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Description
Technical data for the American UNEF 3/8-32 hand tap
| Technical data | ||
|---|---|---|
| Nominal diameter d1 inches/mm | 3/8" | 9.52 mm |
| Thread pitch P inches/mm | 32 threads per inch | 0.79 mm |
| Overall length l1 | 63 mm | |
| Flute length l2 | 20 mm | |
| Shank diameter d2 | 7 mm | |
| Square drive size a | 5.5 mm | |
| Tap drill diameter | 8.75 mm | |
| Form C chamfer | (2÷3) P | |
| Thread angle G | 60° | |
| Thread standard | UNEF DIN 2184 | |
Roughing tap and finishing tap – how much material each removes
Division of work in the 1–2 set
The UNEF 3/8-32 thread is cut into the wall of a hole with a diameter of 8.75 mm. The thread groove extends 0.385 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 3/8-32 hole – sequence 1, 2
Hand tapping step by step
Thread UNEF 3/8-32 in four steps
ØDrilling
Drill bit 8.75 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 3/8-32 thread diameter is 3/8 inches, or 9.52 mm. A diameter of 9.52 mm minus a pitch of 0.79 mm gives 8.73 mm – in practice, use a 8.75 mm drill bit. 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 23.8 mm, or 2.5 times the thread diameter (2.5 x 9.52 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
Unscrew the roughing tap and remove the 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 the full profile within tolerance 2B. When it screws in smoothly all the way, the thread is ready – check it with a UNEF 3/8-32 screw.
UNEF 3/8-32 Fine-Thread Hand Tap Set
Hand Tap UNEF 3/8-32 is designed for precision cutting of internal threads in the system UNEF (Unified National Extra Fine thread series), used for short engagement lengths and thin-walled housings; the 60° profile meets the requirements of American thread fine-thread.
HSS construction with ground geometry for hand operation promotes even guidance and surface roughness control. A pilot hole of 11/32 inch (8.73 mm) is recommended for a class 2B fit, along with an entry chamfer of 1.5–2 threads. Lubrication and periodic reversal are recommended during cutting for effective chip breaking; typical materials include structural steels, cast iron, copper alloys, and aluminum. A two-stage sequence (roughing tap + finishing tap) can be used for greater process predictability.
- Clean 60° profile — reduced burrs and improved surface quality at the exit.
- Stable guidance — the ground edge and optimized rake angle reduce instantaneous cutting torque in blind and through holes.
- Interchangeability according to standards — GO/NO-GO inspection and the target class 2B facilitate integration with UN/UNEF standard fasteners.
- 2-piece tap set — distributing the load between the roughing tap and finishing tap improves axial alignment and edge life.
Hand Tap Set UNEF 3/8-32 provides a repeatable fine thread for precision connections in thin-walled structures and applications with limited engagement length.
Frequently asked questions
UNEF (Unified National Extra Fine) is a fine-thread variant of the system American thread with a 60° profile and an increased number of threads per inch. It is selected where engagement length is limited or in thin-walled components — especially in pipe connections and hydraulic and pneumatic fittings, inspection and measuring equipment, and precision fasteners — when high pull-out resistance and very fine adjustment are required.
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