How are holes drilled in metal and how are internal and external screw threads cut by hand?
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.
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What this dot point is asking
Many metal jobs need holes and screw threads. The SQA expects you to describe drilling a hole with a twist drill (and countersinking), and cutting threads by hand: an internal thread with a tap, and an external thread with a die. You should know the order of operations, the safety points, and why cutting fluid and the forward-then-back action matter.
Drilling a hole
A twist drill held in a drill chuck cuts a round hole.
- Centre punch the marked centre first so the drill point locates and does not wander.
- Choose the correct drill size and clamp or hold the work securely (never hold thin work by hand).
- Let the drill do the cutting with steady pressure; use cutting fluid on steel to keep the drill cool.
- Countersinking uses a cone-shaped countersink bit after drilling to cut a recess so a countersunk screw head sits flush with the surface.
Cutting an internal thread: the tap
An internal thread lets a bolt screw into a hole.
- Drill the tapping-size hole.
- Fit the taper (first) tap into a tap wrench and start it square in the hole; check it is upright from two directions so the thread is not crooked.
- Turn forward about one turn, then back about half a turn to break the swarf, using cutting fluid.
- Follow with the second and plug (bottoming) tap to cut the thread to full depth, then test with the bolt.
Cutting an external thread: the die
An external thread turns a plain rod into a bolt or threaded stud.
Try this
Q1. Name the tool used to cut an external screw thread on a rod. [1 mark]
- Cue. A die (held in a die stock).
Q2. State why a hole is centre punched before drilling. [1 mark]
- Cue. So the drill point locates in the dent and does not wander off the marked centre.
Q3. Explain the difference between a tapping hole and a clearance hole. [2 marks]
- Cue. A tapping hole is slightly smaller than the bolt so a thread can be cut into it; a clearance hole is slightly larger so the bolt passes straight through.
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 marksA bracket needs a hole drilled and an internal screw thread cut so a bolt can be screwed in. Describe how you would cut the internal thread.Show worked answer →
Award up to 4 marks, 1 per correct stage. First drill the correct tapping-size hole through the bracket with a twist drill (a tapping hole is slightly smaller than the bolt size so there is metal left for the thread) (1). Fit the first tap (taper tap) into a tap wrench and start it square in the hole, checking it is upright so the thread is not crooked (1). Turn the tap about one full turn forward, then quarter to half a turn back to break off the swarf, using cutting fluid to keep it cutting cleanly (1). Follow with the second and plug taps if needed to finish the thread to depth, then test with the bolt (1). A good answer names the tapping hole, the tap and the tap wrench and explains the forward-then-back action.
SQA-style Explain3 marksExplain why a tap or die is turned forward then backward a little, and why cutting fluid is used when threading steel.Show worked answer →
Award up to 2 marks for the forward-and-back action and 1 for the cutting fluid, to a maximum of 3. Turning forward then back a little breaks off the small chip (swarf) being cut (1), so it does not pack into the cutting edges and clog or jam the tap, which could snap it inside the hole (1). Cutting fluid lubricates and cools the tap or die, reducing friction and heat so the cutting edges stay sharp, the thread is cleaner and the tool is less likely to break (1). Snapping a tap inside a hole is hard to remove, which is why this care is taken.
Related dot points
- Measuring and marking out: using a steel rule, engineer's try square, scriber, odd-leg (jenny) callipers, dividers and a centre punch from a datum edge to transfer a drawing onto metal accurately.
How SQA National 5 Practical Metalworking expects you to measure and mark out a metal workpiece: working from a datum edge with a steel rule, engineer's try square, scriber, odd-leg callipers, dividers and a centre punch so that holes and lines are accurate before cutting.
- 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.
- 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.
- Mechanical joining and forming: joining metal with rivets (including pop rivets) and threaded fasteners (nuts, bolts and machine screws), choosing temporary or permanent joints, and bending and forming bar and rod to shape.
How SQA National 5 Practical Metalworking covers joining metal without heat: riveting (including pop rivets) and threaded fasteners (nuts, bolts, machine screws), the difference between temporary and permanent joints, and bending and forming bar and rod to shape.