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Description
Technical data for the UNC 1/4-20 hand tap
| Technical data | ||
|---|---|---|
| Nominal Diameter d1 Inches/mm/Decimal | UNC 1/4-20 | |
| Thread Pitch P in Inches and mm | 20 threads per inch | 1.27 mm |
| Thread Diameter d1 mm / Decimal | 6.35 mm / 0.2500 in | |
| Thread angle G | 60° | |
| Thread Standard | UNC ANSI B-1.1 | |
| Overall Length l1 | 50 mm | |
| Flute Length l2 | 18 mm | |
| Shank Diameter d2 | 6 mm | |
| Square Drive Width Across Flats a | 4.9 mm | |
| Tap Drill Hole | 5.1 mm | |
Roughing, intermediate, and finishing taps – how much material each removes
Division of work in the 1–2–3 set
The UNC 1/4-20 thread is cut into the wall of a 5.1 mm diameter hole. The thread groove extends 0.625 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 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 | turns easily and produces the full 2B 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 UNC 1/4-20 hole – sequence 1, 2, 3
Hand tapping step by step
Thread UNC 1/4-20 in five steps
ØDrilling
Drill bit 5.1 mm
1Roughing tap
Removes approx. 60%
2Intermediate
Removes approx. 30%
3Finishing tap
Removes approx. 10%
✓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 equals 25.4 mm, and 25.4 : 20 threads = 1.27 mm. The diameter of the UNC 1/4-20 thread is 1/4 inch, or 6.35 mm. The diameter of 6.35 mm minus the pitch of 1.27 mm gives 5.08 mm – in practice, use a 5.1 mm drill bit. 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 9.5 mm, or 1.5 times the thread diameter (1.5 x 6.35 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
Back out the roughing tap and remove the chips from the hole. Screw the intermediate tap into the cut threads by hand – it should engage them naturally. It turns easily at first and starts cutting only deeper in the hole. The finishing tap enters with almost no resistance and cuts the final portion of the profile only at the end. When it screws in smoothly all the way, the thread is complete – check it with a UNC 1/4-20 screw.
UNC 1/4-20 hand tap – set of 3 pcs for American inch threads
UNC 1/4-20 hand tap is a professional set of three tools for cutting internal threads to the American Unified National Coarse – UNC standard (unified coarse thread). The set includes HSS roughing, intermediate, and finishing taps and complies with DIN 352.
Key advantages of the UNC 1/4-20 inch hand tap
- Standard 2B tolerance for internal threads – class 2B offers an optimal balance between precision and ease of assembly, guaranteeing a proper fit with class 2A screws
- Durability from high-speed steel (HSS) – enables machining of materials with a tensile strength up to 800 MPa
- Progressive thread cutting – 3 taps divide the cutting work into stages, minimizing the risk of tool breakage
- Compliance with DIN 352 – verified manufacturing quality and dimensions
- Universal application – ideal for structural steels, aluminum, brass, and cast iron
The UNC 1/4-20 inch thread (6.35 mm diameter, 20 threads per inch, 60° thread angle) is one of the most popular sizes in the American standard. It is found in machinery imported from the USA, photographic equipment, furniture with adjustable feet, and agricultural equipment. The American UNC 1/4-20 inch tap is suitable for servicing American vehicles and maintaining foreign-made machinery.
A set chosen by maintenance departments and workshops servicing equipment built to inch standards.
Frequently asked questions
Calculate the hole size using the formula: d = D - P (nominal diameter minus pitch). For UNC 1/4-20: d = 6.35 mm - 1.27 mm = 5.08 mm. Use a 5.1 mm drill bit.
Identify a UNC 1/4-20 thread by its 6.35 mm diameter and 20 threads per inch. Measure with a caliper and check the pitch with a thread pitch gauge. Its 60° angle distinguishes it from Whitworth (55°).
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