Fig. 8 shows a hacksaw being used to cut a piece of mild steel bar held in a vice. The saw is commonly used in school workshops for cutting materials to siz...

(a) State the number of teeth per inch (TPI) on a hacksaw blade suitable for cutting this mild steel bar [1]

A blade with 18 TPI or 24 TPI is suitable [1] (the accepted range for general metalwork is 14-32 TPI). A finer blade (higher TPI) gives a smoother cut and keeps more teeth in contact with the material, preventing the blade from snagging. For a solid mild steel bar of moderate section, 18 or 24 TPI provides a good balance between cutting speed and finish quality.

(b) Give the reason why the blade teeth should point away from the handle [1]

The hacksaw is designed to cut on the forward (push) stroke, so the teeth must face forward, pointing away from the handle [1]. This means the blade is in tension during the cutting stroke, which keeps it straight and prevents it from buckling. Pulling a tensioned blade back through the cut on the return stroke allows the teeth to clear the swarf.

(c) State one reason why the blade should be correctly tensioned [1]

Any one from the following for [1]:

• To prevent the blade from wandering, producing a crooked or inaccurate cut - a loose blade flexes sideways in the kerf.
• To prevent the blade from breaking - an under-tensioned blade can twist and snap; an over-tensioned blade can also fracture.
• To ensure efficient cutting - correct tension keeps the full length of the blade's teeth in contact with the workpiece on each stroke.