Material Removal
Continuous chip
made due to: Ductile work materials High cutting speeds Small feeds and depths Sharp cutting edge Low tool-chip friction
Cutting Temperature
Approximately 98% of energy in machining is converted into heat, which cases temperatures to be very high at the tool-chip. Remaining 2% energy is retained as elastic energy in the chip. High cutting temperatures reduce tool life, produce hot chips (safety hazard), and can cause inaccuracies due to thermal expansion
Cutting Force and Thrust Force
Because friction, shear and normal forces can't be directly measured, cutting force and thrust force are measured | sin(a) cos(a) |_______| F | | cos(a) -sin(a) | | F_c | = | N | | cos(p) -sin(p) | | F_t | = |F_s| | sin(p) cos(p) | -----------| F_n|
Fundamentals of cutting
Chip, Rake angle (alpha), cutting speed (v), shear plane angle (phi), chip ratio (r), depth (d)
Types of chip in machining
Discontinuous chip Continuous chip Continuous chip with Built-up edge (BUE) Serrated chip
Drilling
Drill bit creates a round hole
Forces acting on chip
Friction force F and normal force to friction N on the tool = resultant R Shear force F_s and Normal force to shear F_n on the chip = resultant R'
Machining in Manufacturing
Generally done after other manufacturing processes, which create the general shape. Machining does finishing touches (shape, dimensions, finish, special geometric features)
Material Removal Rate (MRR)
MRR = v*f*d v = velocity of tool (relative to workpiece) f = feed (how much being cut in velocity direction) d = depth of cut (how far into surface is being cut)
Material Removal Processes
Machining, Abrasive, Nontraditional processes
Machining Operations
Most Important: Turning, Drilling, Milling Others: shaping, broaching, sawing
Power
P_c = F_c*v P_c is cutting power F_c is cutting force v is cutting speed in Horsepower HP_c = P_c/33000
Gross power
P_g = P_c/E where E is efficiency (of machine)
Milling
Rotating multi-blade cutting edge tool is moved across workpiece Peripheral milling - axis of tool parallel to workpiece surface (tank wheel rolling over) Face milling - axis of tool perpendicular to workpiece surface (waxing the floor)
Serrated chip
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
Shear Stress
Shear stress acting along the shear plane S = F_s/A_s where A_s is the shear plane A_s = t_o*w/sin(phi)
Orthogonal cutting model
Simplified 2-D model of machining
Turning
Single point cutting tool removes material from a rotating (turning) workpiece to make cylindrical shape
Cutting temperature equations
T = (0.4U/(rho*C))((v*t_o/K)^.33) T is temperature rise at tool-chip interface U is specific energy v is cutting speed rho*C is volumetric specific heat of work material K is thermal diffusivity of work material OR T = K*v^m
Unit Power
Useful to convert power into power per unit volume of metal cut rate U = P_u = P_c/MRR = F_c/(t_o*w) where MRR is material removal rate U is specific energy
Benefits of Machining
Variety of work materials can be machined (most frequently metals). Variety of part shapes and special geometries (screw threads, accurate round holes, very straight edges and surfaces) Good dimensional accuracy and surface finish
Disadvantage of Machining
Wasteful of material Time consuming
Coefficient of Friction
between tool and chip mu = F/N
Finishing
completes part geometry to final dimensions, tolerances, and clean finish low feeds and depths, high cutting speeds (v)
Shear strain (gamma) in chip formation
gamma = cot(phi) + tan(phi - alpha) shear plane is more realistically a shear zone fyi
Discontinuous chip
made due to: Brittle work materials Low cutting speeds Large feed and depth of cut High tool-chip friction
Continuous chip with built up edge (BUE)
made due to: Ductile materials Low-to-medium cutting speeds Tool-chip friction causes potions of chip to adhere to rake face BUE forms, then breaks off, cyclically
Abrasive processes
material removal by hard, abrasive particles grinding
Machining
material removal by sharp cutting tool turning, milling, drilling
Friction Angle
mu = tan(beta) angle between F and N
Merchant Equation
phi = 45 + alpha/2 - beta/2 shear plane angle that minimizes energy
Chip Thickness Ratio (r)
r = t_o/t_c t_o = depth t_c = chip thickness chi is always thicker than depth, so chip thickness ratio is always less than 1 (0 < r < 1)
Roughing
removes large amounts of material from the workpart in order to create shape close to final geometry high feed and depth, low speed (v)
Shear Plane Angle (phi)
tan (phi) = (r*cos(alpha))/(1-r*sin(alpha)) r = chip thickness ratio alpha = rake angle Angle at which the shear occurs when chips form. Higher shear plane angle means smaller shear plane, which means lower shear force, cutting forces, power, and temperature (see merchant equation)
Nontraditional processes
various energy forms other than sharp cutting tool to remove material
