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Description
Technical data for the 1/8-40 BSW Whitworth hand tap
| Technical data | |
|---|---|
| Nominal diameter d1 | BSW 1/8-40 |
| Thread diameter | 3.175 mm / 1/8 in |
| Thread pitch P | 40 threads per inch / 0.635 mm |
| Overall length l1 | 37 mm |
| Flute length l2 | 12 mm |
| Shank diameter d2 | 4 mm |
| Square drive size a | 3.15 mm |
| Thread angle | 55° |
| Thread standard | BS 84 (Whitworth) |
| Tap drill hole | 2.55 mm |
Roughing, intermediate, and finishing taps – how much material each removes
Division of work in the 1–2–3 set
The BSW 1/8-40 thread is cut into the wall of a hole with a diameter of 2.55 mm. The thread groove extends 0.31 mm into the wall. A single tap would have to cut the entire groove at once, requiring high manual torque and risking breakage. The set divides this work into three layers.
The drawing shows the entire set in operation. All three taps have the same shank and the same length. They differ in chamfer length: No. 1 has a taper spanning 7–10 threads, while No. 3 spans only 1–2 threads. The detail views at the hole wall show how the thread groove deepens after each tap.
| Tap | Chamfer | Material removed | How it works in the hole |
|---|---|---|---|
| No. 1 – roughing tap | long, 7–10 threads | approx. 60% | enters smoothly and establishes thread alignment |
| No. 2 – intermediate | medium, 3–5 threads | approx. 30% | screws into the previously cut threads and starts cutting only deeper in the hole |
| No. 3 – finishing tap | short, 1–2 threads | approx. 10% + sizing | cuts smoothly and produces the full thread profile |
How to identify the tap numbers in the set
The chamfer is the tapered end of the tap. No. 1 has the longest chamfer and No. 3 the shortest – the difference is immediately visible when they are placed side by side. No. 1 and No. 2 have shallower thread profiles; only No. 3 has the full profile. The number can also be identified on the shank: No. 1 has one ring, No. 2 has two, and No. 3 has none. Keep the set together and always start with the roughing tap.
How to tap a BSW 1/8-40 hole – sequence 1, 2, 3
Hand tapping step by step
Thread BSW 1/8-40 in five steps
ØDrilling
Drill bit 2.55 mm
1Roughing tap
Removes approx. 60%
2Intermediate
Removes approx. 30%
3Finishing tap
Removes approx. 10%
✓Finished thread
Thread BSW 1/8-40
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 : 40 threads = 0.635 mm. The BSW 1/8-40 thread diameter is 1/8 in, or 3.175 mm. The Whitworth profile has a thread height equal to 0.64 times the pitch, so the minor thread diameter is 3.175 mm minus 1.28 × 0.635 mm, or 2.36 mm. The 2.55 mm drill bit listed in the table is larger: the thread in the hole has approximately 77% of the full height, and the tap removes less material. An undersized hole is a common cause of taps twisting off in the workpiece.
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 4.8 mm, or 1.5 times the thread diameter (1.5 x 3.175 mm, shown as 1.5D in the drawing). Drill the hole deeper than the thread. The clearance below the thread accommodates chips and the finishing tap chamfer.
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 and No. 3 – deepening and full profile
Unscrew the roughing tap and remove the chips from the hole. Screw the intermediate tap into the cut threads by hand; it should engage them on its own. It turns easily at first and only starts cutting deeper in the hole. The finishing tap enters with almost no resistance and cuts the final few percent of the profile only at the end. When it screws in smoothly all the way, the thread is complete; check it with a BSW 1/8-40 screw.
1/8-40 BSW Whitworth hand tap
1/8-40 BSW Whitworth hand taps for cutting internal Whitworth coarse threads, type BSW, diameter 1/8 - 40, BS-84:1956, in through holes and blind holes, with a 55° thread angle. They are sold as a set comprising three taps: a starter tap with a 8P chamfer, a roughing tap with a 4P chamfer, and a finishing tap with an 2P chamfer. The tools are made from modern, high-grade high-speed steel HSS – SW7M.
This modern high-speed steel delivers measurable benefits when tapping holes. Its main advantages are high cutting productivity, significantly greater resistance to demanding machining conditions, and increased resistance of the tap cutting edge to potential breakage, chipping, cracking, dulling, and other types of damage. Thanks to the high-speed steel properties described above, 1/8-40 BSW hand taps are suitable for machining materials in categories P1.2 – P1.3 (unalloyed steels, metals, metal alloys, plastics). Inch pipe threads are used for all types of mechanical connections in equipment and fittings. They are most commonly cut on pipes and pipe connection components.
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| Courier GLS | Przelew24 / Prepayment | 14,76 zł gross | Delivery time 1 working day |
| Courier GLS | Cash on delivery | 17,26 zł gross | Lead time 1 working day |
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