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Description
Technical data for the American UNEF 1/4-32 hand tap
| Technical data | ||
|---|---|---|
| Nominal diameter d1 inches/mm | 1/4" | 6.35 mm |
| Thread pitch P inches/mm | 32 threads per inch | 0.79 mm |
| Overall length l1 | 56 mm | |
| Flute length l2 | 14 mm | |
| Shank diameter d2 | 6 mm | |
| Square drive size a | 4.9 mm | |
| Tap drill diameter | 5.55 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 1/4-32 thread is cut into the wall of a hole with a diameter of 5.55 mm. The thread groove extends 0.4 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 1/4-32 hole – sequence 1, 2
Hand tapping step by step
Thread UNEF 1/4-32 in four steps
ØDrilling
Drill bit 5.55 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 1/4-32 thread diameter is 1/4 inches, or 6.35 mm. A diameter of 6.35 mm minus a pitch of 0.79 mm gives 5.56 mm – in practice, use a 5.55 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 15.9 mm, or 2.5 times the thread diameter (2.5 x 6.35 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 1/4-32 screw.
UNEF 1/4-32 Fine Thread Hand Tap Set
Hand Tap UNEF 1/4-32 is designed to cut precision internal threads in the system UNEF (Unified National Extra Fine thread series), used where engagement length is limited or walls are thin. The design is optimized for hand operation and provides stable guidance and a repeatable profile in blind and through holes, with an emphasis on consistent surface quality after tapping.
The cutting geometry and carefully finished edges promote a smooth process, reducing torque spikes and the risk of jamming in the final stage. Hand Tap Set UNEF 1/4-32 works with tap wrenches, making it easier to maintain axial alignment and control the surface roughness of the bore; suitable lubricants and periodic reversal are recommended for effective chip evacuation and cutting edge protection.
- Precise profile control — the UNEF fine-thread series makes it easier to obtain clean profile edges at the entry and exit.
- Process stability — the roughing tap + finishing tap sequence distributes the load and improves guidance in materials of varying ductility.
- Compatibility with inch-based components — American thread ensures interchangeability with fasteners used in instruments, fittings, and precision equipment.
- High repeatability — axial alignment control and finish quality reduce the need for additional corrections after tapping.
Thanks to its balanced geometry Hand Tap UNEF 1/4-32 performs well in thin-walled components and wherever a fine pitch is required for precise connection adjustment.
Frequently asked questions
UNEF (Unified National Extra Fine thread series) is a fine-thread variant American thread with a 60° profile and an increased number of threads per inch (e.g., 7/16-28). It is used where little engagement length is available or walls are thin — mainly in thin-walled pipe connections, hydraulic and pneumatic fittings, measuring equipment, and precision fasteners — to increase pull-out resistance and allow very fine connection adjustment.
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