UNEF: the American extra fine thread
UNEF stands for Unified Extra Fine and is the finest of the American thread series. It is used where a joint has to be adjustable in small increments or particularly resistant to vibration: in aerospace, in hydraulics, on measuring instruments. VÖLKEL makes 41 machine taps for it to ISO 529 from HSSE, in form B and form C, from No. 12 x 32 to UNEF 2" x 18.
UNC, UNF, UNEF: the difference is the pitch
All three series share the same profile and a 60° flank angle, like the metric ISO thread. They differ only in threads per inch. At 1/2 inch that means 13 threads for UNC, 20 for UNF and 28 for UNEF. The finer the thread, the less it loosens under vibration and the finer the adjustment, but the more sensitive it is to dirt and damage.
Threads per inch across the series
Depending on diameter, UNEF uses 32, 28, 24, 20 or 18 threads per inch: 32 on the small diameters up to 3/8 inch, 28 at 7/16 and 1/2 inch, 24 from 9/16 to 11/16 inch, 20 from 3/4 to 1 inch and 18 above that. The pitch in millimetres is 25.4 divided by the thread count.
Form B, form C and material
Form B with a spiral point lead of four to five threads pushes chips forward and is made for through holes. Form C with a 35° spiral flute lifts them out to the rear and belongs in blind holes. HSSE covers the alloyed and stainless steels that UNEF threads usually sit in.
The coarser series are among the machine taps, hand tools under hand taps, complete imperial sets under UNC and UNF. The designations are explained in detail in the article thread tap tolerances at a glance.
All sizes at a glance
41 articles in 1 series. The article number links straight to the tool.
Machine Tap ISO 529 HSSE
| Threadnominal size | Tapping drillØ in mm | Form B | Form C 35° Spiral Flute |
|---|---|---|---|
| UNEF No. 12 x 32 | 4.75 | 83320 | 83370 |
| UNEF 1/4 x 32 | 5.6 | 83321 | 83371 |
| UNEF 5/16 x 32 | 7.2 | 83322 | 83372 |
| UNEF 3/8 x 32 | 8.8 | 83323 | 83373 |
| UNEF 7/16 x 28 | 10.25 | 83324 | 83374 |
| UNEF 1/2 x 28 | 11.8 | 83325 | 83375 |
| UNEF 9/16 x 24 | 13.3 | 83326 | 83376 |
| UNEF 5/8 x 24 | 14.9 | 83327 | 83377 |
| UNEF 11/16 x 24 | 16.5 | 83328 | 83378 |
| UNEF 3/4 x 20 | 17.75 | 83329 | 83379 |
| UNEF 13/16 x 20 | 19.5 | 83330 | 83380 |
| UNEF 7/8 x 20 | 21 | 83331 | 83381 |
| UNEF 15/16 x 20 | 22.5 | 83332 | 83382 |
| UNEF 1" x 20 | 24.25 | 83333 | 83383 |
| UNEF 1.1/16 x 18 | 25.7 | 83334 | – |
| UNEF 1.1/8 x 18 | 27.25 | 83335 | – |
| UNEF 1.3/16 x 18 | 28.75 | 83336 | – |
| UNEF 1.1/4 x 18 | 30.5 | 83337 | – |
| UNEF 1.5/16 x 18 | 32 | 83338 | – |
| UNEF 1.3/8 x 18 | 33.5 | 83339 | – |
| UNEF 1.7/16 x 18 | 35.2 | 83340 | – |
| UNEF 1.1/2 x 18 | 36.8 | 83341 | – |
| UNEF 1.9/16 x 18 | 38.4 | 83342 | – |
| UNEF 1.5/8 x 18 | 40 | 83343 | – |
| UNEF 1.11/16 x 18 | 41.5 | 83344 | – |
| UNEF 1.3/4 x 18 | 43 | 83345 | – |
| UNEF 2" x 18 | 49.4 | 83346 | – |
Tapping drill: drill diameter recommended by VÖLKEL (Technical Information “Tapping drill sizes”).
