THEORY OF METAL MACHINING SME Video – Machining Processes Turning & Lathe Basics (vts_05) Milling & Machining Center Basics (vts_03) Machining Technology Theory of Chip Formation Merchant Equation Power and Energy Relationships Cutting Temperature In-class Assignment ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Classification of Material Removal Processes ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Tool with negative rake angle Machining Cutting action involves shear deformation Cross‑sectional view Tool with negative rake angle ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Why Machining is Important Good dimensional accuracy Good surface finish Disadvantages Wasteful of material Time consuming Machining performed after other processes Casting Forging ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Machining Operations Turning Drilling Peripheral milling Face milling ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Multiple Cutting Edge Tools Cutting Tools Single-Point Tools Multiple Cutting Edge Tools ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Cutting Conditions for Turning MRR = v f d v = cutting speed f = feed d = depth of cut ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Roughing vs. Finishing Several roughing operations Followed by one or two finishing cuts Roughing - removes large amounts of material High feeds and depths Low speeds Finishing - completes part geometry Low feeds and depths High cutting speeds ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Machine Tools Power‑driven machine Performs a machining operation Including grinding Functions in machining Holds workpart Positions tool relative to work Provides power at speed, feed, and depth Machine tools – applies to machines that perform metal forming operations ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Orthogonal Cutting Model Orthogonal cutting: (a) as a three‑dimensional process. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Chip Thickness Ratio & Shear Plane Angle r = chip thickness ratio to = thickness prior to chip formation tc = chip thickness after separation Chip thickness after cut greater than before Based on orthogonal model Shear plane angle  where r = chip ratio, and  = rake angle ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Shear Strain in Chip Formation  = tan( - ) + cot  where  = shear strain  = shear plane angle  = rake angle of cutting tool ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

In-class Example In an orthogonal cutting operation, the tool has a rake angle = 15. The chip thickness before the cut = 0.30 mm and the cut yields a deformed chip thickness = 0.65 mm. Calculate (a) the shear plane angle and (b) the shear strain for the operation. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

SME Video ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

In-class Assignment In an orthogonal cutting operation, the 0.250 in wide tool has a rake angle of 5. The lathe is set so the chip thickness before the cut is 0.010 in. After the cut, the deformed chip thickness is measured to be 0.027 in. Calculate (a) the shear plane angle and (b) the shear strain for the operation. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Production Scheduling Machine-Machine Diagrams ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Time Permitting Content ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Approximation of Turning by Orthogonal Cutting Turning operation ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Chip Formation Figure 21.8 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. ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Basic Types of Chip in Machining Ductile materials High cutting speeds Small feeds Small depths Sharp cutting edge Low tool‑chip friction Brittle materials Low cutting speeds Large feed Large depth of cut High tool‑chip friction ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Basic Types of Chip in Machining 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 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 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Forces Acting on Chip Friction force F and Normal force to friction N Shear force Fs and Normal force to shear Fn Coefficient of friction between tool and chip: Friction angle related to coefficient of friction as follows: ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Shear Stress Shear stress acting along the shear plane: where As = area of the shear plane Shear stress = shear strength of work material during cutting ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Cutting Force and Thrust Force 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 Figure 21.10 Forces in metal cutting: (b) forces acting on the tool that can be measured ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Forces in Metal Cutting F = Fc sin + Ft cos N = Fc cos ‑ Ft sin Fs = Fc cos ‑ Ft sin Fn = Fc sin + Ft cos ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

The Merchant Equation Work material will select a shear plane angle  that minimizes energy, given by Derived by Eugene Merchant To increase shear plane angle Increase the rake angle Reduce the friction angle (or coefficient of friction) ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Effect of Higher Shear Plane Angle Higher shear plane angle means smaller shear plane which means lower shear force, cutting forces, power, and temperature Figure 21.12 Effect of shear plane angle  : (a) higher  with a resulting lower shear plane area; (b) smaller  with a corresponding larger shear plane area. Note that the rake angle is larger in (a), which tends to increase shear angle according to the Merchant equation ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Power and Energy Relationships Power is traditional expressed as horsepower (dividing ft‑lb/min by 33,000) where HPc = cutting horsepower, hp Gross power to operate the machine tool Pg or HPg is given by where E = mechanical efficiency of machine tool Typical E for machine tools  90% ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Unit Power and Specific Energy Power per unit volume rate of metal cut Called unit power, Pu or unit horsepower, HPu or where RMR = material removal rate Unit power is also known as the specific energy U Units for specific energy are typically N‑m/mm3 or J/mm3 (in‑lb/in3) ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Energy Relationships Distribution of total cutting energy Correction factor for unit horsepower calculations ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

Cutting Temperature Analytical method derived by Nathan Cook Experimental method derived by Ken Trigger T = K vm Experimentally measured cutting temperatures ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e