RRB ALP Machinist Trade — Complete CBT-2 Study Guide

• Milling uses a rotating multi-point cutter while the workpiece is fed against it (typically on a table that moves in X, Y, sometimes Z axes) — produces flat surfaces, slots, contours, and complex profiles that a lathe (single-point, rotating workpiece) cannot. • Up-milling (conventional milling — cutter rotation opposes feed direction) vs Down-milling (climb milling — cutter rotation matches feed direction); climb milling generally gives a better surface finish and longer tool life but requires a rigid machine/setup free of backlash to avoid the cutter "grabbing" the workpiece. • Grinding uses an abrasive wheel (bonded abrasive grains) rotating at high speed to remove very small amounts of material, achieving fine surface finish and tight dimensional tolerances — commonly used as a finishing operation after turning/milling. • Grinding wheel specification includes abrasive type (aluminum oxide for steel, silicon carbide for cast iron/non-ferrous), grit size (coarser for fast stock removal, finer for smooth finish), and bond type/hardness (determines how readily worn/dulled grains release to expose fresh cutting edges).

• Shaper machine produces flat/contoured surfaces via a reciprocating (back-and-forth) single-point tool moving horizontally across a stationary workpiece — the cutting stroke removes material, the return stroke is non-cutting (idle). • Quick-return mechanism in a shaper makes the return (idle) stroke faster than the cutting stroke, improving productivity by minimizing time spent on the non-productive stroke. • Slotter machine works similarly to a shaper but with VERTICAL reciprocating tool motion instead of horizontal — well suited for cutting internal keyways, slots, and non-circular internal profiles that a shaper's horizontal motion cannot reach. • Both shaper and slotter have largely been superseded by milling machines and CNC equipment in modern high-volume production but remain relevant for certain low-volume, specific-profile jobs and in ITI/trade training curricula.

• CNC (Computer Numerical Control) machines execute pre-programmed instructions (G-code) to control tool movement with high precision and repeatability, without constant manual operator intervention for each cut. • G-code consists of standardized commands: G00 (rapid positioning, non-cutting move), G01 (linear cutting move at a programmed feed rate), G02/G03 (circular interpolation, clockwise/counter-clockwise arc cutting) — these are the most fundamental CNC programming codes. • CNC machines use a defined coordinate system (typically Cartesian X, Y, Z axes) with an established work-zero/origin point — accurate work-offset setting is essential before running a program, since an incorrect zero point causes the entire job to be machined in the wrong location. • Advantages of CNC over manual machines: higher repeatability/accuracy across a production run, reduced operator fatigue/error, and the ability to machine complex profiles (via programmed toolpaths) that would be extremely difficult manually.

• Gear cutting methods: Form milling (uses a cutter shaped to match the gear tooth profile, simple but limited to one specific gear size/tooth-count per cutter), Hobbing (a rotating hob cutter generates teeth via a continuous generating action, highly efficient and widely used for volume production), Gear shaping (uses a reciprocating cutter shaped like a gear, suited for internal gears and gears close to a shoulder that hobbing cannot reach). • Module (m) is the standard metric measure of gear tooth size: m = Pitch circle diameter / Number of teeth — gears must have MATCHING module to mesh correctly together. • Pressure angle (commonly 20° in modern gear standards, older standard was 14.5°) is the angle between the gear tooth profile and a line tangent to the pitch circle — affects tooth strength and smoothness of engagement; a higher pressure angle gives a stronger tooth root but slightly more sliding friction. • Backlash (a small intentional clearance between meshing gear teeth) prevents jamming from thermal expansion or minor manufacturing variation, but excessive backlash causes noise/vibration and reduced positioning accuracy in precision applications.