Cutting internal threads with taps and external threads with dies; tap drill size.
Taps – for internal threads
A tap is a hardened tool with threads and flutes, used to cut internal threads in a hole. Hand taps come in a set of three:
| Tap | Chamfer (taper) at the start | Use |
|---|---|---|
| Taper tap (first) | 8 – 10 threads chamfered | Starts the thread easily and straight |
| Intermediate / second tap | 3 – 5 threads | Cuts the threads deeper |
| Bottoming / plug tap | 1 – 2 threads | Cuts full threads to the bottom of a blind hole |
Taps are turned with a tap wrench (double-ended adjustable or T-type).
Tap drill size
Tap drill size = Major (nominal) diameter − Pitch
M10 × 1.5 → 10 − 1.5 = 8.5 mm drill
M8 × 1.25 → 8 − 1.25 = 6.75 ≈ 6.8 mm drill
M10 × 1.5 → 10 − 1.5 = 8.5 mm drill
M8 × 1.25 → 8 − 1.25 = 6.75 ≈ 6.8 mm drill
Tapping method
- Drill the correct tap-drill hole and chamfer its mouth.
- Start with the taper tap, square to the job (check with a try square).
- Turn ½ turn forward and ¼ turn back to break the chips.
- Use cutting oil; finish with second and bottoming taps.
Dies – for external threads
A die cuts external threads on bolts and rods. It is held in a die stock (die holder).
- Circular split die – has a slit; size can be adjusted slightly with screws in the die stock.
- Die nut – for cleaning damaged threads.
- Chamfer the rod end before starting; the tapered side of the die goes first.
Quick Revision
- Taps → internal threads; dies → external threads.
- Set of taps: taper, second, bottoming.
- Tap drill = major dia − pitch (M10 → 8.5 mm).
- Turn ½ forward, ¼ back to break chips.
Your progress is saved on this device.