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Page 22-1

Chapter 23Machining Processes Used to Produce

Various Shapes

Alexandra Schnning, Ph.D.Mechanical Engineering

University of North Florida

Figures byManufacturing Engineering and Technology

Kalpakijan and Schmid


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Introduction

What is milling?A manufacturing process in which a rotating,

multitooth cutter removes material whiletraveling along various axes with respect to the

workpiece.Other processes will be discussed, such as Planing, shaping, broaching, sawing, filing, and gear

manufacturing.


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Milling operations

Slab milling (Figure a) Arbor, cutter

Face milling (Figure b) Spindle and cutter

End milling (Figure c) Spindle, shank, end mill

Figure 23.3 A typicalpart that can beproduced on a millingmachine equippedwith computercontrols. Such partscan be madeefficiently andrepetitively on

computer numericalcontrol (CNC)machines, without theneed for refixturing orreclamping the part.


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Slab Milling

Also called peripheral millingThe axis of cutter rotation is parallel to the workpiece

surface to be machinedThe cutter has a teeth along its circumference where

each tooth acts as a single point cutting toolConventional vs. Climb milling

Conventional milling Also called up-milling: the rotation of the cutter is such that it first

engages the workpiece at the bottom. The cut is not a function of the surface characteristics Common method of milling. Proper clamping is necessary to prevent the upward rotation of the

cutter.

Climb milling Also called down-milling: the rotation is such that the cutter first

engages the workpiece at the top. Cutting forces holds the workpiece in place. However, a rigid setup is

important since there are high impact forces.


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Conventional and Climb Milling

Figure 23.4 (a) Schematic illustration of conventional milling and climb milling. (b) Slab milling operation,showing depth of cut, d, feed per tooth,f, chip depth of cut, tc, and workpiece speed, v. (c) Schematicillustration of cutter travel distance lcto reach full depth of cut.


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Milling Parameters

The velocity at the point ofcontact

The chip thickness can be found

Feed per tooth

Cutting time

Material Removal Rate

Definitions of symbols

Definition of Symbols tc: chip thickness f: feed per tooth of cutter

D: depth of cut

N: angular speed in rpm

n: number of teeth on cutter periphery

v: linear speed (feed rate) t: cutting time

L: length of the workpiece

Lc: extent of the cutter's first contactwith the workpiece (illustration on next

page)

w: width of cut

v D

v

t c2 f d

D

f v

N n

rad

slength

N = angular speed in rpm. This

version of the formula is used in the

book. It includes the unit conversion

from rpm to radiance per minute.

The units of the velocity aredistance/min.

It is assumed that lc


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Illustration of Lc

Lc

Lc: extent of the cutter'sfirst contact with the

workpiece


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Summary of Milling Parameters and Formulas

TABLE 23.1

N = Rotational speed of the milling cutter, rpmf = Feed, mm/tooth or in./tooth

D = Cutter diameter, mm or in.n = Number of teeth on cutterv = Linear speed of the workpiece or feed rate, mm/min or in./minV = Surface speed of cutter, m/min or ft/min

=D N

f = Feed per tooth, mm/tooth or in/tooth=v /N n

l = Length of cut, mm or in.t = Cutting time, s or min

=( l+lc) v, where l

c=extent of the cutters first contact with workpiece

MRR = mm3

/min or in.3

/min

=w d v, where wis the width of cut

Torque = N-m or lb-ft(F

c) (D/2)

Power = kW or hp

= (Torque) (), where = 2Nradians/min

Note: The units given are those that are commonly used; however, appropriate units must

be used in the formulas.


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Example

A slab milling operation is beingcarried out on a 12-in long, 4 inwide annealed mild steel block ata feed f=0.01 in/tooth and adepth of cut d=1/8 in. The cutteris D=2in in diameter, and has 20straight teeth, rotates at N =100rpm, and is wider than theblock to be machined. Calculate

the material Removal rate,estimate the power and torquerequired for this operation, andcalculate the cutting time. Given:

w=4in L=12 in f=0.01 in/tooth d=1/8 in D=2 in n = 20 teeth N = 100 rpm Cutter width > block width

Specific energy: 1.1 hp*min/in3

Table 20:2 (annealed mildsteel)

Find Material Removal Rate Power Torque Cutting time

secondst 37.5t

L Lc( )

n60

inLc 0.5Lc D d

lb fTorque 577.7Torque PowerN

330002

hpPower 11Power spec_enrg MRR

in3

minMRR 10MRR w d v

in

min

v 20v f N n


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Face Milling

Cutter is mounted on a spindlehaving an external axis ofrotation perpendicular to theworkpiece surface

Workpiece moves along astraight path at a linear speed, v.

Direction of cutter

Conventional milling (Fig. c) up-milling Climb milling (Fig. b)

down-milling

Leaves feed marks on themachined surface

Terminology in figure Lead angles: 0-45o. Low anglelow vertical force

Figure 23.8 Terminology for a

face-milling cutter.


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Face Milling

Figure 23.6 A face-milling cutter with indexable inserts.

Source: Courtesy of Ingersoll Cutting Tool Company.

v

d w


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Cutter and Insert Position in Face Milling

Figure 23.10 (a) Relative positionof the cutter and insert as it firstengages the workpiece in facemilling, (b) insert positionstowards the end of the cut, and (c)examples of exit angles of insert,showing desirable (positive ornegative angle) and undesirable(zero angle) positions. In all

figures, the cutter spindle isperpendicular to the page.

Third example in figure c: the

insert exits the workpiece

suddenly as opposed to exiting

with an angle.


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End milling

The cutter, called an endmill, rotates about an axis

perpendicular to theworkpiece surface(typicallycan be at an

angle)Ball nose: A type of end

mill in which the bottomsurface is rounded Used in the production of

curved surfaces for diesand molds


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Other Milling Operations and Cutters

Straddle milling: two or morecutters are mounted on anarbor and are used to machinetwo parallel surfaces on theworkpiece Easier to keep tolerances than if

milling one surface at a time

Form milling: producescurved profiles. Also used inmachining gear teeth.

Circular cutters can be usedfor slotting and slitting. Slitting saws are typically

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