Manufacturing Processes II: Machining, Machine Tools, Advanced Manufacturing and CIM

In orthogonal cutting the cutting edge is perpendicular to the cutting velocity and the chip flows in the plane of the paper, so the analysis is two-dimensional; in oblique cutting the edge is inclined at an inclination angle i and the chip flows sideways (by Stabler's rule the chip-flow angle is roughly equal to i). Most real turning is oblique, but orthogonal theory is what GATE computes with. The tool signature of a single-point tool in the American (ASA) system lists seven elements in a fixed order: back rake, side rake, end relief, side relief, end cutting-edge angle, side cutting-edge angle and nose radius — for example 8-14-6-6-6-15-1. The orthogonal rake system (ORS) specifies the same tool in planes normal to the cutting edge instead.

Merchant's analysis treats the chip as formed by shear on a single plane inclined at the shear angle φ. With uncut thickness t and chip thickness t_c, the chip thickness ratio r = t/t_c = sin φ/cos(φ − α), so tan φ = r cos α/(1 − r sin α), with α the rake angle. The shear strain is γ = cot φ + tan(φ − α). From the measured cutting force F_c and thrust force F_t: friction force F = F_c sin α + F_t cos α, normal force N = F_c cos α − F_t sin α, and μ = tan β = F/N; shear force F_s = F_c cos φ − F_t sin φ, which divided by the shear-plane area bt/sin φ gives the shear stress. Minimising energy gives Merchant's relation 2φ + β − α = 90°: a larger rake or a smaller friction angle raises the shear angle, thins the chip and cuts the force.

The specific cutting energy is the energy to remove unit volume, u = F_c/(bt) = F_c V/(MRR), usually quoted in J/mm³ (1 J/mm³ = 1000 N/mm², since 1 N·mm = 10⁻³ J). Cutting power is P = F_c V; thrust does almost no work because the tool barely moves in its direction. Specific energy rises as uncut thickness falls — the size effect — which is why fine finishing cuts are energy-expensive per unit volume.

The three machining parameters are cutting speed V = πDN (surface speed, m/min), feed f (mm/rev in turning, mm/tooth in milling) and depth of cut d. In turning, MRR ≈ V f d: at V = 100 m/min, f = 0.2 mm/rev and d = 2 mm, MRR = 100 000 × 0.2 × 2 = 40 000 mm³/min. The ideal surface roughness left by a tool of nose radius r is R_max = f²/(8r) (and R_a ≈ f²/(32r)), so halving the feed quarters the roughness, while speed has no place in the ideal formula at all.

Tool wear: flank wear on the clearance face, measured as the wear-land width VB, grows steadily and is the usual tool-life criterion; crater wear on the rake face, driven by diffusion at the high temperature of the chip-tool contact, dominates at high speed; notch wear, chipping and plastic deformation are the others. Taylor's tool-life equation VTⁿ = C links speed and life, with n rising from HSS through carbides to ceramics; the extended form VTⁿfᵃdᵇ = K adds feed and depth. Heat is generated in the primary shear zone, at the chip-tool interface and at the flank; most of it leaves in the chip, and the rake-face temperature is what limits speed. Cutting fluids cool (water-based emulsions at high speed) and lubricate (oils at low speed, where built-up edge is the problem). Machinability — the ease of machining judged by tool life, forces, finish and chip form — is often quoted as an index relative to a free-cutting reference steel.

Economics of machining. Raising speed shortens machining time but also tool life, so cost per piece has a minimum. With Taylor exponent n, tool-change time t_c, machine-and-labour cost rate C_m and cost per cutting edge C_t: the tool life for minimum cost is T = (1/n − 1)(t_c + C_t/C_m), and for maximum production rate T = (1/n − 1)t_c. The maximum-production life is always shorter, so its speed is always higher, and the economic operating range lies between the two speeds. For n = 0.25, t_c = 2 min and C_t/C_m = 18 min, the minimum-cost life is 3 × 20 = 60 min.

• Turning reduces a rotating workpiece's diameter; machining time for length L is L/(fN). Taper turning by offsetting the tailstock needs an offset = L(D − d)/(2l) for a taper of length l on a job of length L; by swivelling the compound rest the half-angle is tan α = (D − d)/(2l), suited to short, steep tapers; a taper-turning attachment handles long gentle tapers.
• Thread cutting on a lathe couples the carriage to the spindle through the lead screw: the change-gear ratio (driver/driven) equals pitch of the job divided by pitch of the lead screw.
• Drilling with a twist drill of diameter D: MRR = (πD²/4) f N, and time = (hole depth + approach, about 0.3D to 0.5D for the point) / (fN). Boring enlarges and trues an existing hole with a single-point tool; reaming finishes it to size.
• Milling: table feed f_m = f_z z N (feed per tooth × teeth × rpm) and MRR = w d f_m. In up (conventional) milling the cutter rotates against the feed and the chip starts thin; in down (climb) milling it rotates with the feed, the chip starts thick, finish and tool life are better, but backlash in the feed screw must be eliminated. Cutter travel for time must include the approach.
• Gear cutting: form milling with a cutter shaped to the tooth space (one space at a time, indexing between); gear hobbing, a generating process in which a worm-like hob and blank rotate in mesh — fast and accurate for spur and helical gears; gear shaping with a reciprocating pinion-shaped cutter, which can also cut internal and cluster gears. Thread production: cutting on a lathe, chasing, tapping and die threading, milling, grinding, and thread rolling, a cold-forming process that is fast, wasteless and gives stronger threads through unbroken grain flow.

Grinding removes material by many tiny, randomly oriented abrasive grains with large negative rake, so specific energy is very high. A wheel is specified as, for example, A 46 K 5 V: abrasive (A aluminium oxide for steels, C silicon carbide for cast iron and non-ferrous), grain size (larger number, finer grit), grade (A soft to Z hard — the strength of the bond's hold on the grain), structure (grain spacing, dense to open) and bond (V vitrified, B resinoid, R rubber, M metal). The rule to remember: a soft wheel for hard work, a hard wheel for soft work, so that dull grains break away and fresh edges appear. A wheel glazes when its dulled grains do not break out, and loads when chips clog its pores; dressing restores sharpness and truing restores its geometry.

Honing uses bonded abrasive sticks rotating and reciprocating inside a bore, correcting roundness and taper and leaving a cross-hatched finish that holds oil (engine cylinders). Lapping rubs the work against a lap charged with loose fine abrasive to achieve extreme flatness and fit (gauge blocks, valve seats). Superfinishing oscillates a fine stone at low pressure over a rotating surface to remove the thin damaged layer and leave a mirror finish. None of the three removes much material; they correct form and finish after grinding.

The centre lathe has a bed carrying the headstock (spindle and speed gearbox), tailstock and carriage (saddle, cross slide, compound rest, tool post, apron). Its kinematics: the spindle gives the primary cutting motion; the feed rod drives the carriage for turning feeds, and the lead screw, engaged by the half-nut, for threading. Spindle speeds are usually in a geometric progression, N_k+1/N_k = φ = (N_max/N_min)^1/(z−1) for z speeds, so that the cutting speed error is the same fraction at every step. Milling machines are horizontal (arbor-mounted cutters), vertical (end mills and face mills) or universal (swivelling table for helical work); the dividing head with a 40 : 1 worm ratio indexes by 40/N turns of the crank for N divisions. Drilling machines are bench, pillar, radial (arm swings to reach any point on a large job) and gang or multi-spindle. The shaper reciprocates a single-point tool over a stationary job; its crank-and-slotted-lever mechanism gives a quick return, the ratio of cutting to return time being the ratio of the crank angles swept in each stroke.

A jig locates and holds the work AND guides the tool, through hardened drill bushes; a fixture locates and holds the work but does not guide the tool (milling, turning, welding and inspection fixtures). A free body has 12 degrees of freedom — translation and rotation, each in two senses, along three axes. The 3-2-1 principle locates a prismatic part on three points on the primary face, two on the secondary and one on the tertiary, restraining 9 of the 12; the clamps restrain the remaining 3. Design principles: locate on machined datum surfaces and never over-locate; clamp against the locators and on rigid sections, away from the cutting forces' line of action where possible; design for fool-proofing so the part cannot be loaded wrongly; provide chip clearance and easy loading; keep the fixture rigid and light. Locators include pins, a round and a diamond pin pair for two holes, V-blocks for cylinders and nests; clamps include strap, cam, toggle, screw and hydraulic. Jigs are template, plate, channel, box, leaf and indexing types.

ECM removal rate. By Faraday's law the mass dissolved per second is AI/(zF), with A the atomic weight, z the valency and F = 96 485 C/mol; dividing by density gives the volumetric rate. For iron (A = 55.85 g/mol, z = 2, ρ = 7.87 g/cm³) at 1000 A: 55.85 × 1000/(2 × 96 485) = 0.2894 g/s, i.e. 0.03678 cm³/s = 36.8 mm³/s. Hardness does not appear anywhere in the formula — which is the reason ECM exists.

Additive manufacturing builds a part layer by layer from a CAD model: the solid is exported as a tessellated STL file, oriented, given supports where needed, and sliced into layers whose thickness trades build time against the stair-step finish on inclined faces. The processes the syllabus names: stereolithography (vat photopolymerisation — a UV laser cures liquid photopolymer resin layer by layer; fine detail, needs supports and post-curing); fused filament fabrication (material extrusion — a heated nozzle deposits thermoplastic filament; cheap, anisotropic, visible layers); selective laser sintering (powder-bed fusion of polymer powder by a laser; the unfused powder supports the part, so no support structures are needed); selective laser melting (powder-bed fusion that fully melts metal powder to near-full density; supports are needed to anchor and conduct heat); and directed energy deposition (a laser, electron beam or arc melts powder or wire as it is fed into a melt pool — used for large parts, cladding and repair).

CAD represents geometry as wireframe (edges only, ambiguous), surface (faces, no inside) or solid models — constructive solid geometry (Boolean combinations of primitives) or boundary representation (faces, edges and vertices with topology) — which alone can give mass properties and interference checks; parametric, feature-based modelling drives today's systems. CAM turns that geometry into tool paths and process plans. CNC executes a part program on the machine: G-codes set motion (G00 rapid positioning, G01 linear interpolation, G02 clockwise and G03 counter-clockwise circular interpolation, G90 absolute and G91 incremental dimensions) and M-codes switch functions (M03 spindle clockwise, M05 spindle stop, M06 tool change, M08 coolant on, M30 end of program). Open-loop systems drive stepper motors without feedback; closed-loop systems measure position with encoders or scales. The basic length unit is the smallest movement the control can command. DNC links several CNC machines to one computer.

Automation is fixed (hard) — dedicated transfer lines, highest rate, no flexibility, for very high volume; programmable — CNC and robots reprogrammed between batches; or flexible — changeover with no lost production time, for a variety of parts in medium volume. Industrial robots are classified by the joints of their arm: Cartesian or gantry (three prismatic, rectangular work envelope), cylindrical (one revolute, two prismatic), polar or spherical (two revolute, one prismatic), SCARA (two parallel revolute joints and a vertical prismatic one — compliant in the horizontal plane, stiff vertically, ideal for assembly) and articulated (all revolute, the most dexterous, like a human arm). Drives are electric servo (accurate, clean, most common), hydraulic (heavy payloads) and pneumatic (cheap, fast, pick-and-place). Controls: point-to-point (spot welding, loading) versus continuous-path (arc welding, painting); programming by teach pendant, lead-through or offline; feedback from internal and external sensors; the end effector is a gripper or a tool.

Group technology groups parts with similar shape or processing into part families, identified by classification and coding (the Opitz code's digits describe the part's form and then its size, material and accuracy) or by production flow analysis of routings, often by rank-order clustering of a machine-part incidence matrix. Cellular manufacturing then dedicates a cell of dissimilar machines to each family, cutting material handling, throughput time and work-in-process relative to a process layout. A flexible manufacturing system is a group of CNC workstations linked by automated material handling and storage (conveyors, AGVs, pallet changers) under central computer control, able to process a mix of part types in any order. The Internet of Things puts sensors and network connectivity on machines, so condition data — spindle load, vibration, temperature, cycle counts — stream to the cloud for monitoring and analytics. A digital twin is a virtual model of a specific physical asset or process, kept synchronised with it by that live data, used to monitor, predict failures and try changes virtually before making them on the shop floor.