Machinability & Physics of Metal Cutting

Objectives Be able explain machinability Understand how material is removed from and the effects it has on the surface finish of a work-piece Analyze and troubleshoot chip formation

What is machinability? Machinability is the ease with which a material can be machined, the wear created to the cutting edge, surface finish obtained and power consumption.

What factors play a role in machinability? Mechanical properties of material (Hardness, Yield, Shear, etc.) Geometry of tool tip (Rake angle, relief angle) Tool Type (HSS, Carbide, CBD, PCD)

Stainless Steel / Super alloy Machinability Chart Carbon Steel 1015 72% 1018 78% 1020 1022 1030 70% 1040 64% 1042 1050 54% 1095 42% 1117 91% 1137 1141 1141 annealed 81% 1144 76% 1144 annealed 85% 1144 stressproof 83% 1212 100% 1213 136% 12L14 170% 1215 Alloy Steel 2355 annealed 70% 4130 annealed 72% 4140 annealed 66% 4142 annealed 41L42 annealed 77% 4150 annealed 60% 4340 annealed 57% 4620 4820 annealed 49% 52100 annealed 40% 6150 annealed 8620 86L20 9310 annealed 51% Stainless Steel / Super alloy 302 annealed 45% 303 annealed 78% 304 annealed 316 annealed 321 annealed 36% 347 annealed 410 annealed 54% 416 annealed 110% 420 annealed 430 annealed 431 annealed 440A 15-5PH condition A 48% 17-4PH condition A A286 aged 33% Hastelloy X 19% Tool Steels A-2 42% A-6 33% D-2 27% D-3 M-2 39% O-1 O-2 Cast Iron ASTM class 25 55% ASTM class 30 48% ASTM class 35 ASTM class 40 ASTM class 45 36% ASTM class 50 Aluminum aluminum, cold drawn 360% aluminum, cast 450% aluminum, die cast 76%

Traditional Metal Cutting To machine, the tool or work-piece must rotate or move in a linear direction. In order to remove material, the cutting tool must engage the work-piece. The rate of speed at which this engagement occurs is called Feed-Rate. Feed-Rate controls the thickness of the chip and is a major factor in the type of chip that is formed. Depth of cut, also known as in-feed is the depth of material set to be removed by the cutting tool.

Chip Removal in Traditional Machining Operations

Physics of Chip Removal Metal chips are sheared not “cut” from the work piece

Physics of Chip Removal Deformation Zone / Plastic Deformation

Physics of Chip Removal Elongation of the crystalline structure occur in the direction of the cut.

Physics of Chip Removal Plastic Flow

Physics of Chip Removal Rupture Chip

Physics of Chip Removal Affect of rake angle on chip thickness

Chip Formation Three basic types of chips in machining Discontinuous chip Continuous chip Continuous chip with Built up edge

Chip Formation Discontinuous chip Brittle work materials (e.g. Cast Irons) Low Cutting Speed Large feed and depth of cut High tool chip friction (small rake angle)

Chip Formation Continuous chip Ductile work materials (e.g. Low carbon steel) High cutting speeds Small feeds and depths of cut Sharp cutting edge on tool Low tool-chip Friction

Chip Formation Continuous chip with BUE (Built up edge) Ductile materials Too low rake angle Lack of cutting fluid Feed rate Tool life affected

Chip Formation BUE breaking off tool face onto new work surface

Questions?