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Description
Technical data for the UNC 1-8 hand tap
| Technical data | ||
|---|---|---|
| Nominal Diameter d1 Inches/mm/Decimal | UNC 1-8 | |
| Thread Pitch P in Inches and mm | 8 threads per inch | 3.17 mm |
| Thread Diameter d1 mm / Decimal | 25.40 mm / 1.00 in | |
| Thread angle G | 60° | |
| Thread Standard | UNC ANSI B-1.1 | |
| Overall Length l1 | 110 mm | |
| Flute Length l2 | 36 mm | |
| Shank Diameter d2 | 18 mm | |
| Square Drive Width Across Flats a | 14.5 mm | |
| Tap Drill Hole | 22.25 mm | |
Roughing, intermediate, and finishing taps – how much material each removes
Division of work in the 1–2–3 set
The UNC 1-8 thread is cut into the wall of a hole with a diameter of 22.25 mm. The thread groove extends 1.575 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 1-8 hole – sequence 1, 2, 3
Hand tapping step by step
Thread UNC 1-8 in five steps
ØDrilling
Drill bit 22.25 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 is 25.4 mm, and 25.4 : 8 threads ≈ 3.18 mm. The UNC 1-8 thread diameter is 1 inch, or 25.4 mm. The diameter of 25.4 mm minus the pitch of 3.18 mm gives 22.22 mm – in practice, use a 22.25 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 38.1 mm, or 1.5 times the thread diameter (1.5 x 25.4 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 naturally. It turns easily at first and only starts cutting deeper in the hole. The finishing tap enters with almost no resistance and only cuts the last portion of the profile at the end. When it screws in smoothly all the way, the thread is ready – check it with a UNC 1-8 screw.
UNC 1-8 hand tap – set of 3 taps for large coarse inch threads
UNC 1-8 hand tap is a three-piece set of professional tools for cutting large-diameter internal Unified National Coarse – UNC threads. The set includes HSS roughing, intermediate, and finishing taps manufactured to DIN 352.
Features of the UNC 1-8 tap
- Three-piece set – dividing the cutting process into three stages protects the tools when cutting a coarse pitch
- Large diameter of 1 inch (25.40 mm) – designed for connections carrying the heaviest loads
- 8 threads per inch (3.18 mm pitch) – coarse thread profile for quick assembly and high strength
- UNC standard – unified coarse thread for steel structures and machinery
- HSS high-speed steel – efficient machining of structural steels up to 800 MPa
The UNC 1-8 thread is used in heavy steel structures, construction machinery, and load-bearing components built to American standards. UNC 1-8 inch tap is chosen by workshops servicing Caterpillar and John Deere machines and large structures manufactured in the USA.
Frequently asked questions
Calculate the hole diameter using the formula: d = D - P. For UNC 1-8: d = 25.40 mm - 3.18 mm = 22.22 mm. Use a 22.2 mm or 22.5 mm drill bit before starting to tap.
UNC 1-8 is found in heavy construction equipment (Caterpillar, Komatsu), agricultural machinery (John Deere, Case), steel structures built to American standards, and industrial fasteners. It is one of the largest standard inch sizes.
You need a caliper and a thread pitch gauge. External thread (on a shaft): measure the outside diameter – it should be 25.40 mm (1 inch). Internal thread (in a hole): measure the hole diameter – it should be approximately 22.2 mm. Then place the thread pitch gauge against the threads – the 8 TPI leaf fits this thread. A 60° thread angle confirms the American standard.
Product accessories
Metal drill bit 22.25 NWKc HSS DIN 345 Werko
Reference: 10480 In stock > 10 items
€23.68
€29.13 brutto
Tapered countersink 30/90 DIN 335 B HSS
Reference: 925362 In stock > 10 items
€38.16
€46.93 brutto
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