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