Machining and machine tools

In short

Machining removes unwanted material from a workpiece as chips, using a harder cutting tool carried on a machine tool, to produce the required shape, size and surface finish. The common conventional processes are turning, drilling, boring, milling, shaping, planing, slotting, sawing and broaching, each on its own class of machine tool. The subject matters because almost every engineered part in India - automobile shafts and gears, pump casings, valve bodies, railway components, defence and aerospace fittings - gets its final dimensions on a machine tool. It carries 6-10 marks in GATE and ISRO/BARC/SSC-JE papers (machining time, MRR, gear-train and indexing calculations) and is standard ground in interviews at Maruti, Tata Motors, Bharat Forge, L&T, BHEL, HAL, Ashok Leyland and tool rooms such as IGTR Aurangabad and CITD Hyderabad.

Explained

What a machine tool is and how it is built

• A machine tool is a power-driven machine that holds the work and the tool, provides the relative motion between them, and holds them rigidly against the cutting forces.
• Three motions define any machining operation: primary (cutting) motion supplies the cutting speed, feed motion moves the tool into fresh material, and auxiliary motions position/index the tool or work.
• Essential requirements: rigidity (to resist deflection and chatter), accuracy of guideways and spindles, vibration damping, wide speed/feed range, safety and ease of operation.
• Structure: bed, column, base and housings are usually grey cast iron (FG 200/FG 260 to IS 210) because graphite flakes damp vibration; large or welded structures use steel; polymer concrete (epoxy granite) is used in precision machines.
• Guideways: sliding (V, flat, dovetail - simple, good damping, stick-slip at low speed), antifriction (LM) guideways with rolling balls/rollers (low friction, high speed, costlier), and hydrostatic (oil film, no wear, for heavy precision machines).
• Spindle: carried in preloaded antifriction or hydrodynamic bearings; measured by runout and stiffness. Standard taper noses - Morse, metric and ISO/BT/HSK tool shanks.
• Speed and feed drives: stepped drives use a gearbox in geometric progression (ratio φ) so the percentage speed loss is uniform across the range; stepless drives use variable-frequency AC drives (now the norm) or earlier mechanical/hydraulic variators.
• Degrees of freedom and accuracy terms: geometric accuracy (alignment tests to IS 2063 / ISO 230 series), positioning accuracy, repeatability, and stiffness (N/µm) decide what tolerance a machine can hold.

The lathe - parts, operations and calculations

• A centre lathe rotates the work about a horizontal axis (primary motion) while a single-point tool is fed axially or radially - the mother machine of the workshop.
• Main parts: bed (with inverted-V and flat guideways), headstock (spindle, all-geared or cone-pulley drive, back gear), tailstock (dead/live centre, drilling), carriage (saddle, cross-slide, compound rest, tool post), apron (feed clutches, half-nut), feed rod (for turning feeds) and lead screw (for thread cutting only), quick-change gearbox.
• Specification of a lathe: height of centres, swing over bed and over carriage, distance between centres, bed length, spindle bore and nose, number/range of speeds and feeds, main motor power.
• Work holding: 3-jaw self-centring chuck (round/hex bar), 4-jaw independent chuck (irregular, off-centre work), face plate, between centres with a catch plate and driving dog, collet chuck (small accurate bar), mandrel (machining the OD of a bored part concentric with the bore), steady and follower rests for slender work (L/D above about 8-10).
• Operations: turning (straight, step, taper), facing, parting/grooving, chamfering, knurling, drilling and reaming on the lathe, boring, thread cutting, form turning, spinning and polishing.
• Taper turning methods: (1) form tool - short tapers; (2) compound rest swivelling - any angle but short length, hand feed; (3) tailstock offset - long, small-angle tapers on work held between centres; (4) taper turning attachment - long tapers with power feed, best accuracy; (5) combined feeds on CNC.
• Thread cutting: the lead screw is coupled to the spindle through a gear train so that the tool advances exactly one pitch per spindle revolution; use the thread chasing dial / half-nut to re-engage at the same point, and infeed the tool along the compound rest set at half the thread angle (29-30° for a 60° thread).
• Lathe types: centre (engine) lathe, speed lathe, capstan (hexagonal turret on a short ram), turret (turret on a saddle, heavier work, bar or chucking), automatic (single-spindle automat, Swiss-type sliding head, multi-spindle), vertical turret lathe for large discs, copying and CNC turning centres.
• Capstan/turret lathes carry preset multi-tool stations with bar stops and dead-length stops, cutting idle time drastically - the mass-production answer before CNC.

Drilling, boring, reaming and allied hole making

• Drilling makes or enlarges a hole with a rotating multi-point (usually two-flute) tool; the drill rotates and feeds while the work is clamped.
• Twist drill geometry: point angle 118° (general), 135° (hard material), helix/rake angle about 30°, lip relief (clearance) 8-12°, chisel edge angle 120-135°, web, margin, land, flutes for chip evacuation; cutting speed is zero at the axis, hence a chisel edge that extrudes rather than cuts.
• Drilling machines: bench/sensitive, upright (pillar) with power feed, radial arm, gang (several spindles in a row for sequential operations), multi-spindle (all holes at once), deep-hole (gun drill with through-coolant), CNC drilling and tapping centres.
• Related operations: reaming (0.1-0.4 mm stock, improves size, roundness and finish - cannot correct hole position), boring (single-point, corrects location and size), counterboring (flat seat for a cap screw head), countersinking (82-90° cone for screws/rivets), spot facing, tapping (internal threads), trepanning (cuts a large hole leaving a core), centre drilling (locates and provides a bearing surface for a centre).
• Hole quality order (loose to tight): drilled → reamed/bored → ground/honed. Drilled holes typically IT11-IT13; reamed IT7-IT8.
• Boring machines: horizontal boring machine (spindle supported both ends by an outboard column for long holes), vertical boring, jig boring (extremely accurate hole location, formerly the tool-room standard, largely replaced by precision CNC).

Milling, shaping, planing, slotting, sawing and broaching

• Milling uses a rotating multi-tooth cutter while the work is fed past it; each tooth cuts intermittently, so chip load and impact matter.
• Up (conventional) milling: feed opposes cutter rotation; chip thickness starts at zero, tooth rubs and work-hardens the surface, tends to lift the work, no backlash problem - safe on older machines.
• Down (climb) milling: cutter rotation and feed are in the same direction; chip thickness is maximum at entry, better finish and tool life, pushes work onto the table, but needs a backlash-free (preloaded ball-screw) feed or the cutter snatches the work - standard on CNC.
• Milling machines: column-and-knee horizontal (plain, universal with a swivelling table for helical work), vertical (fixed-head or turret/Bridgeport type), bed-type (production, rigid, no knee), planer-type (large work), special - thread, cam, profile, duplicating; and vertical/horizontal machining centres with ATC.
• Milling cutters: plain slab, side-and-face, end mill, slot drill, face mill (indexable inserts), form (gear, convex, concave), T-slot, dovetail, angle, slitting saw, fly cutter. Cutter mounting: arbor, stub arbor, collet, shell-mill adapter.
• Milling operations: slab/face milling, end milling, slotting, straddle, gang milling (several cutters on one arbor), profile, pocket, gear cutting (form cutter on a dividing head), helical milling.
• Dividing head / indexing: divides a circle for gear teeth, splines, bolt holes. Simple indexing uses a 40:1 worm-worm wheel ratio - crank turns = 40/N.
• Shaping: single-point tool reciprocates (cutting on the forward stroke), work feeds intermittently; quick-return mechanism (crank-and-slotted-link or Whitworth) makes the return stroke faster.
• Planing: the work reciprocates on a long table and the tool feeds across - used for very large flat surfaces; shaper = small work, moving tool; planer = large work, moving work.
• Slotting machine: a vertical shaper, for internal keyways, splines and slots.
• Broaching: a multi-tooth tool with progressively rising teeth is pushed or pulled through/over the work, producing the full form in one pass - internal keyways, splines, non-circular holes, external fir-tree slots on turbine discs. Teeth: roughing, semi-finishing, finishing; rise per tooth (RPT) 0.025-0.1 mm; pull broaching is commonest; very high productivity but a costly dedicated tool, only for large batches.
• Sawing: power hacksaw, band saw (continuous blade, contour cutting), circular/cold saw, abrasive disc - the usual stock-cutting first operation.

Process planning, allowances, fluids and economics

• Sequence generally runs: stock cutting → facing/centring → rough machining (heavy depth, coarse feed) → semi-finish → heat treatment if any → finish machining → grinding → inspection. Rough first, finish last, so that distortion from stress relief is removed.
• Machining allowance on castings/forgings is typically 2-6 mm per surface depending on size and process; too little exposes blow holes and hard skin, too much wastes time and tool life.
• Locating principle: 3-2-1 (six-point) location; machine from the same datum as the designer dimensioned from, to avoid tolerance stack-up.
• Cutting fluids: functions - cool, lubricate, flush chips, inhibit corrosion. Types: straight (neat) oils, soluble-oil emulsions, semi-synthetic and synthetic fluids, MQL (minimum quantity lubrication) and dry/cryogenic machining.
• Machine tool selection and economics: cost per piece = machining cost + tool cost + handling + setup/batch; CNC wins as batch size, part complexity and tolerance rise; dedicated automats/broaching win in very high volume; a centre lathe or shaper remains cheapest for single pieces and repairs.
• Indian standards and practice: IS 2063 series and ISO 230 for machine tool test/alignment, IS 1570 for steels, IS 919/IS 2709 for ISO limits and fits, IS 3073 for surface roughness, IS 10719 for tool nomenclature; machine guarding, lighting and worker safety under the Occupational Safety, Health and Working Conditions Code, 2020 (OSH Code, in force 21 November 2025, which repealed the Factories Act, 1948) and its rules, which carry the former Act's Sections 21-24 (fencing of machinery, work on or near machinery in motion, employment of young persons on dangerous machines, striking gear and devices for cutting off power; hoists and lifts were Section 28) and the state Factories Rules saved under the Code; machine tools are made in India by HMT Machine Tools, Ace Micromatic, Jyoti CNC, Bharat Fritz Werner, LMW, Lokesh and Praga, with IMTMA as the industry body and IMTEX as the flagship exhibition.

Key points

• Machining - subtractive shaping by chip removal; gives the tightest tolerances and best finish of the primary processes.
• Machine tool requirements - rigidity, accuracy, damping, wide speed/feed range, safety; grey cast iron beds for vibration damping.
• Cutting speed V - surface speed in m/min, set by material and tool; feed f - mm/rev (turning, drilling) or mm/tooth (milling); depth of cut - mm.
• Lead screw vs feed rod - lead screw is used only for thread cutting; the feed rod drives normal turning feeds, preserving lead-screw accuracy.
• 3-jaw vs 4-jaw chuck - self-centring for round/hexagonal bar; independent jaws for irregular or deliberately offset work.
• Taper turning - form tool, compound rest, tailstock offset, taper attachment; only the attachment gives long tapers with power feed.
• Capstan vs turret lathe - turret on a short ram (light bar work) vs turret on a saddle (heavy chucking work).
• Twist drill - 118° point angle, ~30° helix, 8-12° lip relief; zero cutting speed at the centre, so the chisel edge extrudes material.
• Reaming improves size and finish but cannot correct hole location; boring can.
• Up milling vs down milling - up milling is safe with backlash; down milling gives better finish and tool life but needs a preloaded ball screw.
• Shaper vs planer - shaper moves the tool over small work; planer moves large work past the tool.
• Broaching produces the complete internal form in one stroke; rise per tooth 0.025-0.1 mm; economic only in large batches.
• Simple indexing - crank turns = 40/N on a standard 40:1 dividing head.
• MRR - turning ≈ π D f d N (or 1000·V·f·d); milling = w × d × feed rate; drilling = (π/4)d²× f × N.
• Machining time = length of travel (including approach and overtravel) ÷ feed rate, × number of passes.
• Cutting fluid cools, lubricates, flushes chips and prevents corrosion; MQL and dry machining reduce disposal cost.
• Rough before finish - heavy cuts first for productivity, light finishing cuts last for tolerance and finish.
• OSH Code, 2020 (in force 21 November 2025, replacing the Factories Act 1948) governs guarding of machinery and safe work near moving parts in Indian workshops.

Formulas and examples

Example 1 - speed, passes, time and MRR in turning. A 60 mm diameter MS bar is turned to 54 mm over 300 mm length at V = 90 m/min, f = 0.25 mm/rev, depth of cut 1.5 mm.

• Passes = (60 − 54)/(2 × 1.5) = 2.
• N = 1000 × 90/(π × 60) = 90000/188.50 = 477.5 rpm (set 480 rpm).
• Time per pass = 300/(0.25 × 477.5) = 300/119.4 = 2.51 min; two passes = 5.03 min.
• MRR = 1000 × 90 × 0.25 × 1.5 = 33,750 mm³/min (33.75 cm³/min).
• At a specific energy of 2 J/mm³, power = 2 × 33,750/60/1000 = 1.13 kW.

Example 2 - drilling. Drill a 20 mm hole, 50 mm deep, in cast iron at V = 30 m/min, f = 0.3 mm/rev.

• N = 1000 × 30/(π × 20) = 30000/62.83 = 477.5 rpm.
• Travel = 50 + 0.3 × 20 = 56 mm; T = 56/(0.3 × 477.5) = 56/143.25 = 0.391 min ≈ 23.5 s.
• MRR = (π/4) × 20² × 0.3 × 477.5 = 314.16 × 143.25 = 44,993 mm³/min ≈ 45 cm³/min.

Example 3 - slab milling. A 100 mm diameter, 8-tooth slab cutter mills a 200 mm long, 60 mm wide block at depth 4 mm, V = 60 m/min, feed/tooth 0.1 mm.

• N = 1000 × 60/(π × 100) = 60000/314.16 = 191 rpm.
• Table feed Fm = 0.1 × 8 × 191 = 152.8 mm/min.
• Approach A = √(4 × (100 − 4)) = √384 = 19.6 mm; take overtravel O = 5 mm.
• T = (200 + 19.6 + 5)/152.8 = 224.6/152.8 = 1.47 min.
• MRR = 60 × 4 × 152.8 = 36,672 mm³/min.

Example 4 - taper by tailstock set-over. A 400 mm long bar held between centres needs a taper from 40 mm to 32 mm over 250 mm of its length.

• S = 400 × (40 − 32)/(2 × 250) = 400 × 8/500 = 6.4 mm set-over.
• Half-taper angle = tan⁻¹((40 − 32)/(2 × 250)) = tan⁻¹(0.016) = 0.92°; if the compound rest were used instead it would be swivelled to 0.92°, but the 250 mm length exceeds its travel - hence the set-over or an attachment.

Example 5 - indexing. Mill 24 teeth on a blank with a 40:1 dividing head: crank turns = 40/24 = 1⅔ = 1 turn + 2/3, i.e. 1 turn plus 12 holes on an 18-hole circle (or 16 holes on a 24-hole circle).

Where it is used

• Automotive and auto components: crankshafts, camshafts, connecting rods, gearbox shafts, brake drums and hubs on CNC turning and machining centres at Maruti Suzuki, Tata Motors, Bosch, Sundaram-Clayton, Rane and Bharat Forge.
• Heavy and power engineering: BHEL and L&T machine turbine casings, valve bodies and large flanges on vertical boring mills and planer-type millers; railway workshops (ICF, RCF, Chittaranjan) machine axles, wheels and brake gear.
• Aerospace and defence: HAL, BrahMos, Ordnance Factories - fir-tree slot broaching on turbine discs, deep-hole drilling of gun barrels, precision boring of hydraulic actuators.
• Pumps, valves and general engineering clusters: Coimbatore, Rajkot, Ludhiana, Batala and Kolhapur SMEs running capstan lathes, radial drills and milling machines for high-volume turned parts.
• Tool rooms and job roles: machinist/turner/machine operator (ITI), production engineer, process planner (setting speeds, feeds and cycle times), tool and die maker, maintenance engineer for machine tool geometry and alignment; training at IGTR Aurangabad/Indore, CITD Hyderabad, NTTF and Government Tool Rooms.
• Machine tool manufacture itself: HMT, Ace Micromatic, Jyoti CNC, LMW, Bharat Fritz Werner - design of beds, guideways, spindles and drives.

Interview questions

• Q: Why does a lathe have both a feed rod and a lead screw? A: The lead screw is used only for thread cutting, where the tool must advance exactly one pitch per spindle revolution; routine feeds use the feed rod so the lead screw's accuracy is not worn away.
• Q: When would you choose a 4-jaw chuck over a 3-jaw? A: For irregular, square or rectangular work, for castings, and whenever a deliberate eccentricity or very accurate centring by dial indicator is needed, since each jaw moves independently.
• Q: Which taper turning method would you use for a long, small-angle taper and why? A: A taper turning attachment (or tailstock set-over), because the compound rest has too little travel; the attachment also allows power feed, giving a better and more consistent surface.
• Q: Why can reaming not correct a misplaced hole? A: A reamer is guided by the existing hole and takes a very small, uniform stock, so it follows the original axis; correcting location needs single-point boring.
• Q: Difference between up milling and down milling, and which is preferred on CNC? A: In up milling feed opposes cutter rotation and chip thickness starts at zero; in down milling they act together with maximum chip thickness at entry. Down milling is preferred on CNC because preloaded ball screws remove backlash, and it gives better finish and tool life.
• Q: Why is the cutting speed of a twist drill zero at its axis? A: Cutting speed is πDN/1000 and the diameter is zero at the axis, so the chisel edge extrudes rather than cuts, which is why thrust rises steeply and web thinning helps.
• Q: What is the difference between a shaper and a planer? A: In a shaper the tool reciprocates over a stationary (intermittently fed) small workpiece; in a planer the large workpiece reciprocates on a table while the tool feeds across.
• Q: Why are machine tool beds made of grey cast iron? A: It is cheap to cast in complex shapes, has high compressive strength, and its graphite flakes damp vibration - which suppresses chatter and improves surface finish.
• Q: What is rise per tooth in broaching and what limits it? A: The radial step between successive teeth (typically 0.025-0.1 mm); it is limited by the allowable pull force, chip space in the gullet and the required surface finish.
• Q: How many crank turns for 36 teeth on a 40:1 dividing head? A: 40/36 = 1 turn plus 4/36 = 1/9, i.e. 1 turn plus 2 holes on an 18-hole circle.
• Q: Why are machine tool speeds arranged in geometric progression? A: So the percentage drop in cutting speed when you step down to the next available spindle speed is the same throughout the range, giving uniform loss of productivity for any work diameter.
• Q: A part must be turned, milled and drilled in a batch of 50,000. What would you choose? A: A CNC turning centre plus a machining centre with fixtures, or dedicated automats/special-purpose machines, because at that volume the setup and tooling cost is spread thinly and cycle time dominates cost per piece.

Common mistakes

• Using the diameter in metres (or the speed in m/s) in V = πDN/1000, and getting the spindle speed out by a factor of 1,000.
• Forgetting the drill point allowance (0.3 d) and the approach/overtravel in milling, and under-quoting machining time.
• Taking the number of passes from the diameter difference instead of the radial difference - (D₁ − D₂)/(2d), not /d.
• Using feed per tooth as the table feed in milling, or ignoring the number of teeth z.
• Confusing counterboring with countersinking, or claiming reaming corrects hole position.
• Setting the compound rest to the full taper angle instead of the half-angle, and setting it to the thread's full 60° instead of 29-30° for thread cutting.
• Choosing down milling on an old worn machine with backlash, which lets the cutter snatch the work and break the teeth.

Related topics

Theory of metal cutting · CNC machining and CAM · Grinding and finishing processes · Jigs and fixtures · Non-traditional machining · Metrology and quality control