RRB ALP Turner Trade — Complete CBT-2 Study Guide

• A lathe rotates the workpiece against a stationary (but positionable) cutting tool to remove material and produce cylindrical, tapered, or contoured shapes — it is the fundamental machine tool of the turning trade. • Main lathe parts: Headstock (houses the spindle and drive mechanism, holds/rotates the workpiece), Tailstock (supports the free end of long workpieces, holds drills/centers), Carriage (holds and moves the cutting tool along the bed), Bed (the base providing rigid alignment for all other parts). • Common lathe operations: Turning (reduces diameter along the workpiece length), Facing (cuts a flat surface perpendicular to the axis at the workpiece end), Taper turning (produces a gradually changing diameter), Knurling (creates a textured grip pattern on the surface), Parting (cuts off a finished piece from the bar stock). • Cutting speed, feed, and depth of cut are the three primary machining parameters — cutting speed depends on material and tool type, and must be neither too high (causes rapid tool wear/overheating) nor too low (reduces productivity).

• Single-point cutting tool geometry includes rake angle (affects cutting force and chip flow — positive rake reduces cutting force but weakens the tool edge, negative rake strengthens the edge for interrupted/heavy cuts), clearance angle (prevents the tool flank from rubbing against the finished surface), and cutting edge angle (affects chip formation direction). • Tool materials: High Speed Steel (HSS — tough, retains hardness at moderate temperatures, common for general-purpose work), Carbide tools (much harder, retain cutting edge at higher temperatures/speeds, used for high-volume/hard-material production work) — carbide tools generally allow much higher cutting speeds than HSS. • Tool wear types: Flank wear (gradual wear on the tool's side face, most common, measurable and predictable), Crater wear (wear on the tool's top face from chip friction, common at high cutting speeds/temperatures) — both eventually degrade surface finish and dimensional accuracy if the tool isn't replaced/reground in time. • Cutting fluid/coolant reduces friction and heat at the tool-workpiece interface, extending tool life and improving surface finish, especially critical when machining tougher materials like alloy steels.

• Screw threads are cut on a lathe using the lead screw (which controls carriage feed rate) engaged via change gears to produce a specific thread pitch — the ratio of change gears determines threads-per-inch or pitch (mm) produced. • Thread terminology: Pitch (distance between adjacent thread crests), Major diameter (largest diameter of an external thread), Minor diameter (smallest diameter of an external thread), Lead (axial distance the nut/screw advances in ONE full turn — equals pitch for a single-start thread). • Common thread forms: V-thread/ISO Metric (60° included angle, most common general-purpose thread), Square thread and ACME thread (flatter profiles, used where high axial load transmission is needed, e.g. lead screws/jacks — lower friction than a V-thread for power transmission). • Thread cutting requires multiple passes at progressively increasing depth of cut, following the SAME starting point each pass (using a thread dial indicator to re-engage the half-nut at the correct carriage position) to avoid cutting a double/overlapping thread.

• Vernier caliper (Least Count typically 0.02mm) measures external, internal, and depth dimensions on turned components — essential for checking diameters against drawing tolerances during and after turning. • Micrometer (Least Count typically 0.01mm) gives finer precision than a vernier caliper, commonly used for critical diameter checks on precision-turned shafts/pins. • Surface finish/roughness affects both function (bearing fits, sealing surfaces) and appearance — finer feed rates and sharper tool edges generally produce a smoother turned surface finish. • Dial gauge (dial indicator), mounted on a stand or attached to the lathe, checks for concentricity/run-out of a rotating workpiece — high run-out indicates the workpiece is not running true, which will produce an out-of-round or eccentric finished part.