A Snapped M8 Tap in an Aluminium Blind Hole
An apprentice must produce 30 aluminium 6061 brackets, each with a blind M8 × 1.25 hole in a 12 mm thick plate; the drawing calls for 12 mm of full thread. He drills every hole with a 6.0 mm drill because it is the nearest one in the box, taps dry with a taper tap only, and runs the tap in until it will not turn. At the fifth hole the tap squeals and snaps off flush with the surface. The store has 6.8 mm, 7.0 mm and 7.5 mm drills, a full set of taper, second and plug taps, tapping paste and kerosene, and a tap extractor set. The job is behind schedule and the supervisor suggests drilling the hole out to 8.5 mm and 'using a bigger bolt'.
1. Calculate the correct tapping drill size for M8 × 1.25 and explain what the 6.0 mm drill did to the tap.
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Tapping drill = major diameter − pitch = 8.00 − 1.25 = 6.75 mm, so the correct choice from the store is 6.8 mm (the standard preferred size).
With a 6.0 mm hole the tap has to remove far more material than it was designed for. Approximate thread engagement is 100 × (major − drill) ÷ (1.0825 × pitch):
- 6.8 mm drill → about 89 % thread, a normal strong thread;
- 6.0 mm drill → well over 100 %, meaning the tap must cut full-depth flanks and rub on the minor diameter it is trying to generate.
That has three consequences:
- Torque rises sharply, and it rises faster than thread strength, so the tap shank - the weakest section - fails first.
- Aluminium 6061 galls; dry tapping smears metal onto the flanks, further raising torque, which is exactly the squeal heard before the break.
- Chips cannot clear from a 6.0 mm hole, so they pack under the flutes and wedge the tap.
The 8.5 mm suggestion is wrong: 8.5 mm is the tapping drill for M10 × 1.5. It would erase the M8 thread completely and leave a plain hole, scrapping the bracket.
2. Describe the correct tap sequence and depth for this blind hole, with the reasoning behind each depth.
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Sequence: drill 6.8 mm to a depth of at least 16–18 mm, countersink or chamfer the mouth, then taper → second → plug tap, lubricated with kerosene or tapping paste suitable for aluminium.
Why the depths:
- Full thread must be 12 mm, but a plug tap has a chamfered lead of about 3–4 threads (approximately 4–5 mm) that cuts incomplete threads. So the thread must continue past 12 mm by the lead length.
- Chips need clearance below the thread, otherwise the plug tap bottoms on packed swarf and snaps. Total drilled depth ≈ 12 mm full thread + 5 mm lead + 3 mm chip clearance ≈ 18–20 mm - but the plate is only 12 mm thick, so the hole is through-drilled or the design must be questioned. This is the key judgement in the case: 12 mm of full thread in a 12 mm plate with a blind hole is not achievable; the drawing should be referred back, or the hole made through and the thread limited to 10 mm with a plug tap.
- Each tap goes in only as far as its own lead allows; two turns forward, quarter turn back, to break the chip.
3. Explain how the broken tap should be removed and list the preventive controls so the remaining 25 brackets are not scrapped.
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Removal, in order of risk:
- Apply penetrating oil, use the tap extractor fingers, and rock gently in both directions - never force it further in.
- If the tap is proud, grip the stub with a small vice-grip and work it out.
- If it is flush and jammed, take the part to spark erosion (EDM) or a specialist tap disintegrator. Do not hand-drill a broken HSS tap with a jobber drill: the drill skids into the aluminium and destroys the hole.
- Chemical removal (acid that attacks steel but not aluminium) is a last resort, done only under supervision with PPE and waste disposal.
Prevention for the remaining 25:
- Use the correct 6.8 mm drill, sharp, with the point ground symmetrically.
- Lubricate with kerosene or tapping paste on every hole.
- Use the taper-second-plug sequence, two turns in and a quarter turn back, clearing chips with compressed air and a brush.
- Fit a tapping guide or use a machine tap in a tapping head for alignment.
- Add a go/no-go thread plug gauge check at the first-off, and record the drill size on the route card so the mistake cannot repeat silently.