Experiment p1 Metal Cutting Process

8/14/2019 Experiment p1 Metal Cutting Process

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P1-1

EXPERIMENT P1

METAL CUTTING PROCESS

Objectives

To learn about machining equipment and the metal cutting process. To better understand the relationship between metal cutting parameters and other process

issues that impact cutting force values. To observe and understand the progression of tool wear and its effect on the process.

Introduction to Metal Cutting (Theoretical Background)

Figure 1 shows the geometry of the cutting zone and outlines the notation used to describe a bar turning operation.


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ME 3M02 Experiment P1: Metal Cutting Processes P 1-2


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Flank face wear and Cutting (rake) face crater wear can be characterized using parameters asoutlined in Figure 3.

Figure 3: Tool Wear Characterization (Elbestawi, 2002)

a) Flank wear - flank wear width VB, b) Crater wear - crater depth KT,c) Tool nose deformation ND, (Not shown in figure)d) Wear notch (Oxidation) VN

Cutting (Rake) Face

Flank Face


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Values VB, KT depend on cutting conditions such as: material machined (machinabilityrating - tensile strength, hardness, and microstructure), cutting speed V (m/min), feed s

(mm/rev), approach angle , geometry of tool (especially rake angle ) and length of machiningtime.

VB = F l (mat, V, s, , t) (1)

KT = F 2 (mat, V, s, , t) (2)

Choosing maximum admissible wear values, VB lim, KT lim , the tool life T is obtained:

from Eqn. (1): T 1 = N1 (VB lim, mat, V, s) (3)

from Eqn. (2): T 2 = N2 (KT lim, mat, V, s) (4)

The smaller value of either T 1 or T 2 determines the end of tool life. Figure 4 describes atypical wear curve of a cutting tool.


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Effect of Cutting Speed on the Cutting Process

A change in the cutting speed also impacts the cutting process. This is due to the fact thatthe temperature in the secondary deformation zone (close to the rake face of the tool)continuously increases with cutting speed and therefore the deformation process in this zonechanges as a whole. Figure 5 captures the effect of a change in cutting speed (V) on tool wear (VB) and tool life. Generally a tool wear limit of VB* is set (typical 0.25-0.35 mm) to determinetool life T.

Figure 5: Effect of Cutting Speed (V) on Tool Wear VB and Tool Life (Bhattacharyya, 1984)

The most apparent phenomenon related to these changes in the cutting zone is theoccurrence of the Built Up Edge (BUE) on the tool. This phenomenon is shown in Figure 6.With 1045 steel BUE starts occurring at speeds of about V=10 m/min. The probability of BUEoccuring increases with cutting speed until a speed is reached at which it is a maximum then withfurther increases in V the probability of BUE decreases. After this point a thin built-up-layer mayformed on the face of the tool that varies with further increases in speed.


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Related to these changes are changes in friction between the chip and the tool, whichconsequently changes the shear plane angle and impacts the components of the cutting force as

well. These are best expressed as changes of the chip ratio C, changes of surface roughness andof the magnitude and direction of the cutting forces.

Cutting Force Measurements

Usually the cutting forces are measured in a three-dimensional cut, i.e., both the major and minor cutting edges are active as is the case in bar turning. In such cases it is desirable toreport the three principal force components as shown in Figure 7. Please refer to Figures 8-10for details on the cutting force tool dynamometer.

Cutting force F t , which is in the tangential direction F z Feed force F a , which is in the axial direction along the workpiece F x Thrust force F r , which is in the radial direction F y

Figure 7: Cutting Forces in Turning (Stephenson and Agapiou, 1996)

References:

Bhattacharyya, A., Metal Cutting Theory and Practice, Central Book Publishers, Calcutta, 1984.

Boothroyd, G. and Knight, W. A., Fundamentals of Machining and Machine Tools, MarcelDekker, New York, 1989.


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Experimental Apparatus

Lathe: Colchester Carbide Cutting InsertsTool holder Micrometers, stop watchTool room microscope and tool fixture (flank wear)Cutting force tool dynamometer (strain gauge based system)

Figure 8. Colchester Lathe

Table 1: Dynamometer Calibration

Axis Calibrated Sensitivity

Spindle(Workpieceholding jaws)

Cross Slide(Tool postmounting

point)

CuttingTool Insert

Tail Stock (Workpiecesupport)

Dynamometer


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Safety Rules

This material is largely taken from Mr. Ron Lodewyks, Machine Shop / Project LaboratorySafety Training Program.

1) Keep in mind that machinery capable of cutting metal is inherently dangerous.2) Develop and use common sense when using the equipment (think before you act).3) Keep in mind that rotating equipment stores significant amounts of energy and represents

a serious entanglement hazard. Remember if it looks unsafe it probably is. Be sure to

discuss the operation of the lathe in detail with the TA before conducting the tests.4) Do not operate the machine until you have had detailed instruction from the TA.5) Make sure the workpiece is adequately clamped in the spindle for the job being

performed. Rotating parts with a slight imbalance can generate large forces, alsometal cutting generates large forces. The clamping force must be high enough toresist movement under these conditions. Make sure the TA checks the part clamping

before starting the machine. (Never leave the chuck key in the spindle.) 6) Make sure the tool holder and cutting insert are adequately clamped for the job being

performed.7) Always wear safety glasses when around the machine.8) Long hair should be tied back to avoid being caught in the revolving parts of the machine

(Keep loose items away from rotating objects) .9) Loose clothing, rings or watches should not be worn when operating machine tools to

avoid having them getting caught in the machine.10) Wear long pants (preferably cotton) as metal cuttings removed from the workpiece during

the machining process can reach temperatures in excess of 300C and can burn.11) Watch out for sharp edges on the part, tool and on the chips. Use gloves when handling

sharp objects but remove them when operating the machine to avoid them gettingcaught in the rotating machinery.

12) Do not use rags near the rotating machines when the spindle is running. Rags can becaught in the rotating spindle and the result can be serious injury. Rags may be usedfor material handling and for cleaning purposes provided there are no rotating hazardsnearby.

13) Use extreme caution with the chips produced during machining. Chips are sharp andsome chips are long stringers which can easily be caught up in the spindle and thrown

with great force. In general do not clear chips away from the work area when themachine is in motion. If it is necessary ask the TA how this can be best done. (If necessary there is a long handled tool available for pushing the chips out of the way.)

14) Sandals and opened toed shoes are not permitted in the lab.


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Test Procedure

1) For the supplied material and a given depth of cut, feed rate and cutting speed measure:a. two components of cutting force using the dynamometer (F t, F a).

b. the deformed chip thickness, procedure outlined below, to find the chip ratioi. Collect chips from each set of experiments

c. Make observations on the surface roughness of the workpieced. State of the tool (tool wear)

i. Be sure to collect the data as soon as the cutting force stabilizes so as to

minimize the accumulated tool wear on the cutting edge.

2) Next conduct a sensitivity analysis, repeating step 1 while adjusting each of the threecritical parameters (depth of cut, feed rate and cutting speed) up by one increment,then up by double the increment and then down by an increment, then down bydouble the increment. (ie. nominal , nominal + increment, nominal + 2 X increments, etc (see form 1). Total of 12 more cutting tests over the nominal)

3) Repeat the test this time till the tool wears out (VB*= 0.3 mm) for the set of conditionswhich supplied the highest measured cutting force (highest wear condition) this timecarefully monitor the tool wear versus the actual distance traveled by the cutting edge.Interrupt the cutting process periodically to measure the tool wear. (TA to supply areasonable distance). Record the BUE and tool wear in Form 2a as well as the chipand force data in Form 2b

Details on Testing

Experimental conditions (verify with TA before proceeding): Workpiece material: medium carbon steel 1045, initial diameter 150 mm, final diameter approximately 100 mm, length 750 mm".Tool materials: Uncoated Carbide Insert, Geometry ( = 0 o, = 5 o, r n = 0.8mm)

Depth of Cut a = 1.5 mm (0.060 in) (increment 0.5 mm, 0.020 in)

Feed s r = 0 .25 mm (0.010 in/rev) (increment 80 m, 0.003 in/rev)Speed v = 120 m/min (400 feet/min) (increment 15 m/min, 50 feet/min)

Cutting force measurement:Cutting forces are measured by means of a cutting force dynamometer. The lab instructor

will explain the design and calibration of the dynamometer used in the test. The detailed


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measure on the chip given the irregular nature of the surfaces. To avoid a potentially inaccuratemeasurement for h 2 the value used in calculation is based on an average of measured values.

In this lab, the thickness is measured using a pointed-tip micrometer. Two measurements will bemade from different chips and the average will be used to calculate the chip ratio.

Another method that is sometimes used is as follows. The mean deformed chip thickness can beestimated by measuring the length of the chip l 2, its weight W, the width of cut b (assuming noside spread) and knowing the specific density of 1045 steel.

22 lb

W h =

Requirements :

Required Graphs:C vs. depth of cutC vs. feed

C vs. speedFt, F a & F Resultant vs. depth of cutFt, F a & F Resultant vs. feedFt, F a & F Resultant vs. speed

VB vs. Actual cutting distance (L)Ft, F a & F Resultant vs. Actual cutting distance (L)

Where the actual cutting distance (L) is the distance that the tool tip travels, taking into accountthe rotation of the workpiece. A is the linear distance that the tool travels in the direction of theworkpieces axis and S r is feed per revolution.

RadiusS A

Lr

= 2

Discussion of Results

Using metal cutting theory discuss the results summarized in the graphs. Comment on whatchanges occur in the chip ratio C and how the change in cutting geometry affects force. Relatethis information to final part quality and metal removal rate (productivity).


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P1-11 Form 1) Cutting test results Date: ______________ Name: ___________________________________

Test Depth

of cut

Feed per

revDia

CuttingSpeed/ Actual

Force Chip ratio calculation Comments

a Sr d V= DN Fx Fz Fresult h1 h2,1 h2,2 C#

mm (in) mm (in) mm m/min(ft/min) N N N mm mm mm(tool wear and built up edge formation)

1 2.0 (.080) 0.25 (0.01) 120 (400)

2 1 (0.04) 0.25 (0.01) 120 (400)

3 2.5 (0.1) 0.25 (0.01) 120 (400)

4 0.5 (0.02) 0.25 (0.01) 120 (400)

5 1.5 (.060) 0.25 (0.01) 120 (400)

6 1.5 (0.06) 0.40 (0.016) 120 (400)

7 1.5 (0.06) 0.10 (0.004) 120 (400)

8 1.5 (0.06) 0.25 (0.01) 140 (450)

9 1.5 (0.06) 0.25 (0.01) 160 (500)

10 1.5 (0.06) 0.25 (0.01) 110 (350)

11 1.5 (0.06) 0.25 (0.01) 90 (300)


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Form 2a) Tool Life Tests Date: _______________ Name: _____________________________________

At the end of every time interval, make a sketch of the tools cutting edge that you observe in the microscope, see examples EX in the bottomright corner


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Form 2b) Tool life tests Date: ______________ Name: ___________________________________

Conditions:Depth of cut = , Feed per rev = , N = __________, Dia = _________, Speed= ____________ ( DN)

Timeinterval Time

Cuttingfeed

travel

Actualcutting

distance

Force Tool Wear Comments

T A L Fx Fz Fresult VB KT#sec mm mm N N N mm mm

(BUE, Crater Wear, Surface Roughness etc.)

1

2

3

45

6

7

8

9

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Experiment p1 Metal Cutting Process

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