Lathe and Turning Operations Questions and Answers

Answer: D

The preferred option is a four jaw chuck.

Each jaw of a four jaw chuck is moved independently by its own screw, so the jaws can be set to grip irregular or odd-shaped work and to position it off-centre, for example to turn an eccentric diameter.

A three jaw chuck is self-centring: all jaws move together, which suits round and hexagonal stock but cannot grip irregular shapes securely. Live and dead centres with a lathe dog hold work between centres, which needs centre holes on the axis of a reasonably regular part.

Answer: D

The answer is None of these: every method listed can turn a taper.

The compound rest is swivelled to half the included angle and fed by hand, which suits short, steep tapers. The tailstock offset method moves the tailstock centre sideways so that work between centres is turned at a slight angle, which suits long, gentle tapers. A form tool ground to the taper angle plunges straight in to produce a short taper or chamfer.

The fourth common method, the taper turning attachment, is simply not listed. Since none of the three listed methods is unusable, none of them can be the answer.

Answer: C

Lathe beds are made of cast iron because of its high damping capacity.

The graphite flakes in grey cast iron absorb vibration, so chatter from the cutting tool and the motor dies out in the bed instead of being passed to the work. Cast iron is also rigid, strong in compression, easy to cast into the complex shape of a bed, and wears well on the slideways.

Weight and cost are side effects, not the engineering reason. 'It does not resist compressive load' is false: cast iron is strong in compression, and that is a further reason it suits a bed.

Answer: D

Knurling is done on the lathe machine.

Knurling presses a straight or diamond pattern into a cylindrical surface, such as a tool handle or a thumb screw, to give a better grip. The work revolves in the lathe while a knurling tool, carrying hardened knurled wheels, is held in the tool post and forced against it with a slow speed and a coarse feed. The metal is displaced rather than cut.

A grinding machine finishes surfaces with an abrasive wheel, a milling machine cuts with a rotating multi-tooth cutter, and a drilling machine makes holes. None of them rotates the work against a fixed forming roller the way a lathe does.

Answer: C

The lead screw uses a trapezoidal thread.

A lead screw transmits motion and power to the carriage during thread cutting, so it needs a strong thread that engages a split (half) nut smoothly. The trapezoidal form, the metric equivalent of the Acme thread with a 30° included angle, is stronger than a square thread, easy to cut, and lets the half nut engage and disengage easily.

'Spur' describes gears, not threads. Every screw thread is a helix, so 'helical thread' names no particular form. A saw tooth (buttress) thread carries load in one direction only, but a lead screw has to drive the carriage both ways.

Answer: B

The spindle nose of a lathe headstock uses a Morse taper.

The bore of the headstock spindle, like the tailstock barrel, is ground to a standard Morse taper, a self-holding taper of roughly 1 in 20 on the diameter. Live centres, drill chucks and other tooling with a matching Morse shank are held by friction in it and are easily knocked out.

A taper pin uses a 1 in 50 taper for locating and fastening parts, not for spindle bores. The Brown and Sharpe taper is used mainly on milling and grinding machine spindles. 'Pitch taper' is not a standard taper.

Answer: A

The tailstock offset is 10 mm.

When only part of the job is tapered, the offset is worked out over the whole length between centres:

\[ \begin{aligned} \text{offset} &= \frac{D - d}{2} \times \frac{L}{l} \\ &= \frac{50 - 40}{2} \times \frac{200}{100} = 5 \times 2 = 10\ \text{mm} \end{aligned} \]

Here L is the total length of the job and l the length of the taper. 5 mm would be right only if the taper ran the full length of the job, and 2.5 mm and 20 mm come from halving or doubling it again by mistake.

Answer: C

Large, heavy and irregular work is held on a face plate.

A face plate is a large disc screwed onto the spindle nose, with radial slots through which the work is bolted or clamped directly. Because the work is fixed with bolts, plates and angle plates rather than gripped by jaws, almost any shape can be set up, including castings and odd shapes that will not fit between centres.

A three jaw chuck grips only round or hexagonal work, and a two jaw chuck only work of suitable shape. A driving plate does not hold work at all: it drives a lathe carrier on work mounted between centres, which is exactly the case the question excludes.

Answer: B

Lathe beds are usually made of cast iron.

Grey cast iron suits a machine bed: it absorbs vibration well, which reduces chatter; it is stable and keeps its shape; it can be cast into the complex box sections a bed needs; and the graphite in it gives the guideways good wearing and sliding properties. It is also cheap.

Mild steel and stainless steel damp vibration poorly and are expensive to form into a bed, and an aluminium alloy is too flexible and wears too quickly for the ways of a machine tool.

Answer: C

The accessory that supports a long, slender job is the steady rest.

A long thin job held between centres or in a chuck sags and springs away from the tool. A steady rest, clamped to the bed, carries three adjustable jaws that bear on the job part-way along its length and stop it deflecting, so it can be turned, drilled or bored at the end.

A dead centre supports the job only at its end, in the tailstock. A lathe dog and a lathe carrier are the same device: a clamp that transmits the drive from the driving plate to the job; neither gives any support.