Lathe: Parts and Operations
Overview
A lathe is the most fundamental machine tool used in an ITI Fitter workshop. It rotates a workpiece about its axis while a cutting tool removes material to produce cylindrical, conical, or flat surfaces. Understanding lathe parts, their functions, and operations is essential for NCVT Semester 2 examinations. This chapter covers the main parts of a centre lathe, their roles, and all standard operations performed on it, including turning, facing, drilling, boring, knurling, and thread cutting.
Typed Notes
Main Parts of a Centre Lathe
The centre lathe has these major parts: (1) Bed — cast iron base with hardened V-guideways. (2) Headstock — fixed left-side housing with spindle, chuck, and speed-change gears. (3) Tailstock — right-side sliding unit for support and holding drilling tools. (4) Carriage — moves longitudinally; includes saddle, cross-slide, compound rest, and tool post. (5) Apron — front face of carriage containing half-nut, feed rod, lead screw engagement levers. (6) Lead screw — long threaded rod used for thread-cutting operations. (7) Feed rod — used for automatic feed during plain turning and facing.
Lathe Specifications and Cutting Tools
A lathe is specified by: (1) Swing over bed — maximum diameter of workpiece that can rotate without touching the bed. (2) Distance between centres — maximum length of workpiece that can be held. (3) Bed length. (4) Spindle bore diameter. Common lathe cutting tools include: Single-point HSS or carbide turning tools, parting/grooving tools, boring bars, knurling tools, and threading tools. Tool angles — rake angle, clearance angle, and cutting angle — affect surface finish and tool life. HSS tools are used for general work; carbide-tipped tools for harder materials.
Standard Lathe Operations
Key operations performed on a lathe: (1) Plain/Straight Turning — reduces diameter along the length. (2) Facing — produces a flat surface at the end of the workpiece; tool moves perpendicular to the axis. (3) Taper Turning — produces conical surface; done by swiveling compound rest, offsetting tailstock, or using taper turning attachment. (4) Drilling/Boring — drill held in tailstock; boring bar enlarges existing holes. (5) Knurling — produces diamond or straight pattern on surface for grip using a knurling tool. (6) Thread Cutting — lead screw engaged via half-nut; pitch controlled by change gears. (7) Parting/Grooving — cuts off or makes grooves using a narrow parting tool.
Work-Holding and Tool-Holding Devices
Work-holding devices: 3-jaw self-centering chuck (most common, auto-centers round bars), 4-jaw independent chuck (each jaw adjusted separately — used for square, rectangular, or eccentric jobs), face plate (for odd-shaped heavy jobs), lathe centres (live centre in headstock, dead centre in tailstock — used with dogs for between-centre work), mandrel (for holding hollow workpieces), steady rest and follower rest (support long slender workpieces to prevent deflection). Tool-holding: tool post (single or four-way), boring bar holder, drill chuck in tailstock. Correct tool height (on centre line) is critical for accurate cutting.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Lathe Bed and Headstock
- Bed material and guideway types (V and flat)
- Headstock spindle and speed-change mechanism
- Chuck mounting on spindle nose
- Spindle bore and its significance
A lathe with swing over bed of 200 mm can machine a shaft of maximum 200 mm diameter — commonly seen in ITI workshop lathes like HMT NH-22.
02Tailstock and Carriage Assembly
- Tailstock offset for taper turning
- Carriage components: saddle, cross-slide, compound rest
- Apron: lead screw, feed rod, half-nut lever
- Tool post types: single, four-way turret
To turn a taper of included angle 10°, the compound rest is swiveled to 5° (half the included angle) — a direct numerical asked in NCVT papers.
03Work-Holding Devices
- 3-jaw self-centering chuck for round/hex bars
- 4-jaw independent chuck for eccentric/square jobs
- Lathe centres (live and dead) with carrier dog
- Steady rest and follower rest for long shafts
A 500 mm long, 20 mm diameter mild steel shaft needs a steady rest to prevent bending/vibration during turning — standard workshop practice.
04Lathe Operations
- Facing and plain turning (longitudinal feed)
- Taper turning methods (compound rest, tailstock offset, attachment)
- Thread cutting using lead screw and change gears
- Knurling, parting, drilling, and boring
To cut a metric thread of pitch 2 mm, the change gears are set so that for one revolution of spindle, the lead screw advances the carriage by exactly 2 mm.
05Lathe Specifications and Cutting Tools
- Swing over bed and distance between centres
- HSS vs carbide-tipped single-point tools
- Tool angles: rake, clearance, cutting edge
- Feed, depth of cut, and cutting speed (RPM)
Cutting speed for mild steel with HSS tool ≈ 30 m/min; if workpiece diameter is 60 mm, RPM = (1000 × 30)/(π × 60) ≈ 159 RPM — a formula-based NCVT question.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01The headstock is fixed on the left side of the lathe bed and houses the main spindle, driven by a motor through a gearbox — it holds and rotates the workpiece.
- 02The tailstock is mounted on the right side of the lathe bed and can slide along the bed; it supports long workpieces and holds tools like drills and reamers.
- 03Carriage (saddle + cross-slide + compound rest + tool post) moves along the bed and holds the cutting tool; the apron contains the feed mechanism gears and levers.
- 04Lathe bed is the backbone/base of the lathe made of cast iron; it has flat or V-shaped guideways (hardened and ground) on which the carriage and tailstock slide.
- 05Chuck types used on lathe: 3-jaw self-centering chuck (for round/hexagonal work), 4-jaw independent chuck (for irregular/eccentric work), magnetic chuck, and collet chuck.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
What is the function of the lead screw in a lathe, and how does it differ from the feed rod?
Name four methods of taper turning on a lathe and state which method is used for long, gradual tapers.
Differentiate between a 3-jaw chuck and a 4-jaw chuck with respect to their use in a lathe.
What is knurling, and what type of tool is used for this operation on a lathe?
How is a lathe machine specified? Give two main parameters with an example.
The centre lathe is the king of machine tools in a Fitter workshop. Its major parts — bed, headstock, tailstock, carriage, and apron — work together to perform turning, facing, taper turning, thread cutting, drilling, boring, knurling, and parting operations. Work is held in chucks, between centres, or on face plates. Lathe is specified by swing over bed and distance between centres. Mastering these facts ensures high scores in NCVT Semester 2 exams.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Which part of the lathe is used to engage the lead screw for thread cutting?
Q2.The compound rest of a lathe is swiveled to 30° to cut a taper. What is the included angle of the taper?
Q3.Which chuck is most suitable for holding a square cross-section workpiece on a lathe?
Q4.What is the material of the lathe bed?
Mark karo — dashboard par progress dikhega.