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Description
Technical data for the UNF No 12-28 hand tap
| Technical data | ||
|---|---|---|
| Nominal Diameter d1 Inches/mm/Decimal | UNF No 12-28 | |
| Thread Pitch P in Inches and mm | 28 threads per inch | 0.91 mm |
| Thread Diameter d1 mm / Decimal | 5.49 mm / 0.2160 in | |
| Thread angle G | 60° | |
| Thread Standard | UNF ANSI B-1.1 | |
| Overall Length l1 | 62 mm | |
| Flute Length l2 | 17 mm | |
| Shank Diameter d2 | 5.6 mm | |
| Square Drive Width Across Flats a | 4.9 mm | |
| Tap Drill Hole | 4.6 mm | |
Roughing tap and finishing tap – how much material each removes
Division of work in the 1–2 set
The UNF No 12-28 thread is formed in the wall of a 4.6 mm diameter hole. The thread groove extends 0.445 mm into the wall. A single tap would have to cut the entire groove at once – by hand, this creates 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 UNF No 12-28 hole – sequence 1, 2
Hand tapping step by step
Thread UNF No 12-28 in four steps
ØDrilling
Drill bit 4.6 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. For inch threads, pitch is specified in threads per inch, so first convert it to millimeters: an inch is 25.4 mm, and 25.4 ÷ 28 threads ≈ 0.91 mm. The UNF No 12-28 thread diameter is 0.216 inches, or 5.49 mm. Subtracting the 0.91 mm pitch from the 5.49 mm diameter gives 4.58 mm – in practice, use a 4.6 mm drill bit. An undersized hole is a common cause of taps 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
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 on its own. Continue turning with the same rhythm used for the roughing tap. The finishing tap removes the remaining material and cuts the groove to its full profile within tolerance 2B. When it screws in smoothly all the way, the thread is complete – check it with a UNF No 12-28 screw.
UNF No 12-28 hand tap – set of 2 pcs for fine numbered thread sizes
UNF No 12-28 hand tap is a two-piece set of tools for producing internal threads to the Unified National Fine – UNF standard (unified fine thread). The set includes HSS roughing and finishing taps and complies with DIN 2181.
UNF No 12-28 tap features
- Class 2B tolerance – ensures leak tightness and a precise fit between components
- Diameter No 12 (5.49 mm) – largest numbered size before fractional inch sizes
- 28 threads per inch (0.91 mm pitch) – fine thread profile resists loosening
- Two-stage tapping – roughing and finishing taps ensure optimal distribution of work
- HSS high-speed steel – effective machining of materials up to 800 MPa
The UNF No 12-28 thread is used in aviation, motorsports, and precision mechanisms requiring vibration resistance. UNF No 12-28 fine-thread tap is chosen by workshops specializing in servicing aerospace equipment and race vehicles.
Frequently asked questions
Calculate the hole size using the formula: d = D - P. For UNF No 12-28: d = 5.49 mm - 0.91 mm = 4.58 mm. Drill the hole with a 4.6 mm drill bit before tapping.
Both threads have a diameter of No 12 (5.49 mm), but differ in pitch. UNF No 12-28 has 28 threads per inch, while UNC No 12-24 has only 24 threads per inch. The UNF version is finer and holds better under vibration.
You need a caliper and a thread pitch gauge. External thread (on a shaft): measure the major diameter – it should be 5.49 mm (No 12). Internal thread (in a hole): measure the hole diameter – it should be approximately 4.6 mm. Then place the thread pitch gauge against the threads – the 28 TPI leaf fits this thread. The 60° thread angle confirms the American standard.
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