n (7.3)

where,F = maximum force in broaching operation [kg], A = area of material removed [mm2], =

specific resistance of cutting, and n = number of cutting teeth simultaneously.

As the shape of the teeth varies, the value ofA also varies and therefore the value of the

force as well. In addition there is a change in the number of teeth in cut simultaneously when n is

not integer, generating a fluctuation.

Calculation example. Here is an example to simplify understanding. Suppose you need to

broaching an eight-slot hole, as shown below, on a piece of steel with a thickness of 32 mm and a

specific cutting resistance is 315 kg/mm2. The broach has 12 mm pitch and step forward thinning of

0.05 mm.

8
http://www.brighthub.com/engineering/mechanical/articles/47630.aspx?image=24135


9/110

First step: Calculating the number of teeth cut simultaneously (n):

n = L / p

n = 32/12 = 2,667

n = 3 teeth

Second step: Calculating the area of material to be removed (A)

A = 8 * (5 * 0.05)

A = 2 [mm2]

Third step: Calculation of the maximum force in broaching (Fmax)

Fmax = 2 * 315 * 3

Fmax = 1890 [kg]

Fourth and last step: Calculation of force at the beginning of broaching. At the beginning of the

operation, before joining the scheme, we have only 1 tooth in contact with the piece. In this case the

force is F1 = 630 [kg]. Shortly thereafter comes into the second tooth and the force will be F2 =

1260 [kg].

The most important characteristic of a broach is its RPT (rise per tooth), which changes

forvarious parts of the broach. Here they are defined as: roughing (tr), semi-finishing (ts), and

finishing (tf).

The roughing teeth remove most of the material so the number ofroughing teeth required

dictates how long the broach is. The semi-finishing teeth provide surface finish and the finishing

teeth provide the final finishing. tf is zero so that as the first finishing teeth wear the later ones

continue the sizing function. The maximum RPT is about 0.15 mm for free-machining steels and a

minimum of 0.025 mm for finishing teeth. Forsurfacebroaching the RPT is usually between 0.076

to 0.15 mm and fordiameter broaching is usually between 0.030 to 0.063 mm.

The exact depth depends on many factors, howeverifthe cut is too big it will impart too

much stress into the teeth and the workpiece; if the cut is too small the teeth will not cut but rub

the workpiece. The starting material condition should be 0.51 to 0.63 mm greater than the final

dimension for broaching to be effective.

9


10/110

The rake () determines the primary rake angle and is a parameter of the material being

cut. Forsteel is between 15 and 20 and forcast iron it is between 6 and 8.

The clearance () provides clearance for the teeth so that they don't rub on the workpiece;

it is usually between 1 and 3.

A relatively large pitch and tooth depth are required forroughing teeth to accommodate

greater chip volume in the chip gullet, especially when machining materials produce continuous

chips. Forsemifinishing and finishing teeth, the pitch is reduced to about 60% of that ofroughing

teeth to reduce the overall length of the broach.

The calculated pitch, according to Equation (7.2), should not be greater than l/2, to

provide betterguidance of the tool and to prevent the broach from drifting. To prevent possible

chattering and to obtain better surface finish, the pitch p should be made nonuniform as shown in

Fig.7.8.

Fig.7.8 Nonuniform pitching to prevent chattering, and engagement of more than two teeth to ensure guidance.

To avoid the formation of long chips, especially when broaching profiles and circular

shapes, chip breakers (Fig.7.9) are uniformly cut into the cutting edges of the broach in a

staggered manner. The chip breakers are ground into the broach, parallel to the tool axis. Chip

breakers on alternate teeth are staggered so that one set ofchipbreakers is followed by a cutting

edge. The finishing teeth complete the job.

Chip breakers are not necessary when broaching brittle materials produce discontinuous

chips. They are not used for finishing teeth and small size broaches. The use of chip breakers

10


11/110

reduces the pitch and consequently the overall length of the broach. As a result, the productivity

is enhanced and the tool cost may be reduced.

a)

b)

Fig.7.9 Composite of chipbreakers on a flat broach (a) and on a round broach (b).

Chip breakers are vital on round broaching tools. Without the chip breakers, the tools would

machine ring-shaped chips that would wedge into the tooth gullets and eventually cause the tool to

break. Special chip breaker designs can be used to increase the maximum tooth rise of a broach

without overloading the machine. If deep slots are ground into the lands of the cutting teeth, the

depth of cut can be increased on each tooth without fear ofoverloading.

The sections of the workpiece not machined by the first tooth are picked up by the next

tooth, or the next, by staggering the array of slots along the tool axis.

7.5 Types of broaches

Flat-Bottomed Gullet. Long cuts in ductile materials or interrupted cuts producing two or

more chips, would soon fill a circulargullet with chips. The solution is a flat-bottomed gullet with

extra-wide spacing (Fig.7.10). This provides room fortwo ormore spiral chips or a large quantity of

chip flakes.

Fig.7.10 Flat-bottom gullet.

11


12/110

Keyway Broach. Almost all keyways in machine tools and parts are cut by a keywaybroach

- a narrow, flat bar with cutting teeth spaced along one surface (Fig.7.11). Both external and

internal keyways can be cut with these broaches.

Internal keyways usually require a slottedbushing orhorn to fit the hole, with the keyway

broach pulled through the horn, guided by the slot (Fig.7.12).

Fig.7.11 Pull type broach.

Fig.7.12 Keyway Bushings.

a)a a) b) c)

Fig.7.13 Steps in machinig a keyway: a) Select the right bushing for the bore and insert in the bore of work; b) Insert

broach for the desired width of keyway into the bushing slot and check alignment , place this assembly in the press,

12


13/110

lubricate, push broach through; c) clean broach.

Multiple Spline Keyway Broach. When several keyways are spaced around a hole, the

resulting sections is a multiple-spline cut (Fig.7.14). A single keyway broach can be used to cut all

the splines by indexing the workpiece around a fixture. However, high production work usually

requires a multiple-spline broach. This tool is equivalent to a series of keyway broaches combined in

one tool, with the cutting teeth spaced around the tool diameter. These teeth can be straight sided,

involute, helical, spline or a combination. Helical splines (either straightsided or involute) can be

broached with a helical broach. The teeth are ground in a helical path around the tool axis. The helix

angle corresponds to that required in the work.

Fig.7.14 Multiple spline keyway broach.

Spiral Tooth Broach. The spiral tooth tool for internal broaching basically is a round

broach with teeth on a shear angle (Fig.7.15). The teeth are always engaged in the workpiece

which can reduce vibration.

Fig.7.15 Spiral tooth continuous-contact tool.

Burnishers. Burnishers are broaching tools designed to polish (by cold-working) rather

than cut a hole. The total change in diameter produced by a burnishing operation may be no more

13


14/110

than 0.012 mm to 0.025 mm. Burnishing tools, used when surface finish and accuracy are critical

and relatively short and are generally designed to pushbroaches.

Fig.7.16 Burnisher.

Burnishing buttons sometimes are included behind the finishing-tooth section of a

conventional broaching tool. The burnishing section may be added as a special attachment or

easily replaced shell. These replacement shells are commonly used to reduce tooling costs when

high wear ortoolbreakage is expected. They are also used to improve surface finish.

Carbide Broach Inserts. Broaching tools with brazed carbide broach inserts are frequently

used to machine cast-iron parts (Fig.7.17). Present practice, such as in machining automotive

engine blocks, has moved heavily to the use of disposable, indexable inserts, and this has

drastically cut tooling costs in many applications.

a)

b)

Fig.7.17 a) Brazed carbide inserts; b) indexable carbide broach.

Surface broaches. These Broaches are used to

remove material from an external surface are

commonly known as surface broaches (Fig.7.18).

Such broaches are passed over the workpiece surface

to be cut, or the workpiece passes over the tool on

horizontal, vertical orchain machines to produce flat

14


15/110

Fig.7.18 High Speed Steel (HSS) plane broach. orcontoured surfaces

While some surface broaches are ofsolid construction, most are of built-up design - with

sections, inserts, or indexable tool bits that are assembled end-to-end in a broach holder orsub-

holder. The holder fits on the machine slide and provides rigid alignment and support. The first

tooth of each insert or section of the assembly is ground to conform with the last tooth of the

preceding insert or section. Burnishing inserts are sometimes provided at the end of the holder to

perform their function after the other teeth have completed their operations, but such tools are very

vulnerable to metal pickup and can cause tearing.

Most surface broaches are special and designed for a specific application, but some standard,

general-purpose broaches are available. Broach holders can often be standardized in several sizes to

hold various surface broaches. Only the more common of the many different surface broaches

available are discussed in this section.

Pine-Tree Broaches. Pine-tree broaches cut the complex serrations used to lockturbine

blades into theirrotors (Fig.7.19).

Fig.7.19 Pine tree broach.

Common practice is to use a set of broaches;

the first cuts a straight-sided V-notch in the rotor

rim and is followed by one or more serrated

broaches that progressively widen the notch to the

full pine-tree configuration.

7.6 Broach material

The low cutting speeds used in most broaching operations (2-12 m/min) do not lend

themselves to the advantage of carbide tooling. Accordingly, most broaches are made ofalloy steel

and high-speed steel of high grades (Cr-V-grade), which have less distortion during heat treatment.

This is an important factor in the manufacture of long broaches. Titanium-coated high-speed steel

broaches are becoming more common due to their prolonged tool life.

15


16/110

Recently, carbide-tipped K-type (cobalt group) tools are employed to machine cast iron,

thus allowing highercutting speeds, increased durability, and improved surface finish.

However, carbide-tipped broaches are seldom used for machining steels and forged parts,

as the cutting edges tend to chip in the first stroke due to lack of rigidity of work fixture/tool

combination.

Almost all broaches are made of high-speed tool steels in monolithic construction.

Brazed carbide or disposable inserts are sometimes used for cutting edges, most often on tools used

for broaching cast irons.

Carbides. Most of the carbide cutters used to broach cast iron are used in flat surface

broaching applications, although contoured cast-iron surfaces have been broached successfully.

Surface broaching of pine tree slots has been tried with carbides on high-temperature alloy turbine

wheels, but with little success. The carbide edges tend to chip on the first stroke.

Carbide-Tipped Broaches. Carbide tips are seldom used on conventional steel parts and

forgings. One reason is that good performance is obtained from high-speed-steel tools; another is the

low cutting speeds of most broaching operations do not lend themselves to the advantages of carbide

tooling. The success of carbide tooling on cast irons is due to carbide's resistance to abrasion on the

tool flank below the cutting edge.

Anotherproblem with carbide-tipped tools is that a broaching machine work fixture must be

exceptionally rigid to prevent chipping of the cutting edge. Experimental work with extra-rigid

tools and workpiece fixtures, however, has shown that tool life and surface finish can be greatly

improved with carbide tipped tools, even when used on alloy steel forgings.

Cast high-speed tool steels are almost never used in broaches. One property of the cast tool

materials that prohibits their use in monolithic internal pull broaches is low tensile strength. Most

cast alloys that can attain a hardness of Rockwell C 60 or higher do not have ultimate tensile

strengths much in excess of 85,000 psi.

7.7 Broach sharpening

Broach sharpening is essential, as dull tools require more force, leading to less accuracy

and broach damage. Dull internal broaches have the tendency to drift during cutting.

16


17/110

The clearance angle of the sizing teeth of a broach is made as small as possible (10-20) to

minimize the loss ofsize when it is sharpened.

Also, the finishing orsizing teeth are commonly provided with a land of a small width of

50-200 m to limit the size loss due to sharpening. Most of broaches are sharpenedby grinding

the hookfaces of the broach. The lands must not be reground because this would change the size

of the broach (Fig.7.20). After sharpening, the tooth characteristics such as rake angle, clearance

angle, tooth depth, root radius, RPT, and pitch should not be altered.

Fig.7.20 Sharpening of tool face of a round broach.

The original grinding is the responsibility of the broach producer. Resharpenings are the

responsibility of the user, but the broach tool may be returned to the producer for resharpening in the

producer's plant. With proper care and use most broaches may be sharpened numerous times

17


18/110

However, the high-speed tool-steels used in making broaches include some of the most difficult-to-

grind steels known.

Internal broaches are sharpened by grinding them only on the face. Metal removal on the top

of the teeth changes the dimensions of the broached surface. Grinding on the tool face requires a

small grinding wheel inclined at an angle greater than the face angle because of the geometry

involved. Internal broaches may be retapered to remove abrasion in some cases.

7.8 Commonly broached materials

Broaches have been used on almost every material at one time or another - most of the

known metals and alloys, some plastics, hard rubber, wood, composites, graphite, and so on. Metals

and alloys are, by far, the most commonly broached materials. The products made from the other

materials are not usually made to the stringent dimensional tolerances, or in the quantities, that make

broaching economical.

In general, any material that can be machined can be broached. And the higher the

machinability of the material, the easier it is to broach. In steels, machinability correlates closely

with hardness. That is why workpieces with a high surface hardness, such as produced by previous

work-hardening or scale, require that the first broach tooth cut beneath the scale or hard surface is

possible.

The hardness of the workpiece material also influences the allowable cut per tooth. On

harder metals, it is customary to take a relatively fine finishing cut; on softer nonferrous metals, a

fine surface finish can be achieved with a heavier finishing cut.

Too heavy a cut, however, will tend to overload the broach tool - no matter what material is

being broached. Too fine a cut, on the other hand, tends to interfere with free-cutting action and

increases the tendency of the material to glaze, gall, or tear. Smaller steps can be used for finishing

than for roughing.

Stainless Steels. Stainless steels with hardnesses above Rockwell C 35 can be broached.

Stainless harder than this, however, tends to dull broach teeth fairly fast, reducing the number of

pieces produced between grinds.

The approximate rise per tooth (round broaches) runs from 0.025 mm to 0.125 mm. This

range will cover practically all types of stainless steel. Broaches with hook angles between 12 and

18


19/110

18 usually give the best results. Backoff should be held to a minimum; a 2 angle is preferable, but in

no case should it exceed 5. Chipbreakers should be used.

Cast and Malleable Irons. Cast and malleable irons permit a greater rise per tooth than

even the free-machining steel. Brittle materials such as cast iron call for small hook angles, usually

around 6 degrees to 8 degrees. Backoff angles are the same as for the general run of steels. Usually,

a shorter pitch is permissible in broaching cast irons than in broaching steels because less chip room

is required for the irons.

Brasses and Bronzes. Brasses and bronzes allow a slightly heavier step, or rise per tooth,

than steel. Too heavy a rise, however, will tend to overload the broach. Hook angles usually range

from 0 degrees up to 10 degrees and even higher, increasing with ductility of the metal being

broached. Brittle brasses call for smaller angles, from +5 degrees to -5 degrees. Backoff angles are

usually 2 degrees on the roughing teeth, 1 degree on the semi-finishing teeth, and 0.5 degrees on the

finishing teeth. Some form of chipbreaker is required.

Aluminum and Magnesium. Aluminum and magnesium can be broached with standard tool

design, although special broaches give even better results. A hook angle of 10 degrees to 15 degrees

and a backoff angle between 1 degree and 3 degrees are recommended. Heavier cuts can be taken;

even the finishing teeth can remove as much as 0.05 mm each. If trouble is experienced in

maintaining proper tolerances, the size of the finishing cut can be increased, rather than decreased,

to correct the situation.

Ductility of a Metal. The ductility of a metal has a considerable influence on the selection of

an optimum hook angle for the broach teeth. In general, this angles decreases with decreasing

ductility. Brittle materials, therefore, call for very small hook angles

7.9 Cutting fluids for cutting

The three functional roles of cutting fluids are to provide lubrication for the cutting tool,

to reduce heat at the interface of the tool and the workpiece, and to flush away metal chips and

fines from the cutting zone. In addition, to these roles cutting fluids can improve the surface finish

of the machinedpart.

19


20/110

In broaching, temperature reduction is an important aspect ofincreasing tool life.

Studies have indicated that the reduction of heat at the tool interface by as little as 50

degrees can increase tool life by as much as 50% in addition to increased tool life through

temperature reduction, the sheer angle of the chip is increased thereby reducing the power

requirements and the possibility of part degradation.

While the application of cutting fluids to the point of cut is critical in broaching operations, it

is not always an easy thing to accomplish. An internal broach tool, for example, may receive an

adequate supply of cutting fluid upon entering the workpiece however, upon entering the work the

fluid is retarded. This has been noted in some horizontal broaching operations where surface finish

and cutting tool life are good at the starting half of a horizontal internal shape but poor at the final

half.

During horizontal internal broaching, the flow of cutting fluid into the interior of a

workpiece is restricted by the cutting teeth. Fluid trapped between the tooth spaces flows by gravity

to the lower half of the tool; the upper teeth may be cutting dry in a very short time. The problem

can be resolved by submerging the workpiece in cutting fluid during the entire broaching operation.

When broaching long internal shapes such as rifle barrels, high pressure streams of cutting fluid can

be forced through the bore and around the broach. This helps to accomplish two functional roles.

First it insures the cutting fluid is getting to the cutting zone and that the fluid pressure will flush the

chips out of the workpiece. High temperature alloys and exotic metals can cause special problems

because cutting forces are higher and more heat is generated. One approach is to reduce the cutting

speed of the broach thereby allowing heat transfer by conduction and using a high water based

synthetic which reduces heat.

Many synthetic cutting fluids provide exceptional lubricating properties while also

delivering the advantage of faster cooling than conventional straight oils. In that synthetics have the

viscosity of water (32 viscosity) it is imperative that the broaching tool be fully flooded with the

cutting fluid at the point of the cut. The usual approach to selecting a cutting fluid for a particular

broaching operation involves trying several fluids to determine which give the best performance.

The best performance is a combination of finished part quality, tool life and cutting fluid

compatibility with the broach and disposal requirements. Regardless of the type of product you

ultimately select, you must deliver the cutting fluid to the point of cut in order for the fluid to

perform its function.

20


21/110

The following list of workpiece materials and the cutting fluids used in broaching them

should be considered as a starting point rather than a recommendation.

Material Product

Type

Concentration

Ratio

Low carbon steel Soluble oil 10%Free-machining steel Semi-synthetic 5-10%

High carbon steel Semi-synthetic 5-10%

Alloy steel Soluble oil or straight oil 10%

Tool steel Heavy duty soluble oil 5-10%

Cast iron Synthetic 5-10%

Stainless 300 Heavy duty soluble oil 5-10%

High temperature allow Heavy duty soluble oil 5-10%

Titanium Heavy duty soluble oil 5-10%

7.10 Broaching machines

In comparison with other types of machine tools, broaching machines are notable for their

simple construction and operation. This is due to the fact that the shape of the surface produced in

broaching depends upon the shape and arrangement of the cutting edges on the broach. The only

cutting motion of the broaching machine is the straight line motion of the ram. Broaching

machines have no feed mechanism, as the feed is provided by a gradual increase in the height of

the broach teeth.

Hydraulic drives, developed in the early 1920s, offered pronounced advantages over the

various early mechanical driving methods. Most broaching machines existing today are of

hydraulic drive, accordingly characterizedby smooth running and safe operation.

The choice between vertical and horizontal machines is determined primarily by the

length ofstroke required and the available floorspace. Vertical machines seldom have strokes

greater than 1, 5 mbecause ofceiling limitation. Horizontal machines can have almost any stroke

length; however, they require greaterfloor space.

The main specifications of a broaching machine are as follows:- Maximum pulling or pushing force (capacity) [ton];

- Maximum stroke length [m];

- Broaching speed [m/min];

- Overall dimensions and total weight.

21


22/110

Horizontal broaching machines. Currently, horizontal machines are finding increasing

favor among users because of their long strokes and the limitation that ceiling height places on

vertical machines. About 47% of all broaching machines are horizontal units. Horizontal internal

broaching machines are used mainly for some types up to 3 m, and cutting speeds limited to less

than 12 m/min. Broaching that requires rotation of the broach, as in rifling and spiral splines, is

usually done on horizontal internal broaching machines. Horizontal machines are seldom used for

broaching small holes.

Horizontal surface broaching machines may be hydraulically or electromechanically

driven. In these machines, the broach is always supported in guides. The surface hydraulic

broaching machines are build with capacities up to 40 tons, strokes up to 4, 5 m, and cutting speeds

up to 30 m/min. These machines are basically used in the automotive industry to broach a great

variety ofcast iron parts for nearly 30 years.

On the other hand, the electromechanically driven horizontal surface broaching machines

area available with higher capacities, stroke lengths, and cutting speeds (up to 100 tons, 9 m, and 20

m/min, respectively). Carbide-tipped broaches are used to machine cast iron blocks of internal

combustion engines.

Fig.7.21 Major components of a broaching machine.

Verical broaching machines. These machines are almost all hydraulically driven. They are

used in every area of metal working. Depending on their mode of operation, they may be pull-up,

pull-down, orpush-down units. Fig.7.22 schematically illustrates a pull-down vertical broaching

machine in which the work is placed on the worktable. These machines are capable of machinig

internal shapes to close tolerances by means of special locating fixtures.

22


23/110

They are available with pulling capacities from 2 to 10 tons, strokes from 0, 4 to 2, 3 m, and

cutting speeds up to 24 m/min. When cutting strokes exceed existing factory ceiling clearances,

expensive pits must be dug for the machine so that the operator can workat the factory floor level.

Fig.7.22 A schematic of a pull-down vertical broaching machine.

The pull head (Fig.7.4) is recommended for pull down broaching operations where chips fall

or are washed downward by coolant onto the pull head. The special design of the head provides an

umbrella top to shed falling chips and a hole the length of the puller for chips to wash through.

In Fig.7.23a is shown a progressive broach steps in manufacture gears by cutting teeth ininternal gears and Fig.7.23b shows broaching of an external spurgear using a rotatingbroach.

23


24/110

Fig.7.23 Gear broaching by forming: a) broaching of an internal spur gear using an axial broach; b) broaching of an

external spur gear using a rotating broach.

24


25/110

7.11 Rotary broaching

Rotary broaching is a completely different process. It can cut the same forms as

conventional broaching, but it can be made on screw machine orlathe. A special rotary broaching

tool holder is mounted on the machine turret, and rotary broaching becomesjust another step inthe process. This eliminates the need forsecondary operations to form square holes, hex holes,

splines orgear teeth, or almost any otherinternal orexternal shape you want. Rotary broaching

easily works in blind holes, which is not possible with conventional broaching.

Cutting Principle. A rotarybroaching tool has cutting edges the shape of the hole orform

that is necessary. It mounts in a toolholder that holds the tool at a 1-degree axial tilt in relation to

the center line of the workpiece and has a 1030clearance angle built in (Fig.7.24).

Fig.7.24 How rotary broaching works.

Bearings in the toolholder allow the tool to

rotate freely. The workpiece is turning, and when

the tool comes in contact, it rotates right along

with the workpiece. Because of the 1-degree axial

tilt, the tool appears to wobble as it rotates.

Because of this, rotary broaching is sometimes

called wobble broaching. It is also known as

Swiss broaching.

Fig.7.25 Rotary broaching principle.

As can be seen in Fig.7.25, the broaching

cutterspindle is driven into the part at a 2 degree

included angle of its cone ofrotation. This causes

the broach to cut only on its leading edge, not its

full end surface as it would if it did not have the 1

degree offset. This eases the load of the cut and

creates a shearing, rotational cutting action so that

cutting tool is actually spiraling its way into the

part.

25


26/110

Fig. 7.26 Rotary broaching in action.

Before the rotarybroaching operation, the workpiece needs to be drilled orturned to the

correct diameter for use with the rotary broaching tool. This minimizes the amount ofmaterial

that the tool will cut. Then, the area where the tool will contact the workpiece is countersunkor

chamfered, to allow smooth engagement of the tool. If the chamfer or countersink is not

acceptable in the final part, you can design your process to remove it afterward. Then the part is

ready for broaching. The following describes internal rotary broaching; external is similar.

As the prepared workpiece is turning, the rotary broaching tool/toolholder advances

toward it. Because of the 1-degree axial tilt, only one corner of the tool engages the workpiece at

first. When the tool makes contact, the workpiece drives the tool to rotate in unison with it.

During rotation, first one corner of the tool contacts the workpiece, then the next, and so on,

around and around. As the tool advances into the workpiece, each corner, in turn, cuts into the

metal. This way, bitby bit, the tool cuts a shape that matches the shape of the tool.

Blank preparation. For internal broaching, the hole should be drilled 0.12 0.36 mm

larger than smallest diameter of the broach, and countersinkit at 90 degrees to slightly larger than

largest dimension of broach. Drill the hole as deep as possible to leave room for chip accumulation.

Spiralling of the broached form is caused by the back taper on the broach. Since the

broach is driven by the leading edge of the hole (ID) against the nearest surface of the broach

(BB), the spacebetween the broach and the hole caused by the backclearance allows the broach

to rotate slightly and cut a spiral as shown in Fig.7.27.

26


27/110

Fig.7.27 Spiralling of the broached form.

For external broaching, turn the O.D. to be broached 0.12 0,36 mm smaller than the

largest dimension of the broach and form a 45 degree chamfer on the front end down to at least

the smallest dimension of the broach. This 45 degree angle is essential for easy starting of thebroach; if the part design does not permit it, it can be removed after broaching.

Nothing is more important than having the cutter centered as close as possible to the center

of the workpiece. Improper centersetting will cause uneven hole configurations, oversize holes,

spiralling, excessive cutterwear and excessive holderwear.

Broach geometry. Internal broaches must be ground with a 1-1/4 degree maximum back

clearance on all surfaces. The face should be dished at a 7-10 degree clearance. This can be done

with a carbide centering-type drill. The larger end of the broach should be made to the high side of

the part tolerance since the broach gets smaller as it is sharpened.

External broach dies must be made with maximum 1-1/4 degree back taper (draft) on all

surfaces. The front end of the opening in the die should be made to the low side of the part tolerance

since the hole gets larger as the broach is resharpened.

a) b)

Fig.7.28 Offset internal (a) and external (b) rotating broach tool holder and broach.

27


28/110

This tool is used to produce hexagon, square and other irregular internal shapes in the

workpiece as it is rotating in the spindle of the machine. Minimal turret driving force is required due

to the spindle offset which distributes the load of the cut. The tool holder spindle, which is mounted

in ball bearings, rotates freely with the workpiece and does not require auxiliary driving attachments

(Fig.7.29).

Fig.7.29 Cross section in external rotating broach.

1. Shank 2. Body3. Spindle4. Radial Bearing5. Set Screw7. Clamp Screw & Washer8. Adjusting Screw15. Seal18. Grease Fitting

19. Spindle Nut

a) b)c)

Fig.7.30 Internal broaches; a) square; b) 6 lobed head screw; c) involute splines.

Fig.7.31 Examples of special shapes that have broached.

28


29/110

a) b)

Fig.7.32 a) Driven head with square broach; b) Driven head with serration broach in lathe turret.

The driven attachments shown in Fig.7.32 offer adaptation to CNC lathes that utilize a

driven tool turret. The broaching tools used in the attachment are available for both internal and

external shapes such as squares, hexes, stars, serrations and any number of special geometries.

Usual coolant or cutting oil can be used, however it is typically unnecessary as rotary

broaching produces little heat. Also, excessive coolant in the broach pilot hole can result in

hydraulic lockup during broaching. Vented broaches can solve this problem if it encountered.

Fig.7.33 Rotary broaching tools gallery.

29


30/110

Fig. 7.34 Gallery of part examples obtained by rotary broaching.. Vented broaches can solve this problem if it encountered.

30


31/110

Chapter eight

Finishing operations

8.1 Introduction

As the name of this group of abrasive operations suggests, their objective is to achieve

superior surface finish up to mirror-like finishing and very close dimensional precision. The

finishing operations are assigned as the last operations in the single part production cycle usually

after the conventional or abrasive machining operations, but also after net shape processes such

as powder metallurgy, cold fleshless forging, etc.

The finishing processes discussed in this section include honing, lapping, superfinishing,polishing, and buffing. The typical surface finishes for these operations are presented in the Fig.8.1.

Also presented forcomparison are surface roughness values for fine grit size grinding.

Fig.8.1 Typical surface finishes for finishing operations.

8.2 Honing

Honing is defined as a precision method to carry out grinding work of a workpiece in

contact with the surface of honing stones in an abundance of coolant while simultaneously

providing the honing stones with rotating and reciprocating movements and also applying

pressure on the inner surface of a hole.

31


32/110

As a result of pre-machining operations, the workpiece generally possesses geometrical

inaccuracies, which can only be eliminated by honing. The peaks and valleys can be eliminated

by the continuous contact between honing stone and workpiece. Roundness errors can be

corrected by the honing stones encompassing a large contact area with the workpiece. Honed

surfaces have a highbearing area and are extremely durable and wear resistant.

Some shapes of holes than can be honed are shown in Fig.8.2.

Fig. 8.2 Shape of holes to be honed.

With the honing stones brought into contact with the workpiece on a rectangular cross-

section, honing is carriedoutby rotating,reciprocating and pressurizing (expanding) one or a

few bar-shaped honing stones fitted into a body called mandrel (or hone). Accordingly, unique

cross-hatching (mesh pattern) is formed on the honed surface.

In addition to removing stock, honing involves the correction of errors from previous

machining operations. These errors include:

- geometrical errors such as out-of-roundness, waviness, bell mouth, barrel, taper, rainbow, and

reamer chatter;

- dimensional inaccuracies;

- surface character (roughness, lay pattern, and integrity).

Honing corrects all of these errors with the least possible amount of material removal;

however, it cannot correct hole location orperpendicularity errors. The most frequent application

of honing is the finishing ofinternal cylindrical holes.

32


33/110

However, numerous outside surfaces also canbe honed. Gear teeth, valve components, and

races forantifriction bearings are typical applications ofexternal honing. The hone is allowed to

floatby means oftwo universaljoints so that it follows the axis of the hole (Fig.8.3). Owing to the

fact that the tool floats, the honing sticks are able to exert an equal pressure on all sides of the bore

regardless of the machine vibrations, and therefore, round and straight bores are produced.

Honing requires no chucking oralignment. The process allows the part to float on the

tool, aligning itself with and being supported by the tool. In this way, honing can remove the bore

errors caused by otherless accurate machiningprocesses.

As the tool reciprocates through the bore, the pressure and the resulting penetration ofgrit

is greatest at high spots and consequently the waviness crests are abraded, making the bore

straight and round. After leveling high spots, each section of the bore receives equal abrading

action. The hole axis is usually in the vertical position to eliminate gravity effects on the honing

process; however, for long parts the axis may be horizontal.

Fig.8.3 Floating hone using two universal joints to

permit the bore and the tool to align.

Honingprocess has some advantages such as:

- it is characterized by rapid and economical stock

removal with a minimum ofheat and distortion;

- it generates round and straight holes by

correcting form errors caused by previous

operations;

- it achieves high surface quality and accuracy.

8.2.1 Process capabilities

33


34/110

1. Materials: Although cast iron and steel are the most commonly honed materials, the process

can also be used for finishing materials ranging from softer metals like Al-and Cu-alloys to

extremely hard materials like case nitrided steels or sintered carbides. The process can also be used

for finishing ceramics and plastics.

2. Bore size and shape: Bores as small as 1, 6 mm in diametercan be honed. The maximum bore

diameter is governed by the machine power and its ability to accommodate the workpiece.

Machines powered by motors of up to 37 kW are available that can hone bore up to about 1200 mm

in diameter. Honing bores up to 760 mm in diameter is a common practice. Although most internal

honing is done on simple, straight-through holes, blind holes with a slight taper can also be

honed. It is not feasible to hone the sides of a blind hole flush with the bottom. Bores having

keyways can be honed and so can male or female splines.

3. Stock removal: In honing, a general rule is to remove twice as much as the existing error in

the workpiece. For example, if a cylinder is 50 m out-of-rounds or tapered, a removal of 100 m

will be required for complete cleanup.

The work in preceding operations is usually planned so that the amount of stock removed in

honing is minimized. On the other hand, stock removal of up to 6, 4 mm may be practical for rough

honing in some applications. For instance, as much as 2, 5 mm is honed from the inside diameter of

hydraulic cylinders, because stock removal through honing is more practical and economical than

attaining close preliminary dimensions by grinding or boring. Another example occurs in finishing

bores of long tubes, where even larger amounts as much as 6 mm may be removed by honing,

because it is the only practical method. Such tubes are finished by honing immediately after

drawing. Honing is performed at a rate of 32 cm 3/min from soft steel tubes; for tubes steel-hardened

to 60 HRC, the rate is reduced to 16 cm3/min.

Rough honing is employed before finish honing when large amounts of stock are to be

removed and specific finishes are required. Sticks containing abrasives of 80 grit or even coarser are

used forrough honing to maximize the removal rate. Finish honing is accomplished by abrasives of

180-320 grit or finer.

34


35/110

4. Dimensional accuracy and surface finish. Internal honing to tolerances of2, 5 25 m is

common. Surface roughness Ra of 0,25 0,38 m can be easily obtained by rough honing and

roughness ofless than 0,05 m can be achieved and reproduced in finish honing.

5. Honing sticks. The same designation system ofgrinding wheels is applied to honing sticks.

Honing sticks commonly used may be vitrified, resinoid, or metallic honed. The bond must be

strong enough to hold the grit; however, it must not be so hard as to rub the bore and hence retard

the cutting action.

The grit selection depends generally on the desired rate ofmaterial removal and the degree of

surface finish required. Guide rules for selecting the type of abrasive materials are as follows:

- Al2O3,is widely used forsteels;

- SiC is generally used forcast iron and nonferrous materials;

- CBN is used forall steels (soft and hard), Ni and Co base super alloys, stainless steels, Br-

Cu-alloy, and Zr.

- Diamonds are used for chromium plating, carbides, ceramics, glass, cast iron, brass, bronze, and

surfaces nitrided to depths greater than 30 m.

Cylinder block

Components of an injection pump

Connecting rod

Transmission gear

Fig.8.4 Examples of parts that are obtained by honing with diamond or CBN tools.

35


36/110

In modern manufacturing, Diamond & CBN honing stones represent the most cost effective and

best technological solution. The extreme hardness of Diamond and CBN guarantees long lifetime

cycles for the honing stones, a basic requirement for a production facility with a high degree of

automation and multi shift operation.

On account of the particular hardness of the abrasive material, honing stones of Diamond or CBN

stones have an excellent accuracy of form. This has an impact on the process reliability and the

precision of the honed bore. The honing of very small bores can often only be carried out by

Diamond or CBN.

Some type of honing stones is given as above: (W = Width (mm); H = Height (mm); L =

Length (mm); A = Abrasive height (mm); S = Slot width (mm)).

Standard honing stone: Honing stones having

abrasive layer on steel base. Honing stone is

soldered on honing shoe / carrier.

Solid honing stone: This stone consists only the

abrasive (No steel base) and is also soldered on

the honing shoe/carrier or holdersegment.

Slotted standard honing stone: Same as

standard honing stone but with a slot

longitudinally for better cutting, cooling and

chip removal.

Compact honing stone: Honing stones for

direct fitting into the body of the honing holder.

This type of stone is generally used for very

small bore honing.

36


37/110

8.2.2 Machining parameters

Parameters affecting the performance ofhoningprocess are:

1. Rotation speed. The choice of the optimum surface speeds is influencedby:

- material being honed higher speed can be used for metals that shear easily;

- material hardness harder material requires lower speed;

- surface roughness rougher surfaces that mechanically dress the abrasive stick permit higher

speed;

- number and width of sticks in the hone speed should be decreased as the area ofabrasiveper

unit area to be hone increases;

- finish requirement higher speed usually results in finer surface finish.

2. Reciprocation speed. Reciprocation speed commonly ranges from 1, 5 to 30 m/min for a

variety of metals and alloys (Fig.8.5).

3. Control of cross-hatch angle. The cross-hatch angle 2n (Fig.8.6) obtained on a honed surface

is given by :

u

a

n V

V

tg=

(8.1)

where:

n = half cross-hatch angle;

Va = (axial)reciprocation speed (m/min);

Vu= (peripheral) rotational speed of the honing head (m/min);

37


38/110

Fig.8.5. Schematic illustration of honing operation.

Depending on the material to be honed, the

rotational surface speed typically varies from 15 to

90 m/min. Experience with a particular application

may indicate advantages for higher or lower speeds.

Rotation speeds as high as 183 m/min have been used

successfully. However, a reduction of surface rotation

speed can reduce the number of rejects. Excessive

speeds contribute to decreased dimensional

accuracy, overheating of the workpiece, and glazing

of the abrasive stick. Overheating causes breakdown

of honing fluid and distortion of the workpiece.

Fig.8.6 Honing operation: a) honing head with honing sticks; b) cross-hatched angle.

When rotation and reciprocation speeds are equal, the cross-hatch angle is 900.

Cross hatch angle 300 450 600 900

Stroke speed

1 1 1 1Peripheral speed 3,7 2,4 1,75 1

Forsome applications (engine cylinder bores), the cross-hatch angle is an important feature

that shouldbe noted in specifications.

The cross-hatch scratch pattern left on the wall of cylindrical surfaces tends to retain

lubricating fluids and thus reduce the wear in mating components. In the majority of applications,

38


39/110

although an angle of300 is commonly recommended, any angle within the range 20 - 450 is usually

suitable.

Cutting speed Vsis calculated with equation:

22 uas VVV += (8.2)

4. Honing pressure. It is selected depending on hardness and toughness of the material,

characteristics of honed surface (plain or interrupted by keyways), type of stick, and so on.

Insufficient pressure results in a subnormal rate of metal removal and rough surface finish.

Excessive removal rate and rough finish can cause an increased stickcost as well as decreased

productivity due to time loss offrequent tool exchange.

5. Honing fluids. Lubrication is more critical in honing than in most other material removal

operations. Honing fluids are necessary to act as lubricants, coolants, and remove swarf. No

single honing fluid possesses all requirements needed for honing process. Therefore, mixtures of

two ormore liquids are commonly used.

Water-based solutions are superior as coolants, but they are poor lubricants, have insufficient

viscosity to prevent chatter, and cause rust. Because of this, water-based solutions are seldom used

as honing fluids.Mineral seal oil is effective and widely used for honing. It has a higher viscosity and flash

point than kerosene. It is less likely to cause skin irritation. Mineral oils used for other machining

operations have also proved satisfactory when one part oil is diluted with four parts kerosene.

8.2.3 Honing machines

39


40/110

For the production offewparts, honing may be performed on drill presses orengine lathes

on which arrangements can be made for simultaneous rotating and reciprocating motions. The

stroking can be done manually orpowered depending on the equipment capabilities.

On the other hand, the production honing is done with machines built for the purpose.

These vertical machines are available in a wide range of sizes and designs. Some horizontal

machines operate by manual stroking. In power stroking, the workpiece is usually held stationary

in a rigid fixture, while the hone is rotated and hydraulically powered for stroking, which is

considered beneficial for heavier workpieces.

a)

b)

Fig.8.7 a) Vertical and b) horizontal honing machines.

Both horizontal and vertical honing machines are available. In general honing, no proof is

available that one type of machine gets better results than the other, either in speed or in accuracy

attainable. There are, however, some obvious limitations. For instance, to hone a 3 m long tube, a

vertical machine would have to be at least 8 m tall, and it would be difficult to find a building to fit

it in.

8.3 Design recommendations

40


41/110

On the surface of a workpiece being honed, allowance must be made for the multidirectional

application of the abrading members. Accurate geometric characteristics can be generated only

when the abrading elements can be applied uniformly and repetitiously over the entire area of the

surface to be honed. For example, projections such as shoulders, bosses, etc., must be avoided when

designing a workpiece for the honing operation. The same general rule applies to the honing of other

geometric figures such as spherical surfaces, flat surfaces, and outside diameters. The area adjacent

to and beyond the surface to be honed must be free ofinterferingprojections.

When honing an inside diameter, the abrading elements must overrun the ends of the bore

by an amount equal to one fourth to one-halfof the length of the abrasive used (Fig.8.8).

Fig.8.8 Design recommendations for internal cylindrical

surfaces which are honed.

Keyways, ports, undercuts, and other surface

interruptions frequently present problems on

the surface to be honed. Because an abrading

element has a tendency to overcut whenever

an edge surface is passed over by the abrasive,

some degree of washout, or depression of a

surface, can be expected around the edge of the

surface interruption. When they are essential to

the functional design of the workpiece,

interruptions such as keyways or ports should

be kept as small as the limits of good design

will permit so that the abrading elements can

pass over these interruptions with minimal

effect.

In designing workpieces that require application of the honing process, it is important that

the part have easily identifiable locating surfaces. Also, the part must have convenient clamping

pads that will not cause workpiece distortion during application of the process.

8.4 Lapping

41


42/110

Lapping is a finishing process, which is carried out using loose abrasives. Surface

smoothing is achieved by the abrasive grains that are supported by the lap, which acts as the

cutting tool shank. Extremely high accuracy ofform and dimensions, and a good surface quality

are obtained. Lapping is characterizedby the type of the relative motion between the lap and the

workpiece surface. Lapping is done by charging a lap, which is made of soft material with

abrasive particles and rubbing it over the workpiece surface with a slight pressure as shown in

Fig.8.9.

Fig.8.9 Principles of lapping.

The process is classified into hand and machine lapping, and mechanical and chemo-

mechanical. Lapping can be performed with free orforced lap charging with abrasives. Rotary,

plane, and profile lapping also tackle different workpiece forms.

Lapping is a final operation, which results in four majorrefinements in the workpiece:

- extreme accuracy ofdimensions;

- correction ofminor imperfections ofshape;

- refinement ofsurface finish;

- producing close fitbetween mating surfaces;

Lapping does not require the use ofholding devices; therefore, no workpiece distortion

occurs. Additionally, in normal lapping, less heat is generated than in most of other finishing

operations. This minimizes the possibility ofmetallurgical changes to the machined parts.

When both sides of a flat workpiece are lapped simultaneously, extreme accuracy in

flatness and parallelism, and the reliefof inherent stresses can be achieved.

8.4.1 Process components

42


43/110

Laps: For proper lapping performance, the particular grains must be partially driven into

the lap surface for a certain time. The lap material plays the role similar to the bond in the

grinding wheels. Only the grains that are embedded in the lap perform the abrasive action.

Too hard a lap causes excessive and rapid chipping ofabrasive grains and reduces the

periods during which the grain remain embedded in the lap. This causes a faster wear of the

abrasive mixture and the reduction of material removal rate. In addition, some of the grains are

driven into the workmaterial, thus, deteriorating the machined surface quality.

Too soft laps cause the abrasive grain to be driven too deeply and permanently into the

lap material. Consequently, as the grain corners become worn, the rate of metal removal

diminishes and the quality of surface finish and the surface layerdeteriorates.

A properly selected lap material enables the grains to be temporarily supported, but also

to change theirposition repeatedly. Under such circumstances, each grain may cut with several

corners instead ofone. This favors longerabrasive life, increase in material removal rate, and the

improvement of surface finish and quality of the surface layer.

Laps used for machine lapping are mostly made of ferritic cast iron (120-160 HB) or

pearlitic cast iron (160-200 HB). The former is better for free lap charging and the latter is

recommended for free lap charging. Steel,copper alloys, lead, plastics, mirror, glass, and wood

are also used as lap materials. The lap shape and dimensions should make possible the lapping of

work surface with the necessary accuracy and even distribution of abrasive compound over the

entire workpiece surface. Regarding the lap wearability, cast iron is the best material. The

wearability of cast steel laps is 25 percent higher than that ofcast iron and the wearability of

copper laps is 250percent higher.

Lapping material. Lapping utilizes abrasive mixture in the form ofcompounds. However,

slurries containing abrasives suspended in a cutting fluid are also used. The abrasive mixture is

characterized by the properties of the abrasives, properties of the mixture or the cutting fluid,

and the degree of concentration and regeneration. Aluminum oxide and silicon carbide are

commonly used forlapping steel, cast iron, nonferrous metals, and sintered materials.

In addition, boron carbide is mostly used for lapping sintered carbides, diamond dust is

used forhardened steel and sintered carbides; chromium oxide is used for obtaining the highest

surface quality of steel and copper alloy surfaces. The size of the abrasive dust can be taken as is

shown in table below:

43


44/110

Table 1 The size of the abrasive dust.

Silicon carbide 100 5 m

Boron carbide 60 5 m

Diamond dust 5 0,5 m

Chromium oxide 2 1 m

Compound: The compound to be mixed with the abrasives (vehicles) should ensure

sufficiently the following properties:

- holding the abrasives in a uniform suspension during the lapping operation;

- should not evaporate easily;

- be non-corrosive and non-toxic;

- easily removable by normal cleaning;

- adhere to the lap and, therefore, minimizes the waste of the machining compound;

- respond to temperature variations with the viscosity characteristics desired in a given

application;

Lubricating compounds include grease, tallow, stearin, and wax. In some cases colloidal

graphite is added to improve the lubricating properties. Machine oil, rape oil, and paraffin are

mainly used as lapping fluids where the abrasive dust is suspended.

Fig.8.10 Dependence of the rate of material removal on abrsive concentration and unit pressure.

The optimum degree ofabrasive concentration depends on the unit pressure. It has been

established experimentally that increasing the concentration necessitates a corresponding increase

in the unit pressure if the maximum rate of material removal under given conditions is to be attained

as shown in Fig.8.10. However, after a certain value of abrasive concentration, further increase in

unit pressure does not result in a corresponding increase in material removal rate.

44


45/110

Lapping allowance: The machining allowance in lapping could be of the order of the

mean total height of the surface irregularities Rtm left over from the previous machining operation

(10 20 m). The machining allowance depends on the previous machining operation and the

workpiece material hardness (typical values are shown in Table 2).

Table 2 Lapping allowances for different workpiece materials.

Work material Lapping allowance (mm)

Cast iron 0,2

Aluminum 0,1

Soft steel 0,01 0,02

Ductile steel 0,05 0,50

Hardened steel 0,005 0,020

Glass 0,03

Cemented carbide 0,03 0,05

Bronze 0,03

Lapping can also be used to correct shape and improve dimensional accuracy of the

workpiece where the lapping allowance is increased to 0, 1 mm. In such a case preliminary and

finish lapping must be followed. The initial preliminary pass uses coarse grains and higher unit

pressure; fine grains and a lower unit pressure are used for the finish pass.

The accuracy obtained by lapping depends on the method and time of lapping, initial

accuracy of workpiece, kinetic and geometric accuracy of the lap, etc. Generally the attainable

dimensional accuracy lies in the range of 0,5 m.

Similarly, a high quality ofsurface finish is obtained by lapping depending on the initial

roughness and conditions of the lappingprocess. The lapped surface is usually matt with a surface

roughness of0,08 0,02 m Ra.

8.4.2 Process characteristics

Fig.8.12 shows the main factors that affect the performance of the lapping process, which

include:

Effect of unit pressure. Practical unit pressures are maintained whitin 25 kg/cm2 for Al2O3 and

from 0, 52, 5 kg/cm2 for SiC in preliminary lapping, and whitin limits of 0, 3-1, 2 kg/cm 2 in finish

lapping.

45


46/110

Fig.8.12 Factors affecting lapping performance.

Fig.8.13 shows the effect of unit pressure on the surface roughness and the linear removal

rate. Accordingly, the optimum unit pressure with respect to the linear removal rate p2 is higher than

the optimum pressure with respect to surface roughnessp1.

Fig.8.13 Typical dependence of mean total height of surface irregularities and rate of metal removal unit pressure.

The optimum unit pressure should, therefore, be contained within the limits P1 P2 if the

roughness required lies between Ra1and Ra2. Additionally, if the roughness required Ra>Ra2, the unit

pressure should be taken asP2.

As shown in fig.8.13, increasing the nominal pressure from very low values at constant

concentration and size of abrasives causes a greater depth of cut and thus a larger volume of material

removed by the particular lapping grains. At high pressures, the load acting on the grains exceeds

their compressive strength, which causes crushing and disintegration of the lapping grains. Thus, the

46


47/110

volume cut by each single grain decreases, which is followed by diminishing the rate of material

removal and a rise in surface roughness.

Effect of grain size: As shown in Fig.8.14, surface roughness increases monotonically with

grain size. On the other hand, the rate of material removal (under constant lapping pressure) reaches

a maximum value at a grain size 0g . Accordingly, if the surface roughness greater than Ra0 is

required, grain dimension of 0gg = should be selected. IfRa = Ra1 choose .1gg =

Fig. 8.14 Dependence of linear removal rate and surface roughness on grain size.

Effect of concentration: Increasing the abrasive concentration in the mixture (constant grain

size and unit pressure) results in an increase in the number of grains per unit area, but reduces the

pressure excreted on a grain, which reduces the volume cut by the working grains reach its

maximum as shown in Fig.8.15. At higher concentration, a reduction of linear material removal rate

occurs because the volume cut by the particular grains diminishes quicker than the number of

working grains increase.

At low concentration a reduction of linear material removal rate occurs as the pressure

excreted on the grains exceeds the compressive strength of some of them and those are crushed,

which result in a decrease of material removal rate.

47


48/110

Fig.8.15 Dependence of linear removal rate on grain concentration.

Lapping speed: This is the speed of the lap relative to the workpiece surface. The

dependence of removal rate and surface roughness on the lapping speed is shown in Fig.8.15.

Accordingly, the linear removal rate increases with the lapping speed at higher rate than does the

surface roughness. Table 8.3 shows typical speeds that are applied in machine lapping of plane

surfaces.

Fig.8.15 Dependence of linear removal rate on the lapping speed.

Table 8.3 Lapping speed for machine lapping plane surfaces.

Accuracy level Surface roughness Ra

[m]

Lapping allowance up to

[mm]

Lapping speed [m/min]

Medium 0,16-0,63 0,50 200

Accurate 0,04-0,16 0,25 100-250

Very accurate 0,01-0,04 0,04 10-100

48


49/110

8.4.3 Lapping operations

Lapping between plates:Machine lapping between plates is an economical and productive

(100 parts/h) method of lapping cylindrical surfaces. The machine can be used for lapping parts

such as plug gages, piston pins small valve pistons, cylindrical valves, small engine pistons, roller

and needle bearings, diesel injector valves, plungers, and miscellaneous cylindrical pins (Fig.8.16)

.

Fig.8.16 Two-plate lapping machine with two rotating laps and eccentrically rotating plate holder.

Both hard and soft materials can be lapped, provided that they are rigid enough to acceptpressure oflaps. Because the hardness slows the operation, soft materials lap more rapidly than hard

ones. Additionally, hard materials provide easier control of tolerances.

Some limitations: a part with diameter greater than its length is difficult orimpossible to

machine lap between plates. Parts with shoulders require special fixtures. Parts with keyways,

flates, or interrupted surface are difficult to lap because the variations in lapping pressure that

occur are likely to fall out ofround. If the reliefextends over the entire length of the part, this

method of lapping cannot be used at all.

Thin-wall tubing can be lapped, provided that the deflection due to lapping pressure is

insignificant. Parts that is hollow on one end and solid on the other present problems in obtaining

roundness and straightness. Plugging the hollow end of the part will sometimes solve the problem.

The outside edges of the laps lap at a faster rate than the inside edges. Therefore, it is

expected that the cylindrical workpiece will become tapered. One method of overcoming this

problem consists of using short cycles, while the workpieces are reversed in their slots. In

49


50/110

addition, they are mixed between slots. Taper can also be minimized by positioning the

workholder so that parts in slots are at 150 angles to a radius, as illustrated in Fig.8.17.

Fig.8.17 Lapping setup that minimizes taper for production quantity of cylindrical parts.

Example: The valve needle (high-alloy tool steel of 60-65 HRC) shown in Fig.8.18 is to be

lapped to achieve the accuracy requirements where Ra = 0,05 m, tolerance = 0,13 m, out-of-

roundness = 0,13 m, and a taper = 0,25 m. Discuss the possible alternatives to achieve the

preceding requirement.

Fig.8.18 Lapping of a valve needle using a ring lap.

Solution: There are two alternatives forlapping:

1. For small quantities, a ring lap ofcast iron is used as is shown in Fig.8.18. Each needle is

chucked by its stem and rotated in a lathe at 650 rpm. The cast iron lap is stroked back and forth

over the needle until grinding marks are vanished. The needle is coated with lapping compound

(CrO mixed with spindle oil).

50


51/110


52/110

a) b)

Fig.8.19 a) Planetary fixed-plate double face lapping machine for flat surfaces; b) Dual-face lapping machine using two

bonded laps.

The workpiece is propelled by the carrier in a serpentine path between lap plates on

which abrasives have been charged or continuously fed in the form of slurry. In the planetary

fixed-plate machine (Fig.8.19a), the bottom lap is fixed and the top lap is restrained from

rotating. It is allowed to float to bear on the largest pieces and laps all the pieces to the same size.

The part is dragged between the plates by the carrier and all the power is directed to the flat,

thin carrier plates, exerting high forces on theirthin teeth that may cause edge chipping on fragile

parts.

Fig.8.19b illustrates another dual-face lapping machine, having two-bonded abrasive laps

(400-grit SiC) that are rotated in opposite directions at 88 rpm. The head is airactuated to

provide the lapping pressure to the top lap. The workpiece carrier is eccentrically mounted over

the bottom lap and rotates at 7,5 rpm. The viscous cutting oil is fed to the laps during operation.

The laps are dressed two orthree times during an eight-hour shift.

Fig.8.20 illustrates some typical shapes that can be machined on flat lapping machines.

Symmetrical components (a) and (b) do not require workholders. Asymmetrical components (c)and (d) require workholders. Parts similar to (e) require holders to keep them from tipping.

52


53/110

Fig.8.20 Typical shapes lapped on flat lapping machine.

Tolerances, roughness, flatness, and parallelism. Achieved tolerance of parts havingparallel shapes can be 2,5 m (for small parts) to 25 m (for large parts). It is difficult to

maintain accuracy for parts of uneven configuration. Such parts may require fixtures that

determine the level of accuracy attainable. The flatness may attain a value of 0,3 m and the

achievable surface roughnessRais0,05 m.

Flatparallel surfaces can be lapped on eitherdouble-lap machines, which lap both sides of

the workpiece in a single operation, or the single-lap machines, which require two operations. In

the latter case, extraordinary attention is required to such details as cleanliness and lap flatness.

Flatness of laps must be kept within the required flatness tolerance of the workpiece. In case of lot

production, a parallelism of0,2 m/mm dictates the use of a dual machine. Allowance for stock

removal in this operation should be 1,5-2 times the amount of the part out-of-parallelism plus the

amount of the variation in part size.

Lapping machines for spherical surfaces:these are classified into two classes: single-and

multiple-pieces lapping machines. Single-piece machines have the following two configurations:

a) A single-spindle machine with a vertical spindle that rotates the lap. Ferrous workpieces are

held stationary by a magnetic chuck: those ofnonferrous materials are clamped in a fixture.

A crank is held by the chuckof a lathe, is provided by a ball-end crankpin that fits in a

drilled hole in the back of the lap (Fig.8.21a), rotates over the spherical surface of the workpiece.

The workpiece is in line with the spindle of the lathe. The lap should be heavy enough to provide

the required lapping pressure.

53


54/110

Fig.8.21 Lapping of spherical surfaces: a) single-spindle machine and b) two-spindle machine.

b) A two-spindle machine. One spindle holds and rotates the workpiece, while the other holds

the lap in a floating position and oscillates it through an angle large enough to lap the requiredarea of the surface (Fig.8.21b).

Vibratory lapping: To increase the linear material removal rate by lapping additional

vibration is applied to the lap as shown in Fig.8.22.

Fig.8.22 Vibratory lapping.

Under such conditions, the material

removal rates rises by 30-40 percent

but the height of surface irregularities

increases by 50-100 percent. Vibratory

lapping is, therefore suitable as a

preliminary lapping process or when

the surface required is not smooth.

The abrasive mixture of boron carbide

or diamond dust is used for longer

abrasive life and material removal rate

requirements.

8.5 Superfinishing

Superfinishing (microhoning) is an abrading process that is used for external surface

refining orcylindrical, flat, and spherical-shaped parts. It is not a dimension-changing process,

but is mainly used forproducing finished surfaces of superfine quality. Only a slight ofstockis

removed (2-30m), which represents the surface roughness (Fig.8.23).

54


55/110

Fig.8.23 Gradual improving a rough surface by superfinishing.

The process ofhoning involves two main motions, whereas superfinishing requires three

or more motions. As a result of these motions, the abrasive path is random and never repeats

itself.

The primary distinction between honing and superfinishing is that in honing, the tool

rotates, while in superfinishing, the workpiece always rotates. The operating principle of the

superfinishing process is illustrated in Fig.8.24. The bonded abrasive stone, whose operating face

complies with the form of the workpiece surface, is subjected to very light pressure. A short

stroke, super-imposed on a reciprocating traverse, is used for superfinishing oflong lengths.

Fig.8.24 Principle of superfinishing process.

8.5.1 Kinematics of superfinishing

In this process, the main aim is to remove the burnt out layer of the surface to improve the

surface finish and to correct the inequalities in geometry. The correction of shape and

dimensional accuracy is not aimed at. This leads to even distribution of the load. It is a slow speed

abrasive machiningprocess. The abrasive stick is made ofvery fine grains. The workis rotated

55


56/110

between centers as in a lathe and abrasive stickholder is reciprocated back and forth with short

strokes but rapidly. The rotational speed ofwork is very low: 2 to 20 m/min. The sticks stroke

may be 2 to 5 mm long which a frequency of up to 1800 stroke/min. A lubricant made of

kerosene oil is used to give fine finish. Superfinishing can be accomplished on a lathe. Special

purpose machines are also available for finishing crank shafts, cam shafts, etc. The superfinishing

process is shown in Fig.8.25.

Fig.8.25 Process parameters in superfinishing.

P= contact pressure between stone and workpiece = 0,5-4,0 kg/cm2;

fa

ndtg

=

a = amplitude of stone vibration [mm];

f = number of strokes per min (about 500 cycles/min);

d= diameter of workpiece [mm];

n = rpm of workpiece;

Vs = traverse speed [m/min];

If the workis longer than the stone, then a traverse motion is also requiredparallel to the

axis ofwork. It should be noted that the stone gradually wears in to the average radius of the part.

The early stages of the operation consist of the abrasion of the peaks and ridges of the workpiece.

The stone will have contact with the workpiece at isolated points. However, as the work

approaches nearer to a true cylinder the area of contact increases thus reducing the pressure on

unit area.

56


57/110

In the straight oscillatory superfinishing (when the feed is parallel to the workpiece axis),

shown in Fig.8.26, the path of the grain is projected on a developed cylindrical workpiece surface.

Fig.8.26 Path of grain motion on work surface and cutting speed components in straight superfinishing.

The compositon of the rotary motion and feed motion yields the path of grain motion PR. As

a result of the periodic (mostly sinusoidal) tool oscillation of an amplitude a and wave length the

resulting grain path motion P passes through the points E-H-F-G-R. The amplitude a may becontrolled; the

chapter seven -broaching

Documents