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Description
Technical data for the UNF No 6-40 hand tap
| Technical data | ||
|---|---|---|
| Nominal Diameter d1 Inches/mm/Decimal | UNF No 6-40 | |
| Thread Pitch P in Inches and mm | 40 threads per inch | 0.64 mm |
| Thread Diameter d1 mm / Decimal | 3.51 mm / 0.1380 in | |
| Thread angle G | 60° | |
| Thread Standard | UNF ANSI B-1.1 | |
| Overall Length l1 | 50 mm | |
| Flute Length l2 | 13 mm | |
| Shank Diameter d2 | 3.55 mm | |
| Square Drive Width Across Flats a | 2.8 mm | |
| Tap Drill Hole | 3 mm | |
Roughing tap and finishing tap – how much material each removes
Division of work in the 1–2 set
The UNF No 6-40 thread is formed in the wall of a 3 mm diameter hole. The thread groove extends 0.255 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 6-40 hole – sequence 1, 2
Hand tapping step by step
Thread UNF No 6-40 in four steps
ØDrilling
Drill bit 3 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 ÷ 40 threads ≈ 0.64 mm. The UNF No 6-40 thread diameter is 0.138 inches, or 3.51 mm. Subtracting the 0.64 mm pitch from the 3.51 mm diameter gives 2.87 mm. The table specifies a 3 mm drill bit: tolerance 2B allows a hole from 2.82 to 3.02 mm, and this drill bit falls within that range. 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 8.8 mm, or 2.5 times the thread diameter (2.5 x 3.51 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 6-40 screw.
UNF No 6-40 hand tap – set of 2 pcs for precision miniature threads
UNF No 6-40 hand tap is a two-piece set of tools for cutting internal threads to the Unified National Fine – UNF standard (unified fine thread). The set includes HSS roughing and finishing taps manufactured to DIN 2181.
UNF No 6-40 fine-thread tap specifications
- Miniature diameter No 6 (3.51 mm) – numbered thread size for precision joints
- 40 threads per inch (0.64 mm pitch) – very fine thread profile ensures leak tightness and vibration resistance
- Two-stage tapping – roughing and finishing taps minimize the load on the delicate tool
- HSS high-speed steel – durability when machining nonferrous metals and steels up to 800 MPa
- Conforms to DIN 2181 – European standard ensures dimensional repeatability
The UNF No 6-40 thread is used in measuring instruments, optics, watchmaking, and precision electronics. UNF No 6-40 miniature tap is chosen by specialists in laboratories, watchmaking workshops, and scientific instrument service centers.
Frequently asked questions
Calculate the diameter using the formula: d = D - P. For UNF No 6-40: d = 3.51 mm - 0.64 mm = 2.87 mm. Use a 2.9 mm drill bit for proper tapping.
Both threads have a diameter of No 6 (3.51 mm), but differ in pitch. UNF No 6-40 has 40 threads per inch, while UNC No 6-32 has only 32 threads per inch. The UNF version is finer and provides better joint sealing.
You need a caliper and a thread pitch gauge. External thread (on a shaft): measure the major diameter – it should be 3.51 mm (No 6). Internal thread (in a hole): measure the hole diameter – it should be approximately 2.9 mm. Then place the thread pitch gauge against the threads – the 40 TPI leaf fits this thread. The 60° thread angle confirms the American standard.
Product accessories
Tapered countersink 6.3/90 DIN 335 C HSS
Reference: 0000004110 In stock > 10 items
€4.21
€5.18 brutto
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