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Description
Technical data for the UNF 1/2-20 left-hand hand tap
| Technical data | ||
|---|---|---|
| Nominal Diameter d1 Inches/mm/Decimal | UNF 1/2-20 LH | |
| Thread Pitch P in Inches and mm | 20 threads per inch | 1.27 mm |
| Thread Diameter d1 mm / Decimal | 12.70 mm / 0.5000 in | |
| Thread angle G | 60° | |
| Thread Standard | UNF ANSI B-1.1 | |
| Overall Length l1 | 70 mm | |
| Flute Length l2 | 20 mm | |
| Shank Diameter d2 | 9 mm | |
| Square Drive Width Across Flats a | 7 mm | |
| Tap Drill Hole | 11.5 mm | |
Roughing tap and finishing tap – how much material each removes
Division of work in the 1–2 set
The UNF 1/2-20 LH thread is cut into the wall of a hole with a diameter of 11.5 mm. The thread groove extends 0.6 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 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 1/2-20 LH hole – sequence 1, 2
Hand tapping step by step
Thread UNF 1/2-20 LH in four steps
ØDrilling
Drill bit 11.5 mm
1Roughing tap
Chamfer 8 threads
2Finishing tap
Chamfer 2 threads
✓Finished thread
Tolerance 2B
Turning sequence: half a turn counterclockwise, a quarter turn clockwise | 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 : 20 threads = 1.27 mm. The UNF 1/2-20 LH thread diameter is 1/2 inch, or 12.7 mm. The diameter of 12.7 mm minus the pitch of 1.27 mm gives 11.43 mm. The table specifies a 11.5 mm drill bit: tolerance 2B allows a hole from 11.33 to 11.61 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 31.8 mm, or 2.5 times the thread diameter (2.5 x 12.7 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 into a tap wrench and position it perpendicular to the surface. Check the angle from two sides, for example with a square. This is a left-hand thread, so turn in the opposite direction to usual: half a turn counterclockwise, then a quarter turn clockwise. 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
Back out the roughing tap and remove the chips from the hole. Screw the finishing tap into the cut threads by hand – it should engage them naturally. Continue turning at the same rhythm as with the roughing tap. The finishing tap removes the remaining material and cuts the groove to the full profile within tolerance 2B. When it screws in smoothly all the way, the thread is complete – check it with a UNF 1/2-20 LH screw.
UNF 1/2-20 left-hand hand tap – set of 2 pcs for left-hand fine threads
Left-hand UNF 1/2-20 hand tap is a two-piece tool set for cutting left-hand internal Unified National Fine – UNF threads. The set contains HSS roughing and finishing taps and complies with DIN 2181. Thread direction: left-hand (LH – Left Hand).
Specifications of the UNF 1/2-20 left-hand tap
- Left-hand thread direction (LH) – thread loosened by turning clockwise
- Popular diameter of 1/2 inch (12.70 mm) – a commonly used size in special designs
- 20 threads per inch (1.27 mm pitch) – fine thread profile resists loosening
- Two-piece set – roughing and finishing taps ensure a precise left-hand thread
- HSS high-speed steel – durability when machining materials up to 800 MPa
The UNF 1/2-20 left-hand thread is used in mechanisms preventing self-loosening, rotating components, and special locking devices. The left-hand UNF 1/2-20 tap is chosen by workshops servicing machines with rotating components and structures requiring left-hand threaded connections.
Frequently asked questions
Calculate the hole diameter using the formula: d = D - P. For UNF 1/2-20: d = 12.70 mm - 1.27 mm = 11.43 mm. Use a 11.5 mm drill bit – the hole diameter is identical to that for the right-hand thread.
Left-hand threads are used where a right-hand thread could loosen during operation. Typical applications: the left side of machine tool spindles, bicycle pedals (left side), gas cylinder valves, tensioning mechanisms, and components rotating counterclockwise.
You need a caliper and a thread pitch gauge. External thread (on a shaft): measure the outside diameter – it should be 12.70 mm (1/2 inch). Internal thread (in a hole): measure the hole diameter – it should be approximately 11.5 mm. Then place the thread pitch gauge against the threads – the 20 TPI leaf fits this thread. A 60° thread angle confirms the American standard. You can identify the thread direction by the slope of the threads.
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