In any machine shop or railway workshop, the ability to name the right tool, read a measurement, and describe the correct operation is not just bookish — it is the difference between a part that fits and one that destroys the machine. For RRB ALP technical aspirants in the fitter trade, this lesson organises every tool, instrument and concept the exam tests.

Definition: The fitter trade covers the skilled bench work of mechanical fitting — marking, measuring, cutting, filing, drilling, threading and assembling metal components to precise dimensions using hand tools and light machines.

Hand tools — identification and use

Hammers

Ball-peen hammer: the most common workshop hammer; the flat face drives punches and chisels, the hemispherical peen peens (spreads) rivets.
Sledge hammer: heavy two-handed hammer for breaking and rough work.
Claw hammer: for driving and pulling nails (woodwork context; less common in fitter trade but tested for recognition).

Files

Files are classed in two ways examiners love:

By shape: flat, hand (tapered in thickness), pillar, half-round, round (rat-tail), triangular (three-square), square, knife.

By cut (tooth pattern):

Single cut: one set of parallel ridges; used for hard materials and finishing.
Double cut: two crossing sets; cuts faster, used for roughing.
Rasp cut: individual teeth punched out; for soft materials (wood, aluminium, rubber).
Curved cut: for sheet metal.

By grade (coarseness): rough → bastard → second cut → smooth → dead smooth.

Cutting and chipping tools

Hacksaw: hand frame with a replaceable blade; forward stroke cuts.
Flat chisel: cuts flat surfaces and shears bar stock.
Cross-cut (cape) chisel: cuts keyways and narrow grooves.
Half-round chisel: cuts oil grooves in bearings.
Diamond-point chisel: cuts V-grooves.

Punches

Centre punch: 90° point; makes a conical indent for drill positioning.
Prick punch: 60° point; finer mark for scribing layout lines — lighter than centre punch.
Drift punch (pin punch): cylindrical tip; drives out pins and rivets.

Spanners and wrenches

Open-end spanner (fixed jaw), ring spanner (gives full contact; won't slip), box spanner (tube type; deep access), adjustable (crescent) spanner (adjustable jaw — not preferred for precision; may round bolt heads), torque wrench (applies measured torque).

Pliers

Flat-nose, round-nose, combination (slip-joint), side-cutting (diagonal), circlip pliers (internal/external).

Measuring instruments and least counts

The least count (LC) — the smallest increment an instrument can reliably read — is the single most-tested measurement concept:

Instrument	Least Count	Range/Use
Steel rule	0.5 mm or 1 mm	General length
Vernier caliper	0.02 mm (metric)	OD, ID, depth, step height
Micrometer (screw gauge)	0.01 mm	Precise OD measurement
Dial indicator/gauge	0.01 mm	Flatness, run-out, roundness
Combination square	1° or ½°	Angles, squareness, depth
Bevel protractor	5' (5 arc-minutes)	Precise angles
Feeler gauge	leaf thickness marked	Clearances, gaps

Vernier caliper reading: main scale reading + (vernier scale coinciding division × LC). For a 50-division vernier: LC = 1 mm / 50 = 0.02 mm.

Micrometer reading: barrel reading (0.5 mm per revolution on a 0.5 mm pitch screw) + thimble reading (÷ 50). The micrometer's precision is because the thimble thread has a pitch of 0.5 mm and 50 divisions on the thimble scale give LC = 0.5/50 = 0.01 mm.

Marking tools and the marking process

Scribers: hardened steel stylus used to scratch layout lines onto metal surfaces.
Surface plate: a precision-flat cast-iron or granite reference; all marking is done relative to this.
V-block: holds cylindrical workpieces horizontal for marking.
Angle plate: holds work at 90° to the surface plate.
Try square: checks squareness of edges.
Marking blue (Prussian blue or whitewash): coating applied to metal before scribing — makes scratched lines visible.
Height gauge: scribes horizontal lines at a set height above the surface plate.

Sequence of marking: clean the surface → apply blue → set up reference → scribe lines → punch at intersections.

Drilling operations

Twist drill has three cutting edges and is the standard for metal. Key angles:

Point angle: 118° for general steel; 90° for aluminium; 60° for very soft metals.
Helix/rake angle: controls chip clearance.
Clearance angle: relieves the heel behind the cutting edge.

Cutting speed (N, rpm): N = (1000 × V) / (π × D), where V = cutting speed in m/min and D = drill diameter in mm. Harder materials → lower cutting speed; larger drills → lower rpm.

Drill sequences for larger holes: centre drill → pilot drill → step up to final size (to reduce cutting force).

Coolants: soluble oil (water-miscible) for steel; neat cutting oil for difficult alloys; kerosene for aluminium and cast iron (to prevent sticking); dry drilling sometimes for cast iron (chips are dry and fine).

Drill wear signs: squealing sound, blue smoke, ragged hole, drill overheating.

Threading — tap and die

Definition: Tap — a hardened fluted cutting tool used to cut internal (female) threads inside a drilled hole.

Definition: Die — a hardened cutting tool in a die stock used to cut external (male) threads on a rod or bolt.

Tap set: three taps used in sequence:

Taper tap (7–10 threads tapered): starts the thread easily.
Plug tap (3–5 threads tapered): follows the taper tap.
Bottoming tap (1–2 threads tapered): cuts threads to the bottom of a blind hole.

Common thread forms:

Thread	Standard	Application
Metric (M)	ISO, thread angle 60°	General engineering in India/Europe
BSW (Whitworth)	55° thread angle	Older British equipment
UNC/UNF	60°, unified coarse/fine	American equipment
ACME	Trapezoid, 29°	Power screws (lathe lead screw)

Drill size before tapping: drill diameter ≈ nominal thread diameter − pitch. For M10 × 1.5 mm: drill = 10 − 1.5 = 8.5 mm.

Fits and tolerances

Tolerance = allowable variation from the nominal dimension. Allowance = intentional (designed) difference between mating parts.

Types of fit:

Clearance fit: shaft is always smaller than the hole; they can move relative to each other. Used for bearings and sliding parts.
Interference (force) fit: shaft is always larger than the hole; assembled by pressing or heating. Used where parts must not move relative to each other (press-fitted gears, bearing outer race).
Transition fit: may result in either small clearance or small interference depending on the actual sizes. Used for locating parts that may need to be disassembled.

Basis systems:

Hole-basis system: hole dimension is kept fixed (standard); shaft is varied. Most common in practice.
Shaft-basis system: shaft dimension is fixed; hole is varied. Used when shafts are bought from standard stock.

BIS/ISO designations: a tolerance is expressed as a letter + number (e.g., H7 for a hole, h6 for a shaft). Capital letters for holes, lower-case for shafts.

Workshop operations and safety

Core fitter operations: marking out → chipping → filing → sawing → drilling → boring (enlarging with a boring bar) → reaming (precision enlarging) → tapping (internal threads) → dieing (external threads) → riveting → scraping → lapping → deburring (removing sharp edges with a file or deburring tool).

Reaming produces a smoother, more accurate hole than drilling alone. A reamer removes only 0.1–0.3 mm.

Safety rules (examiners test these directly):

Always wear PPE: safety goggles (eye protection is non-negotiable near drilling/chipping), safety shoes (steel-toed), gloves when handling materials (not when operating drills/lathes — gloves can be caught).
Never leave tools on machines; never leave a chuck key in a drill chuck.
Use a brush, not hands, to remove metal swarf.
Secure workpieces in a vice; never hold work by hand while drilling.
Right tool for the right job — never use a file without a handle (tang can puncture the palm).

Why it matters: The RRB ALP technical section directly asks you to identify tools, recall least counts and match operations to their descriptions — recognition questions where a prepared fitter scores 8–10 marks in 5 minutes, and where confusion between, say, a tap and a die, or a vernier and a micrometer, costs easy marks.

Real-world example: A railway workshop technician fitting a wheel bearing into a housing must choose between a clearance, transition or interference fit. An interference fit is needed — the bearing must not rotate in its housing. It is pressed in using a hydraulic press. The technician also uses a micrometer to verify the bearing OD and the housing bore are within tolerance before assembly — if the interference is too tight, the housing cracks; too loose, the bearing spins.

Common misconception: Students frequently mix up tap and die. The memory hook: a Tap goes into a hole (like a tap into a pipe socket) — it makes internal threads. A Die wraps around a rod — it makes external threads. They are not interchangeable, and the exam will deliberately offer both as an answer option.

:::compare Vernier caliper vs Micrometer

Feature	Vernier Caliper	Micrometer
Least count	0.02 mm	0.01 mm
What it measures	OD, ID, depth, step	Mainly OD (external diameter)
Range	0–150 mm (common)	0–25, 25–50 mm per instrument
Principle	Sliding scale	Lead screw (0.5 mm pitch)
Speed	Faster (one slide)	Slower (many rotations)
Typical use	Checking dimensions	Precision verification
:::

:::keypoints Key points

Least count: steel rule 0.5/1 mm → vernier caliper 0.02 mm → micrometer 0.01 mm.
Tap = internal threads inside a hole; die = external threads on a rod.
Taper, Plug, Bottoming: the three-tap sequence to full thread depth.
Fits: clearance (moves), interference (force-fit/pressed), transition (in between).
Files classified by shape and by cut (single, double, rasp, curved).
Drill-before-tapping rule: drill diameter ≈ thread major diameter − pitch.
Always wear PPE; never hold work by hand during drilling; never use a file without a handle.
:::
:::memory
"TAPPING puts threads INSIDE; DIEING puts threads OUTSIDE" — say it aloud once and never confuse them again.
:::
:::recap
Identify every hand tool: hammer types, file shapes/cuts, chisel types, punch differences.
Least count is the single most-tested measurement concept — memorise the three benchmark values.
Threading: tap set has three members; know which cuts deepest.
Fits and tolerances determine whether parts move, lock or locate — know all three types.
Safety rules are direct exam questions, not common sense filler.
:::

Fitter Trade (ALP)

Fitter Trade (ALP) — Overview