Fitter Trade (ALP) — Overview

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:

1. Taper tap (7–10 threads tapered): starts the thread easily.
2. Plug tap (3–5 threads tapered): follows the taper tap.
3. 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.

CBT-2 Part B is a 75-question, 60-minute qualifying paper (35% to pass) based on your ITI trade. For the Fitter trade it tests bench work, measuring instruments, tools, fits, and threads — practical shop-floor knowledge.

Measuring instruments (least counts)

Instrument	Least count
Steel rule	0.5 mm
Vernier caliper	0.02 mm
Micrometer (outside)	0.01 mm

Vernier LC = smallest main-scale division ÷ number of vernier divisions.

Files, chisels, drills

• File grades (coarse→fine): rough, bastard, second cut, smooth, dead smooth.
• File shapes: flat, half-round, round, triangular, square, knife-edge.
• Cold chisel cutting angle for mild steel ≈ 60°.
• Twist drill point angle = 118° (general purpose).

Fits & tolerances

• Clearance fit — always a gap (shaft smaller than hole).
• Interference fit — shaft larger than hole (force/press needed).
• Transition fit — may be clearance or slight interference.
• Tolerance = maximum limit − minimum limit; Allowance = intended difference between mating parts.

Exam Tricks & Tips

• 🎯 Vernier least count is 0.02 mm and micrometer 0.01 mm — a very common single-mark question.
• 🎯 Drill point angle 118° and chisel angle ~60° are near-certain recall items.
• 🎯 Remember file order rough → dead smooth; "dead smooth" gives the finest finish.
• 🎯 Clearance = loose, Interference = tight (press fit), Transition = in between.
• 🎯 Thread terms: pitch = distance between adjacent threads; TPI = threads per inch = 1/pitch(in inches).
• ❌ Common mistake: confusing tolerance (a size range) with allowance (the deliberate gap between two parts).

Expected exam pattern

Direct recall on least counts, tool angles, file/chisel types, fit classification, and thread/tap-drill basics — all single-answer objective questions.

Quick recap

Vernier 0.02 mm, micrometer 0.01 mm, drill 118°, chisel ~60°. Know file grades, the three fit types, and pitch vs TPI. Part B only needs 35% — secure these fundamentals.

Cover the answer, recall, then check. 12 cards on Fitter trade for CBT-2 Part B.

Q1. Least count of a standard vernier caliper?
A1. 0.02 mm.

Q2. Least count of an outside micrometer?
A2. 0.01 mm.

Q3. Point angle of a general-purpose twist drill?
A3. 118°.

Q4. Cutting angle of a cold chisel for mild steel?
A4. About 60°.

Q5. Name the three main types of fit.
A5. Clearance, transition, interference.

Q6. Which fit needs a press or force to assemble?
A6. Interference fit (shaft larger than hole).

Q7. Order of file grades, coarse to fine?
A7. Rough, bastard, second cut, smooth, dead smooth.

Q8. What is the pitch of a screw thread?
A8. The axial distance between two adjacent thread crests.

Q9. How is vernier least count calculated?
A9. Smallest main-scale division ÷ number of vernier-scale divisions.

Q10. Difference between tolerance and allowance?
A10. Tolerance = permissible size range of one part; allowance = intended clearance/interference between mating parts.

Q11. Name three file cross-sections.
A11. Flat, half-round, round (also triangular, square).

Q12. TPI stands for and relates to pitch how?
A12. Threads Per Inch; TPI = 1 / pitch (pitch in inches).

Worked Example

Problem: Worked example (Fitter): A metric micrometer has a spindle pitch of 0.5 mm, and its thimble is divided into 50 equal divisions. Find the least count of the micrometer. If the sleeve reads 7.5 mm and the thimble shows the 24th division coinciding, what is the measured value?

Solution:
Step 1 — Least count. Least count = pitch ÷ number of thimble divisions = 0.5 ÷ 50 = 0.01 mm. So each thimble division represents 0.01 mm.
Step 2 — Reading. Main (sleeve) reading = 7.5 mm.
Thimble reading = 24 × least count = 24 × 0.01 = 0.24 mm.
Total measurement = 7.5 + 0.24 = 7.74 mm.

Answer: Least count = 0.01 mm; measured value = 7.74 mm.