Machining and Cutting Tools Questions and Answers

Answer: D

Chips break because of the work hardening of the work material.

As the chip forms, the metal in it is severely deformed in the shear zone and then curls against the tool face. This plastic deformation strain-hardens it, making it harder and less ductile, so the curling chip cracks and breaks into short pieces. Chip breakers on the tool increase the curl to make use of this effect.

Toughness and ductility work the other way: they let the metal deform without fracture, producing long continuous chips. Elasticity is recoverable deformation and plays no part in breaking chips.

Answer: B

The failure is called chip packing.

When drilling deep holes, or drilling soft and gummy materials, the chips cannot escape up the flutes fast enough and become jammed in them. The packed chips increase friction and heat, raise the torque and can break the drill. The cure is to withdraw the drill frequently (peck drilling) to clear the chips, and to use a suitable coolant.

A built-up edge is work material welded onto the cutting edge. Chipping is the breaking away of small pieces of the cutting edge itself. Neither is a failure of chips to pass through the flutes.

Answer: C

Tool chatter is caused by an excessive clearance angle.

The clearance angle stops the flank of the tool rubbing on the work. Making it larger than needed removes metal from under the cutting edge, so the edge is poorly supported, flexes under the cutting force and vibrates, producing chatter marks on the surface and rapid edge wear.

Too small a clearance angle causes rubbing and heat rather than chatter. The rake angle governs the cutting force and chip flow; it is not the cause of chatter that workshop texts give for a single-point tool. Long tool overhang and loose slides are other common causes.

Answer: B

External threads are cut with a Die.

A die is a hardened, threaded ring with cutting edges and chip holes, held in a die stock and screwed onto a rod or bolt blank to cut an external (male) thread by hand.

A tap is its counterpart for internal threads in a drilled hole. A drill bit makes the hole before tapping, and a reamer finishes a hole to an accurate size; neither cuts a thread.

Answer: B

The return stroke is the Idle stroke.

On a shaper the tool cuts only on the forward stroke. On the return stroke it is lifted clear by the clapper box and does no cutting, so the stroke is idle, wasted time that the quick return mechanism shortens.

The cutting stroke is the forward stroke, the opposite of the return. 'Static' and 'dynamic' stroke are not terms used for shaper strokes.

Answer: D

The answer is None of the above, since every statement is correct.

Broaching uses a long toothed tool, the broach, whose successive teeth are each slightly higher than the last, so that one pass removes the full depth and finishes the surface. Broaching may be linear, where the broach is pushed or pulled through or over the work, or rotary, used for internal profiles such as hexagon sockets.

Because none of the first three statements is wrong, none is 'not correct', which leaves 'None of the above'. ('roraty' is the paper's spelling of rotary.)

Answer: B

Discontinuous chips form at low cutting speed and small rake angle.

A discontinuous chip forms when the metal fractures in the shear zone instead of flowing. That happens with brittle materials such as cast iron, and in ductile ones when the cut is unfavourable: at low speed the material has less chance to flow, and a small (or negative) rake angle gives a severe, compressive cut that breaks the chip into segments.

High cutting speed with a large rake angle is the recipe for a continuous chip. Mixed combinations, low speed with a large rake or high speed with a small rake, push in opposite directions and are not the characteristic conditions.

Answer: A

The rake angle for tapping cast iron is about 3 degrees.

Cast iron is brittle and breaks into short, powdery chips, so it needs little rake: a small rake angle, around 0 to 3 degrees, keeps the cutting edges strong and stops them digging in and chipping.

Larger rake angles are for ductile materials that form long chips: about 8 to 12 degrees for steels, and up to about 20 degrees for soft metals such as aluminium. Using them on cast iron would weaken the cutting edges for no gain.

Answer: D

Negative rake is generally given to tools of tungsten carbide.

Carbide is very hard but brittle, and weak in tension and bending. A negative rake angle puts the cutting edge in compression and backs it with more material, so it can withstand the heavy, interrupted cuts and high speeds carbide is used for without chipping.

High speed steel, high carbon steel and other tool steels are tough enough to be given a positive rake, which cuts more freely and needs less power; a negative rake would only make them work harder.

Answer: B

Both tools have the same life at a cutting speed of 40 m/min.

Taylor's equation is \(vT^n = C\). At the same speed \(v\) and the same life \(T\):

\[ \begin{aligned} vT^{0.3} &= 60, \quad vT^{0.6} = 90 \\ T^{0.3} &= \frac{90}{60} = 1.5 \\ v &= \frac{60}{T^{0.3}} = \frac{60}{1.5} = 40 \end{aligned} \]

Check with tool Y: \(T^{0.6} = (T^{0.3})^2 = 2.25\), and \(40 \times 2.25 = 90\). At any other speed one tool outlasts the other, so 45, 50 and 55 m/min do not give equal lives.