Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Manufacturing Processes Cutting (Machining) 절삭가공 Su-Jin Kim School of Mechanical Engineering Gyeongsang National University

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Cutting 1.Cutting mechanics 2.Tool wear 3.Tool material 4.Turning, Turning center 5.Milling, Machining center

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Cutting mechanics ( 절삭역학 ) Chip formation  Shear break off Cutting force = Specific energy x Area Chatter (vibration) Cutting temperature Tool wear Tool life equation

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Chip Formation ( 칩생성 ) Chips are produced by the shearing taking place along a shear plane.

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Cutting Force ( 전단이론 ) According to maximum-shear-stress criterion, yielding occurs when the max shear stress within an element is equal to or exceeds a critical value (shear yield stress). Tool (Assume no friction) Fc σ1σ1 Stock σσ1σ1 τ τ Ф Mohr’s circle Shear angle Shear plane

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Cutting Force (Shear force theory) Shear Force = Shear Stress * Shear Area Shear Area = Width x Depth / sin (Shear Angle) t0t0 φ t 0 /sin( φ ) Tool w FsFs

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Cutting Force (Theory) Resultant force = Shear force / cos (shear angle + friction angle – rake angle) Tool α β-αβ-α ф Workpiece Fs R β Chip

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Cutting Force (Theory) Cutting force = Resultant force x cos (friction angle – rake angle) Shear angle = pi/4 + rake angle/2 – friction angle/2 Tool α β-αβ-α ф Workpiece Fc R Chip

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Cutting Force Rake angle ↑  shear angle ↑, cutting force ↓  chip thickness ↓, cooler chip ↓ Rake angle ↑  tool section ↓  strength at cutting edge ↓, heat conductivity ↓ Relief angle ↑  friction ↓  tool life ↑, surface quality ↑ Relief angle ↑  strength at cutting edge ↓ Nose radius ↓  heat ↓, surface quality ↑ Force ↑< yield stress of stock ↑, cut depth ↑, cut width ↑ Rake angle,α Relief angle, r + Shear angle, φ Nose radius

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Cutting Force Approximation ( 절삭력 ) Cutting force ≈ Specific cutting energy( 비절삭에너지 ) x Cutting area F c ≈ u t A c Cutting power = force x velocity P = F c V Tool Stock AcAc FcFc MaterialSpecific cutting energy (GPa) Tensile strength (MPa) Aluminum alloys Copper alloys Cast irons Steels

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Ex ) Cutting Force Turning steel, depth of cut d = 0.1 mm, feedrate f = 0.01 mm/rev, Specific cutting energy of steel u = 2.7~9.3 GPa. Cutting force? Cutting speed v = 10 m/s. Cutting power? F = u A = u d f = 2.7~9.3 (10^9 N/m^2) x x 10^-6 m^2 = 2.7~9.3 N P = F v = 2.7 ~ 9.3 N x 10 m/s = 27 ~ 93 W

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Chip morphology ( 칩생성 ) Type of chips produced influences surface finish and machining operation. 1.Continuous chips 2.Built-up-edge chips 3.Serrated chips 4.Discontinuous chips Steel: Cast Iron: Stainless:

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Chip breaker Chip breaker  shorter chip GrooveChip breaker

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Chatter (Self-excited vibration) Chatter vibrating with high frequency noise is caused by interaction of chip-removal process with flexibility of the tool. It could be avoided by increasing dynamic stiffness and damping, by decreasing depth of cut and proper selection of spindle speed. Chatter Safe Spindle (rmp) Depth of cut (mm)

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Temperature ( 절삭열 ) Cutting power P=FV  Heat  Increase the temperature of chip, work piece, and tool - Temperature increase = specific heat x mass : dT = c m - Specific heat (kJ/kgK): iron 0.45, aluminium 0.91, copper 0.39 As temperature increases, it will affect the properties of the cutting tool, dimensional accuracy. - Thermal extension: dL = a dT L - Thermal extension coefficient of iron 10 x 10^-6

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Ex) Temperature of chip Material removal rate? m/t = ρ A v = (kg/s) If we assume 100% of cutting power used to heat chip, Temperature of chip? P = Q/t = c dT m/t If workpiece temperature increased 10 ℃, thermal expansion of workpiece?

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Tool wear ( 공구마모 ) Mechanical wear 1.Abrasive wear - hardness 2.Adhesive wear - junction 3.Fatigue wear - crack (toughness) Thermo Chemical wear 1.Diffusion wear ( 확산 ) 2.Solution wear ( 용해 )

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Tool wear The wear behaviour of cutting tools are flank wear(measure width of wear land), crater wear(at high speed, diffusion wear is the major reason, measure depth), nose wear, and chipping of the cutting edge. crater wear flank wear nose wear broken edge

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Tool life (F.W. Taylor, 공구수명 ) Tool-wear relationship for cutting various steels is Tool-life is also effected by depth and feed rate. V : cutting speed / T : time (min) / C : constant. n : exponent depends on cutting conditions HSS , Carbides , TiC insert 0.30, PCD 0.33, TiN insert 0.35, Ceramic coated insert 0.40 Cutting speed V Tool life T Log C -n d : depth of cut, f : feed rate Carbide Ceramic HSS

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Given that n=0.5 and VT n =C, if the V reduced 50%, calculate the increase of tool life. Solution VT 0.5 =C(1) 0.5VT =C(2) (2)/(1) 0.5(T 2/ T) 0.5 =1 T 2 =4T Increase tool life 4 times. Ex Increasing tool life by reducing the cutting speed

Machining Manufacturing Processes © 2012 Su-Jin Kim GNU Surface Finish ( 표면조도 ) Feed marks In turning, peak-to-valley roughness is r r <R f r : feed rate (mm/rev) R : tool nose radius (mm) Tool Stock R frfr rtrt