chapter 20 fundamentals of machining/orthogonal machining (review) ein 3390 manufacturing processes...

Chapter 20Chapter 20

Fundamentals of Fundamentals of Machining/Orthogonal Machining/Orthogonal

MachiningMachining(Review) (Review)

EIN 3390 Manufacturing ProcessesEIN 3390 Manufacturing ProcessesSpring, 2011Spring, 2011

20.2 Fundamentals20.2 Fundamentals

Variables in Processes of Metal Cutting:

• Machine tool selected to perform the processes

• Cutting tool (geometry and material)

• Properties and parameters of workpiece

• Cutting parameters (speed, feed, depth of cut)

• Workpiece holding devices (fixture or jigs)

FIGURE 20-1 The fundamental inputs and outputs to machining processes.


7 basic chip formation processes: shaping, turning, milling, drilling, sawing, broaching, grinding (abrasive)

FIGURE 20-2 The seven basic machining processes used inchip formation.

20.2 Fundamentals20.2 FundamentalsResponsibilities of Engineers

Design (with Material) engineer: • determine geometry and materials of products to meet functional requirements

Manufacturing engineer based on material decision:

• select machine tool• select cutting-tool materials• select workholder parameters,• select cutting parameters

20.2 Fundamentals20.2 FundamentalsCutting Parameters

Speed (V): the primary cutting motion, which relates the velocity of the cutting tool relative to the workpiece.

For turning: V = (D1 Ns) / 12 where, V – feet per min, Ns – revolution per min (rpm), D1

diameter of surface of workpiece, in.

Feed (fr): amount of material removed per revolution or per pass of the tool over the workpiece. In turning, feed is in inches per revolution, and the tool feeds parallel to the rotational axis of the workpiece.

Depth of Cut (DOC): in turning, it is the distance that the tool is plunged into the surface.

DOC = 0.5(D1 – D2) = d

FIGURE 20-3 Turning acylindrical workpiece on a lathe requires you to select the cutting speed, feed, and depth of cut.

20.2 Fundamentals20.2 FundamentalsCutting Tool is

a most critical componentused to cut the work pieceselected before actual values for speed and feeds are determined.

Figure 20-4 gives starting values of cutting speed, feed for a given depth of cut, a given work material, and a given process (turning).

Speed decreases as DOC or feed increaseCutting speed increases with carbide and coated-

carbide tool material.

FIGURE 20-4 Examples of a table for selection of speed and feed for turning. (Source: Metcut’s Machinability Data Handbook.)

20.2 Fundamentals20.2 FundamentalsTo process different metals, the input parameters to the machine tools must be determined.

For the lathe, the input parameters are DOC, feed, and the rpm value of the spindle.

Ns = 12V / ( D1) = ~ 3.8 V/ D1

Most tables are arranged according to the process being used, the material being machined, the hardness, and the cutting-tool material.

The table in Figure 20-4 is used only for solving turning problems in the book.

20.2 Fundamentals20.2 FundamentalsDOC is determined by the amount of metal removed per pass. Roughing cuts are heavier than finishing cuts in terms of DOC and feed and are run at a lower surface speed.

Once cutting speed V has been selected, the next step is to determine the spindle rpm, Ns.

Use V, fr and DOC to estimate the metal removal rate for the process, or MRR.

MRR = ~ 12V fr dwhere d is DOC (depth of cutt).

MRR value is ranged from 0.1 to 600 in3/min.

20.2 Fundamentals20.2 FundamentalsMRR can be used to estimate horsepower needed to perform cut. Another form of MRR is the ratio between the volume of metal removed and the time needed to remove it.

MRR = (volume of cut)/Tm Where Tm – cutting time in min. For turning, Tm = (L + allowance)/ fr Ns

where L – length of the cut. An allowance is usually added to L to allow the tool to enter and exit the cut.

MRR and Tm are commonly referred to as shop equations and are fundamental as the processes.


One of the most common is turning:workpiece is rotated and cutting tool removes material as it moves to the left after setting a depth of cut. A chip is produced which moves up the face of the tool.

FIGURE 20-5 Relationship ofspeed, feed, and depth of cut inturning, boring, facing, andcutoff operations typically doneon a lathe.

20.2 Fundamentals20.2 FundamentalsMilling:

A multiple-tooth process. Two feeds: the amount of metal an individual tooth

removes, called the feed per tooth ft, and the rate at which the table translates pass the rotating tool, called the table feed rate fm in inch per min.

fm = ft n Ns

where n – the number of teeth in a cutter, Ns – the rpm value of the cutter.

Standard tables of speeds and feeds for milling provide values for the recommended cutting speeds and feeds and feeds per tooth, fr.

FIGURE 20-6 Basics of milling processes (slab, face, and end milling) including equations for cutting time and metalremoval rate (MRR).

FIGURE 20-7 Basics of the drilling (hole-making) processes, including equations for cutting time andmetal removal rate (MRR).

FIGURE 20-9 (a) Basics of the shaping process, including equations for cutting time (Tm ) and metal removal rate(MRR). (b) The relationship of the crank rpm Ns to the cutting velocity V.

FIGURE 20-10 Operations and machines used for machining cylindrical surfaces.

FIGURE 20-11 Operations and machines used to generate flat surfaces.

20.3 Energy and Power in Machining20.3 Energy and Power in Machining

Power requirements are important for proper

machine tool selection.

Cutting force data is used to:

properly design machine tools to maintain

desired tolerances.

determine if the workpiece can withstand

cutting forces without distortion.

Cutting Forces and PowerCutting Forces and Power Primary cutting force Fc: acts in the direction of the cutting

velocity vector. Generally the largest force and accounts for 99% of the power required by the process.

Feed Force Ff :acts in the direction of tool feed. The force is

usually about 50% of Fc but accounts for only a small

percentage of the power required because feed rates are

small compared to cutting rate.

Radial or Thrust Force Fr :acts perpendicular to the

machined surface. in the direction of tool feed. The force is

typically about 50% of Ff and contributes very little to the

power required because velocity in the radial direction is

negligible.

FIGURE 20-12 Obliquemachining has three measurablecomponents of forces acting onthe tool. The forces vary withspeed, depth of cut, and feed.

Cutting Forces and PowerCutting Forces and PowerPower = Force x Velocity

P = Fc . V (ft-lb/min)

Horsepower at spindle of machine is:hp = (FcV) / 33,000

Unit, or specific, horsepower HPs:

HPs = hp / (MRR) (hp/in.3/min)

In turning, MRR =~ 12VFrd, then

HPs = Fc / 396,000Frd This is approximate power needed at the spindle to remove a

cubic inch of metal per minute.

Cutting Forces and PowerCutting Forces and PowerSpecific Power

Used to estimate motor horsepower required to perform a machining operation for a given material.

Motor horsepower HPm

HPm = [HPs . MRR . (CF)]/EWhere E – about 0.8, efficiency of machine to overcome friction

and inertia in machine and drive moving parts; MRR – maximum value is usually used; CF – about 1.25, correction factor, used to account for variation in cutting speed, feed, and rake angle.

Cutting Forces and PowerCutting Forces and PowerPrimary cutting force Fc:

Fc =~ [HPs . MRR . 33,000]/VUsed in analysis of deflection and vibration problems in machining and in design of workholding devices.

In general, increasing the speed, feed, depth of cut, will increase power required.

In general, increasing the speed doesn’t increase the cutting force Fc. Speed has strong effect on tool life.

Cutting Forces and PowerCutting Forces and PowerConsidering MRR =~ 12Vfrd, then

dmax =~ (HPm . E)/[12 . HPs V Fr (CF)]

Total specific energy (cutting stiffness) U:

U = (FcV)/(V fr d) = Fc/(fr . d) =Ks (turning)

20.6 Mechanics of Machining 20.6 Mechanics of Machining (statics)(statics)Assume that the result force R acting on the back of the chip is equal and opposite to the resultant force R’ acting on the shear plane.

R is composed of friction force F and normal force N acting on tool-chip interface contact area.

R’ is composed of a shear force Fs and normal force Fn acting on the shear plane area As.

R is also composed of cutting force Fc and tangential (normal) force Ft acting on tool-chip interface contact area. Ft = R sin ( - )

FIGURE 20-20 Free-body diagram of orthogonal chipformation process, showing equilibrium conditionbetween resultant forces R and R.

20.6 Mechanics of Machining 20.6 Mechanics of Machining (statics)(statics) Friction force F and normal force are:

F = Fc sin + Ft cos , N = Fc cos + Ft sin and= tan-1 = tan-1 (F/N),

Where force F and friction coefficient, and – the angle between normal force N and resultant R. If = 0, then F = Ft , and N = Fc . in this case, the friction force and its normal can be directly measured by dynamometer.

R = SQRT (Fc2 + Ft

2 ),Fs = Fc cos - Ft sin , andFn = Fc sin + Ft cos

Where Fs is used to compute the shear stress on the shear plane

20.6 Mechanics of Machining 20.6 Mechanics of Machining (statics)(statics) Shear stress:

s = Fs/As,

Where As - area of the shear plane, As = (t w)/sin

Where t – uncut ship thickness and w – width of workpiece.

s = (Fcsin cos - Ft sin2 )/(tw) psi

for a given metal, shear stress is not sensitive to variations in cutting parameters, tool meterial, or cutting environment.

Fig. 20-22 shows some typical values for flow stress for a variety of metals, plotted against hardness.

20.7 Shear Strain 20.7 Shear Strain & Shear Front & Shear Front

Angle Angle Use Merchant’s chip formation model, a new “stack-of-cards” model as shown in fig. 20-23 is developed. From the model, strain is:

= cossin( + ) cos( + )]

where the angle of the onset of the shear plane, and - the shear front angle.

The special shear energy (shear energy/volume) equals shear stress x shear strain:

Us =

20.7 Shear Strain 20.7 Shear Strain & Shear Front Angle & Shear Front Angle Use minimum energy principle, where will take on value (shear direction) to reduce shear energy to a minimum:

d(Us)/d = 0, Solving the equation above,

= 450 - + , and = 2cossin),

It shows the shear strain is dependent only on the rake angle

20.8 Mechanics of Machining 20.8 Mechanics of Machining

(Dynamics)(Dynamics)Machining is a dynamic process of large strain

and high strain rate.

The process is a closed loop interactive

processes as shown on fig. 20-24.

FIGURE 20-24 Machiningdynamics is a closed-loopinteractive process that createsa force-displacement response.

20.8 Mechanics of Machining (Dynamics)20.8 Mechanics of Machining (Dynamics)

Free vibration is the response to any initial condition or

sudden change. The amplitude of the vibration

decreases with time and occurs at the natural frequency

of the system.

Forced vibration is the response to a periodic (repeating

with time) input. The response and input occur at the

same frequency. The amplitude of the vibration remains

constant for set input condition and is linearly related to

speed


Self-excited vibration is the periodic response to the

system to a constant input. The vibration may grow in

amplitude and occurs near natural frequency of the

system regardless of the input. Chatter due to the

regeneration of waviness in the machining surface is the

most common metal cutting example.


Factors affecting on the stability of machining

Cutting stiffness of workpiece material (machinability),

Ks

Cutting –process parameters (speed, feed, DOC,

total width of chip)

Cutter geometry (rake asd clearance angles, insert

size and shape)

Dynamic characteristics of the machining process

(tooling, machining tool, fixture, and workpiece)


Chip formation and regenerative Chatter

In machining, chip is formed due to shearing of

workpiece material over chip area (A = t x w), which

results in a cutting force.

Magnitude of the resulting cutting force is predominantly

determined by the material cutting stiffness Ks and the

chip area such that F c = Ks t w.

The direction of the cutting force Fc in influenced mainly

by the geometries of rack and clearance angles and

edge prep.

FIGURE 20-27 When theoverlapping cuts get out ofphase with each other, a variablechip thickness is produced,resulting in a change in Fc on thetool or workpiece.


Factors Influencing Chatter:

Cutting stiffness Ks

Speed

FEED

DOC: The primary cause and control of chatter.

Total width of chip

Back rack angle

Clearance angle

Size (nose radius), shape (diamond, triangular,

square, round) and lead angle of insert

Effects of TemperatureEffects of TemperatureEnergy dissipated in cutting is converted to

heat, elevating temperature of chip, workpiece, and tool.

As speed increases, a greater percentage of the heat ends up in the chip.

Three sources of heat:◦ Shear front.◦ Tool-chip interface contact region.◦ Flank of the tool.

FIGURE 20-31 Distribution ofheat generated in machining tothe chip, tool, and workpiece.Heat going to the environmentis not shown. Figure based onthe work of A. O. Schmidt.

FIGURE 20-32 There are three main sources of heat in metal cutting. (1) Primary shear zone. (2) Secondary shear zone tool–chip (T–C) interface. (3) Tool flank. The peak temperature occurs at the center of the interface, in the shaded region.

Effects of TemperatureEffects of Temperature

Excessive temperature affects◦Strength, hardness and wear resistance of cutting tool.

◦Dimensional stability of the part being machined.

◦Machined surface properties due to thermal damage

◦Machine tool, if too excessive.

FIGURE 20-33 The typical relationship of temperature at the tool–chip interface to cutting speed shows a rapid increase. Correspondingly, the tool wears at the interface rapidly with increased temperature, often created by increased speed.

chapter 20 fundamentals of machining/orthogonal machining (review) ein 3390 manufacturing processes...

Documents

cuttingtool material

cutting tool relative

parameters speed

carbide tool material

processes of metal cutting

tool materials

tool geometry

parameters of workpiece