How are holes drilled accurately on a machine, and how is an off-hand grinder used to shape and sharpen metal?
Machine drilling on a pillar or bench drill (work clamped, correct speed, chuck and twist drill) and using an off-hand (bench) grinder to shape, deburr and sharpen tools, with the correct guards and safe practice.
How SQA National 5 Practical Metalworking covers machine drilling on a pillar or bench drill (clamping the work, correct speed, twist drill in a chuck) and using an off-hand bench grinder to shape, deburr and sharpen, with the correct guards, tool rest and safe practice.
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What this dot point is asking
As well as the lathe, machine work uses the pillar (bench) drill to make accurate holes and the off-hand grinder to shape, deburr and sharpen. The SQA expects you to describe drilling on a machine safely (clamping, speed, chuck key) and to know how a grinder is used and guarded. The skill is applied in the practical activity and sampled by the case study.
Machine drilling on the pillar drill
The big variable is speed. A larger-diameter drill must run at a slower speed than a small one, because the edge at the rim of a big drill travels much faster for the same number of turns; too high a speed overheats and blunts it. Steel is also drilled with cutting fluid to cool the drill. The right speed gives a clean hole, long drill life and a controlled break-through; the wrong speed burns the drill tip blue and tears the metal.
When a deep hole is drilled, the drill is eased back out every so often to clear the swarf from the flutes, so chips do not pack in and jam the drill. A piece of scrap wood is placed under the workpiece on the table so the drill can break through cleanly without cutting into the table, and so the underside of the hole is not torn.
Drilling safely and accurately
- Centre punch the marked centre so the drill does not wander.
- Clamp the work so the drill cannot snatch and spin it as it breaks through.
- Remove the chuck key before switching on.
- Set the correct speed for the drill size and material, and feed steadily.
- Let the drill stop before clearing swarf with a brush, not fingers.
The off-hand (bench) grinder
The off-hand grinder spins an abrasive grinding wheel used to shape metal, remove burrs and sharpen cutting tools such as cold chisels and drills. It is called "off-hand" because the work is presented to the wheel by hand, resting on the tool rest, rather than being clamped, so steady, light pressure and good positioning matter.
A grinder usually has two wheels, often a coarser wheel for fast shaping and a finer wheel for finishing and sharpening. The wheel removes metal quickly, so the workpiece heats up: keep it moving across the wheel and dip it in water to cool, otherwise a hardened tool edge can lose its hardness (the colour bands tell you it is getting too hot).
Try this
Q1. State why the chuck key must be removed before a pillar drill is switched on. [1 mark]
- Cue. A key left in the chuck is thrown out as a dangerous flying object when the machine starts.
Q2. Name two jobs an off-hand grinder is used for. [2 marks]
- Cue. Any two of: shaping metal, removing burrs (deburring), sharpening cutting tools.
Q3. Explain why a larger drill is run at a slower speed. [2 marks]
- Cue. Its rim travels faster for the same number of turns, so a slower speed keeps the edge speed safe and stops it overheating and blunting.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA-style Describe4 marksDescribe how you would safely drill a 12 mm hole through a steel plate using a pillar drill.Show worked answer →
Award 1 mark for each correct, relevant step, to a maximum of 4. Centre punch the marked hole position so the drill locates and does not wander (1). Fit the 12 mm twist drill in the chuck, tighten it and remove the chuck key before starting (1). Clamp the plate to the drill table or hold it in a machine vice, never by hand, so the drill cannot snatch and spin it (1). Set the correct (slower) speed for the larger drill and steel, switch on, and feed the drill steadily using cutting fluid; let the drill stop before clearing the swarf with a brush (1). Wearing safety glasses and tying back hair are also creditable safe-practice points.
SQA-style Explain3 marksExplain why a larger drill is run at a slower speed than a small drill, and why the work must be clamped on a pillar drill.Show worked answer →
Award up to 3 marks. A larger drill has a bigger diameter, so its cutting edge travels much faster at the rim for the same number of turns; running it slower keeps the edge speed safe so it does not overheat and blunt or grab (1). A smaller drill needs a higher speed to cut effectively (1). The work must be clamped because as the drill breaks through it can snatch and spin the workpiece; if it were held by hand the spinning plate could injure the operator, so clamping keeps it secure (1).
Related dot points
- The centre lathe: its main parts (headstock, chuck, tailstock, carriage, cross-slide and tool post) and the operations it performs - facing, parallel turning, parting, chamfering and knurling.
How SQA National 5 Practical Metalworking covers the centre lathe: its main parts (headstock, chuck, tailstock, carriage, cross-slide, tool post) and the turning operations it carries out - facing, parallel turning, parting, chamfering and knurling - done safely and accurately.
- Drilling holes with a twist drill and countersinking, and cutting screw threads by hand: an internal thread with a tap and tap wrench, and an external thread with a die and die stock.
How SQA National 5 Practical Metalworking covers making holes and threads: drilling with a twist drill, countersinking, cutting an internal thread with a tap and tap wrench, and cutting an external thread with a die and die stock, all done accurately and safely.
- Bench tools and hand processes: holding work in a bench vice, cutting with a hacksaw, removing metal by filing (cross-filing and draw-filing), cutting with a cold chisel, and the correct hammers (ball pein, cross-pein and claw).
The core hand-tool skills in SQA National 5 Practical Metalworking bench work: holding work in a bench vice, cutting with a hacksaw, removing metal by cross-filing and draw-filing, using a cold chisel, and choosing the right hammer (ball pein, cross-pein, claw).
- Finishing processes: removing sharp edges and burrs (deburring), cleaning and preparing the surface (emery cloth/abrasives), and applying a finish such as paint, lacquer or plating to protect the metal and improve its appearance.
How SQA National 5 Practical Metalworking covers finishing a metal product: removing sharp edges and burrs (deburring), cleaning and preparing the surface with abrasives, and applying a finish such as paint, lacquer or plating to protect the metal and improve its appearance.
- Health and safety in the workshop: identifying hazards, using personal protective equipment (safety glasses, apron, gloves where appropriate), guarding machines, keeping a tidy area, and following safe working practices for hot, sharp and rotating processes.
How SQA National 5 Practical Metalworking expects you to work safely: spotting hazards, using personal protective equipment such as safety glasses and aprons, guarding machines, keeping a tidy workspace, and following safe practices for hot, sharp and rotating processes.