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Description
Technical data for the UNEF No 12-32 hand tap
| Technical data | |
|---|---|
| Nominal diameter d1 | UNEF No 12-32 |
| Thread diameter | 5.49 mm / 0.2160 in |
| Thread pitch P | 32 threads per inch / 0.79 mm |
| Thread angle | 60° |
| Thread standard | UNEF ANSI B-1.1 |
| Tap tolerance | 2B |
| Tap drill hole | 4.7 mm |
Roughing tap and finishing tap – how much material each removes
Division of work in the 1–2 set
The UNEF No 12-32 thread is cut into the wall of a 4.7 mm diameter hole. The thread groove extends 0.395 mm into the wall. A single tap would have to cut the entire groove at once – by hand, this means high resistance and a risk of breakage. The set divides this work into two passes.
The drawing shows the entire set in operation. Both taps have the same shank and the same length. They differ in their chamfer, the tapered tip: No. 1 has a chamfer spanning 8 threads, while No. 2 spans only 2 threads. The detail windows at the hole wall show how the thread groove deepens after each tap.
| Tap | Chamfer | Chamfer angle | Material removed | How it works in the hole |
|---|---|---|---|---|
| No. 1 – roughing tap | long, 8 threads | 5° | the main portion of the groove | enters smoothly and establishes thread alignment |
| No. 2 – finishing tap | short, 2 threads | 20° | the remainder, to the full profile | screws into the cut threads and produces the full 2B profile |
How to distinguish the taps in the set
The chamfer on No. 1 is 4 times as long as the chamfer on No. 2 — the difference is immediately visible when you place them side by side. No. 1 has an incomplete thread profile; only No. 2 has the full profile. The number is also indicated on the shank: No. 1 has one ring, No. 2 has none. Keep the set together and always start with the roughing tap.
How to tap a UNEF No 12-32 hole – sequence 1, 2
Hand tapping step by step
Thread UNEF No 12-32 in four steps
ØDrilling
Drill bit 4.7 mm
1Roughing tap
Chamfer 8 threads
2Finishing tap
Chamfer 2 threads
✓Finished thread
Tolerance 2B
Turning sequence: half a turn clockwise, a quarter turn counterclockwise | Between taps: remove chips from the hole
Start with the hole. Inch thread pitch is specified in threads per inch, so first convert it to millimeters: an inch is 25.4 mm, and 25.4 ÷ 32 threads ≈ 0.79 mm. The UNEF No 12-32 thread diameter is 0.216 inches, or 5.49 mm. A diameter of 5.49 mm minus a pitch of 0.79 mm gives 4.7 mm. An undersized hole is a common cause of a tap twisting off in the material.
Through hole or blind hole – how deep to drill
The set taps both types of holes. In a through hole, the thread extends through the full material thickness. In a blind hole, cut the thread to 13.7 mm, or 2.5 times the thread diameter (2.5 x 5.49 mm, shown as 2.5D in the drawing). Drill the hole deeper than the thread. The finishing tap has a chamfer spanning 2 threads, so approx. 2 incomplete threads remain near the bottom. The extra depth below the thread accommodates them along with the chips.
No. 1 – the roughing tap establishes thread alignment
Fit the roughing tap in the tap wrench and position it perpendicular to the surface. Check the angle from two sides, for example with a square. Turn half a turn clockwise, then a quarter turn counterclockwise. Reversing breaks the chip into short pieces – a slight crunch is normal. The first threads determine whether the thread will be straight.
No. 2 – the finishing tap produces the full profile
Back out the roughing tap and remove chips from the hole. Screw the finishing tap into the cut threads by hand – it should engage them naturally. Continue turning at the same rhythm as with the roughing tap. The finishing tap removes the remaining material and cuts the groove to its full profile within tolerance class 2B. Once it screws in smoothly all the way, the thread is ready – check it with a UNEF No 12-32 screw.
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