Manufacturing Tooling Cutting Tool Design

Nageswara Rao Posinasetti

Elements of Machining

Š Cutting tool Š Tool holding Š Guiding device Š Work piece Š Machine tool

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Chip Formation

Š Cutting tool harder and wear resistant than the workpiece material Š Interference between the tool and workpiece designated as feed and depth of cut Š Relative motion between the tool and workpiece termed as cutting speed or velocity to overcome the resistance

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1 Cutting process parameters

Š The cutting speed, V, is the speed with which the cutting tool moves through the work material. This is generally expressed in feet per minute (fpm) or metres per second (ms-1). Š Feed rate, f, may be defined as the small relative movement per cycle (per revolution or per stroke) of the cutting tool in a direction usually normal to the cutting speed direction. Š Depth of cut, d, is the normal distance between the unmachined surface and the machined surface.

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2 Type of Chips

Š The chip formation in metal cutting could be broadly categorised into three types: – Continuous chip, – Continuous chip with BUE, and – Discontinuous chip

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FIGURE F-14 The three types of chip formation: (a) continuous; (b) continuous with built-up edge; (c) discontinuous (segmented). (J.E. Neely, Practical Metallurgy and Materials of Industry, 2nd ed., Wiley, New York, copyright © 1984.)

Richard R. Kibbe, JohnJanuary E. Neely, 29, 2008 Nageswara Rao Posinasetti 9 Roland O. Meyer, and Warren T. Copyright ©2002 by Pearson Education, Inc. White Upper Saddle River, New Jersey 07458 Machine Tool Practices, 7e All rights reserved.

3 Continuous chip

ŠContinuous chips are normally produced when machining steel or ductile metals at high cutting speeds. ŠThe continuous chip which is like a ribbon flows along the rake face. ŠContinuous chip is possible because of the ductility of metal (steel at high temperature generated due to cutting) flows along the shear plane instead of rupture.

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Continuous chip

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CHIP

TOOL

WORKPIECE

Fig. From P N Rao – Metal Cutting and Machine Tools, Tata McGraw Hill, 2000 January 29, 2008 Nageswara Rao Posinasetti 12

4 Continuous chip

Š It can be assumed that each layer of metal flows along the slip plane till it is stopped by work hardening. Š Each of these layers get welded to the previous ones because of the high temperature, thus forming a continuous chip.

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CHIP

1 2 TOOL 3 4 5

4 3 2 1 9 8 7 6 5

WORKPIECE

Fig. From P N Rao – Metal Cutting and Machine Tools, Tata McGraw Hill, 2000 January 29, 2008 Nageswara Rao Posinasetti 14

Continuous chip

ŠSome ideal conditions that promote continuous chips in metal cutting are: –sharp cutting edge, –small chip thickness (fine feed), –large rake angle, –high cutting speed, –ductile work materials and –less friction between chip tool interface through efficient lubrication.

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5 Continuous chip

Š This is the most desirable form of chip since the surface finish obtained is good and cutting is smooth. Š It also helps in having higher tool life and lower power consumption. Š However, because of the large coils of chips, the chip disposal is a problem.

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Continuous chip

ŠHowever, because of the large coils of chips, the chip disposal is a problem. ŠTo help in this direction various forms of chip breakers have been developed which are in the form of a step or groove in the tool rake face. ŠThe chip breakers allow the chips to be broken into small pieces so that they can be easily disposed off.

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Discontinuous chip

Š When brittle materials like cast iron are cut, the deformed material gets fractured very easily and thus the chip produced is in the form of discontinuous segments. Š In this type the deformed material instead of flowing continuously gets ruptured periodically.

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6 Discontinuous chip

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Discontinuous chip

Š Discontinuous chips are easier from the chip disposal view point. Š However, the cutting force becomes unstable with the variation coinciding with the fracturing cycle.

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12,500

10,000

7,500

Cutting force, Lb 5,000 1.0 2.0 3.0 Distance cut, inches

Fig. From P N Rao – Metal Cutting and Machine Tools, Tata McGraw Hill, 2000 January 29, 2008 Nageswara Rao Posinasetti 21

7 Discontinuous chip

ŠAlso they generally provide better surface finish. ŠHowever, in case of ductile materials they cause poor surface finish and low tool life. ŠHigher depths of cut (large chip thickness), low cutting speeds and small rake angles are likely to produce discontinuous chips.

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Continuous chip with BUE

ŠWhen the friction between tool and chip is high while machining ductile materials, some particles of chip adhere to the tool rake face near the tool tip. ŠWhen such sizeable material piles up on the rake face, it acts as a cutting edge in place of the actual cutting edge. ŠThis is termed as built up edge (BUE).

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Continuous chip with BUE

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8 Continuous chip with BUE

ŠBy virtue of work hardening, BUE is harder than the parent work material. ŠAs the size of BUE grows larger, it becomes unstable and parts of it gets removed while cutting. Š The removed portions of BUE partly adheres to the chip underside and partly to the machined surface.

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Continuous chip with BUE

Fig. From P N Rao – Metal Cutting and Machine Tools, Tata McGraw Hill, 2000 January 29, 2008 Nageswara Rao Posinasetti 26

Continuous chip with BUE

ŠThis causes finished surface to be rough. ŠHowever, since the cutting is being carried by the BUE and not the actual tool tip, the life of the cutting tool increases while cutting with BUE. ŠThat way BUE is not harmful while rough machining.

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9 Continuous chip with BUE

ŠThe conditions that normally induce the formation of BUE are –low cutting speed, –high feed, and –low rake angle. ŠOne of the prerequisites for the formation of BUE is the work hardenability of the workpiece material. ŠHigher the work hardenability, rougher is the machined surface produced.

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11 Manipulating Factors

Š Cutting speed Š Size of cut Š Effect of tool geometry Š Tool material Š Cutting fluid Š Work piece material

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Cutting Forces

Š Cutting force Š Radial force Š Axial force

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Power requirements in Turning

C V f n3 d n4 HP = 33,000

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Table 3.1 (p 47)

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18 Tool Wear

Š Crater wear Š Flank wear

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20 Mechanism of tool wear

Š Abrasive action of hard particles of work material Š Plastic deformation of the cutting edge Š Chemical decomposition of the cutting tool contact surfaces Š Diffusion between work and tool surfaces Š Welding of asperities between work and tool

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Tool Life

Š Criteria for tool life – Change of quality of the machined surface – Change in the magnitude of the cutting force – Change in the cutting temperature – Costs

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Tool Life Equation V T n = C Š V = cutting speed, fpm Š T = tool life, minutes Š C = a constant Š n is a constant – HSS = 0.10 to 0.15 – Carbides = 0.20 to 0.25 – Ceramics = 0.6 to 1.0

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Tool Life Equation V T n f n1 d n2 = K Š V = cutting speed, feet per minute Š T = tool life, minutes Š d = depth of cut, in Š f = feed rate, inches per revolution Š K = a constant Š n1 = exponent for feed (0.5 to 0.8) Š n2 = exponent for depth of cut (0.2 to 0.4)

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23 The following equation was obtained when machining AISI 2340 steel with HSS tool. A 100 minute tool life was obtained using V = 75 fpm, f = 0.0125 ipr, d = 0.10 in.Calculate the effect upon the tool life for a 20% increase in the cutting speed, feed and depth of cut, taking each separately. Calculate the effect of a 20% increase in each of the above parameters taken together. V T 0.13 f 0.77 d 0.37 = 2.035

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Other tool life equations

K = V T n f n1 d n2 Bhn1.25

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