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Theory of metal machining
1.
THEORY OF METAL MACHINING 1.
Overview of Machining Technology 2. Theory of Chip Formation in Metal Machining 3. Force Relationships and the Merchant Equation 4. Power and Energy Relationships in Machining 5. Cutting Temperature ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
2.
Material Removal Processes
A family of shaping operations, the common feature of which is removal of material from a starting work part so the remaining part has the desired geometry Machining – material removal by a sharp cutting tool, e.g., turning, milling, drilling Abrasive processes – material removal by hard, abrasive particles, e.g., grinding Nontraditional processes - various energy forms other than sharp cutting tool to remove material ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
3.
The family tree of
material removal processes ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
4.
Cutting action
involves shear deformation of work material to form a chip, and as chip is removed, new surface is exposed: (a) positive and (b) negative rake tools ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Machining
5.
Why Machining is
Important Variety of work materials can be machined Most frequently used to cut metals Variety of part shapes and special geometric features possible: Screw threads Accurate round holes Very straight edges and surfaces Good dimensional accuracy and surface finish ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
6.
Disadvantages of Machining
Wasteful of material Chips generated in machining are wasted material At least in the unit operation Time consuming A machining operation generally takes longer to shape a given part than alternative shaping processes ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
7.
Machining in the
Manufacturing Sequence Generally performed after other basic manufacturing processes, such as casting, forging, and bar drawing Other processes create the general shape of the starting work part Machining provides the final shape, dimensions, finish, and special geometric details that other processes cannot create ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
8.
Machining Operations Most
important machining operations: Turning Drilling Milling Other machining operations: Shaping and planing Broaching Sawing ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
9.
Single point
cutting tool removes material from a rotating workpiece to form a cylindrical shape ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Turning
10.
Used to
create a round hole, usually by means of a rotating tool (drill bit) with two cutting edges ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Drilling
11.
Rotating multiple-cutting-edge
tool is moved across work to cut a plane or straight surface Two forms: peripheral milling (left) and face milling ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Milling
12.
Cutting Tool Classification 1.
Single-Point Tools One dominant cutting edge Point is usually rounded to form a nose radius Turning uses single point tools 2. Multiple Cutting Edge Tools More than one cutting edge Motion relative to work achieved by rotating Drilling and milling use rotating multiple cutting edge tools ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
13.
Cutting Tools (a)
Single-point tool showing rake face, flank, and tool point; and (b) a helical milling cutter, representative of tools with multiple cutting edges ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
14.
Cutting Conditions in
Machining Three dimensions of a machining process Cutting speed v – primary motion Feed f – secondary motion Depth of cut d – penetration of tool below original work surface For certain operations (e.g., turning), material removal rate RMR can be computed as RMR = v f d ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
15.
Cutting Conditions in
Turning Speed, feed, and depth of cut in a turning operation ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
16.
Roughing vs. Finishing
Cuts In production, several roughing cuts are usually taken on a part, followed by one or two finishing cuts Roughing - removes large amounts of material from starting work part Some material remains for finish cutting High feeds and depths, low speeds Finishing - completes part geometry Final dimensions, tolerances, and finish Low feeds and depths, high cutting speeds ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
17.
Machine Tool A
power-driven machine that performs a machining operation, including grinding Functions in machining: Holds work part Positions tool relative to work Provides power at speed, feed, and depth that have been set The term also applies to machines that perform metal forming operations ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
18.
Simplified 2-D
model of machining that describes the mechanics of machining fairly accurately ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Orthogonal Cutting Model
19.
Chip Thickness Ratio where
r = chip thickness ratio; to = thickness of the chip prior to chip formation; and tc = chip thickness after separation Chip thickness after cut is always greater than before, so chip ratio is always less than 1.0 ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e c o t t r
20.
Determining Shear Plane
Angle Based on the geometric parameters of the orthogonal model, the shear plane angle can be determined as: ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e where r = chip ratio, and = rake angle sin cos tan r r 1
21.
(a) Chip
formation depicted as a series of parallel plates sliding relative to each other, (b) one of the plates isolated to show shear strain, and (c) shear strain triangle used to derive strain equation ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Shear Strain in Chip Formation
22.
Shear Strain Shear
strain in machining can be computed from the following equation, based on the preceding parallel plate model = tan( - ) + cot where = shear strain, = shear plane angle, and = rake angle of cutting tool ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
23.
Actual Chip Formation
More realistic view of chip formation, showing shear zone rather than shear plane Also shown is the secondary shear zone resulting from tool-chip friction ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
24.
Four Basic Types
of Chip in Machining 1. Discontinuous chip 2. Continuous chip 3. Continuous chip with Built-up Edge (BUE) 4. Serrated chip ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
25.
Brittle work
materials Low cutting speeds Large feed and depth of cut High tool-chip friction ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Discontinuous Chip
26.
Ductile work
materials High cutting speeds Small feeds and depths Sharp cutting edge Low tool-chip friction ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Continuous Chip
27.
Ductile materials
Low-to-medium cutting speeds Tool-chip friction causes portions of chip to adhere to rake face BUE forms, then breaks off, cyclically ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Continuous with BUE
28.
Semicontinuous -
saw-tooth appearance Cyclical chip forms with alternating high shear strain then low shear strain Associated with difficult-to- machine metals at high cutting speeds ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Serrated Chip
29.
(a) Friction
force F and Normal force to friction N (b) Shear force Fs and Normal force to shear Fn ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Forces Acting on Chip
30.
Resultant Forces Vector
addition of F and N = resultant R Vector addition of Fs and Fn = resultant R' Forces acting on the chip must be in balance: R' must be equal in magnitude to R R’ must be opposite in direction to R R’’ must be collinear with R ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
31.
Coefficient of Friction
Coefficient of friction between tool and chip ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Friction angle related to coefficient of friction as N F tan
32.
Shear Stress Shear
stress acting along the shear plane where As = area of the shear plane Shear stress = shear strength S of work material during cutting ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e s s F A sin w t A o s
33.
F, N,
Fs, and Fn cannot be directly measured Forces acting on the tool that can be measured: Cutting force Fc and Thrust force Ft ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Cutting Force and Thrust Force
34.
Forces in Metal
Cutting Equations to relate the forces that cannot be measured to the forces that can be measured: F = Fc sin + Ft cos N = Fc cos - Ft sin Fs = Fc cos - Ft sin Fn = Fc sin + Ft cos Based on these calculated force, shear stress and coefficient of friction can be determined ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
35.
The Merchant Equation
Of all the possible angles at which shear deformation can occur, the work material will select a shear plane angle that minimizes energy Derived by Eugene Merchant Based on orthogonal cutting, but validity extends to 3-D machining ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e 2 2 45
36.
What the Merchant
Equation Tells Us To increase shear plane angle Increase the rake angle Reduce the friction angle (or reduce the coefficient of friction) ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e 2 2 45
37.
Higher shear
plane angle means smaller shear plane which means lower shear force, cutting forces, power, and temperature ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e Effect of Higher Shear Plane Angle
38.
Power and Energy
Relationships A machining operation requires power The power to perform machining can be computed from: Pc = Fc v where Pc = cutting power; Fc = cutting force; and v = cutting speed ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
39.
Power and Energy
Relationships In U.S. customary units, power is traditional expressed as horsepower (dividing ft-lb/min by 33,000) ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e where HPc = cutting horsepower, hp 000 33, v F HP c c
40.
Power and Energy
Relationships Gross power to operate the machine tool Pg or HPg is given by or ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e where E = mechanical efficiency of machine tool Typical E for machine tools 90% E P P c g E HP HP c g
41.
Unit Power in
Machining Useful to convert power into power per unit volume rate of metal cut Called unit power, Pu or unit horsepower, HPu or ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e where RMR = material removal rate MR c U R P P = MR c u R HP HP =
42.
Specific Energy in
Machining Unit power is also known as the specific energy U ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e where Units for specific energy are typically N-m/mm3 or J/mm3 (in-lb/in3) w vt v F R P P U o c MR c u = = =
43.
Cutting Temperature Approximately
98% of the energy in machining is converted into heat This can cause temperatures to be very high at the tool-chip The remaining energy (about 2%) is retained as elastic energy in the chip ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
44.
Cutting Temperatures are Important
High cutting temperatures result in the following: Reduce tool life Produce hot chips that pose safety hazards to the machine operator Can cause inaccuracies in part dimensions due to thermal expansion of work material ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
45.
Cutting Temperature Analytical
method derived by Nathan Cook from dimensional analysis using experimental data for various work materials ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e where T = temperature rise at tool-chip interface; U = specific energy; v = cutting speed; to = chip thickness before cut; C = volumetric specific heat of work material; K = thermal diffusivity of work material 333 0 4 0 . . K vt C U T o
46.
Cutting Temperature Experimental
methods can be used to measure temperatures in machining Most frequently used technique is the tool-chip thermocouple Using this method, Ken Trigger determined the speed-temperature relationship to be of the form: T = K vm where T = measured tool-chip interface temperature, and v = cutting speed ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
47.
https://youtu.be/mRuSYQ5Npek https://youtu.be/8SbBC5UWeNI ©2013
John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
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