Cutting, drilling and shaping
This domain examines powered and hand cutting, drilling machines and shaping operations. It tests drill selection, speeds, workholding, coolants and the sequence needed for an accurate hole.
Core rules
- For a drilled hole, mark the centre, centre-punch it, then drill a pilot hole if the final drill is large. Clamp the work or use a machine vice; never hold work by hand under a drill press.
- Select the drill speed from N = 1000S / (pi x D), where S is cutting speed in m/min and D is drill diameter in mm. Use coolant for steel; use paraffin for aluminium where appropriate and follow workshop rules.
- Keep the drill sharp and the correct point angle, usually 118 degrees for general HSS drills. Use a suitable feed and clear long chips by pecking.
- Removing the chuck key immediately after tightening is a safety rule, not a convenience.
- For shaping, choose the tool for the task: hacksaw or bandsaw for stock, shears or nibbler for sheet, file or grinder for edges, countersink for screw heads and reamer for a precise hole.
Worked example
A 10 mm HSS drill is used on mild steel at a cutting speed of 25 m/min. N = 1000 x 25 / (3.142 x 10) = 25 000 / 31.42 = 795.8 rev/min, so set the drill press to about 800 rev/min. Reason: the formula converts surface speed, which is a property of the material and drill, into spindle speed, which is what the machine controls. A larger drill would need lower spindle speed at the same cutting speed.
Traps that cost marks
- Drilling without a centre punch, so the drill wanders.
- Holding work by hand or failing to clamp it.
- Using too high a speed for a large drill.
- Leaving the chuck key in the chuck.
- Using a wood-cutting blade on steel or a blunt drill that overheats.