Thread undercut (relief groove)
The groove at a shoulder where the threading tool or die runs out — so the thread is full right to the end and the tool doesn't hit the face. Dimensions for external metric threads per ISO 4755.
Quick answer
Undercut for M12×1.75 per ISO 4755: diameter dg = d − 2.6 = 9.4 mm, width g1 at least 3 mm, g2 at most 5.25 mm (3P), radius r = 1 mm.
- Undercut diameter dg (h13)
- —
- Width g1, min (at the bottom)
- —
- Width g2, max (= 3P)
- —
- Radius r at the shoulder
- —
Half longitudinal section near the surface, to scale (the undercut depth is not exaggerated). Shoulder on the left, thread on the right.
dg is the bottom diameter of the undercut (slightly under the thread minor diameter, tolerance h13, h12 up to M3). g2 is the full width at the major diameter, g1 the width at the bottom; the transition from the bottom to the thread is at least 30°. r is the radius at the shoulder, against stress concentration.
On a lathe the undercut is cut with a grooving tool with an r corner radius, or a 30° tool: plunge to dg, cut the g1 width, and come out with a 30° chamfer towards the thread.
ISO 4755 table (external threads)
| Pitch P | dg | g1 min | g2 max | r |
|---|---|---|---|---|
| 0.25 | d − 0.4 | 0.4 | 0.75 | 0.12 |
| 0.3 | d − 0.5 | 0.5 | 0.9 | 0.16 |
| 0.35 | d − 0.6 | 0.6 | 1.05 | 0.16 |
| 0.4 | d − 0.7 | 0.6 | 1.2 | 0.2 |
| 0.45 | d − 0.7 | 0.7 | 1.35 | 0.2 |
| 0.5 | d − 0.8 | 0.8 | 1.5 | 0.2 |
| 0.6 | d − 1 | 0.9 | 1.8 | 0.4 |
| 0.7 | d − 1.1 | 1.1 | 2.1 | 0.4 |
| 0.75 | d − 1.2 | 1.2 | 2.25 | 0.4 |
| 0.8 | d − 1.3 | 1.3 | 2.4 | 0.4 |
| 1 | d − 1.6 | 1.6 | 3 | 0.6 |
| 1.25 | d − 2 | 2 | 3.75 | 0.6 |
| 1.5 | d − 2.3 | 2.5 | 4.5 | 0.8 |
| 1.75 | d − 2.6 | 3 | 5.25 | 1 |
| 2 | d − 3 | 3.4 | 6 | 1 |
| 2.5 | d − 3.6 | 4.4 | 7.5 | 1.2 |
| 3 | d − 4.4 | 5.2 | 9 | 1.6 |
| 3.5 | d − 5 | 6.2 | 10.5 | 1.6 |
| 4 | d − 5.7 | 7 | 12 | 2 |
| 4.5 | d − 6.4 | 8* | 13.5 | 2.5* |
| 5 | d − 7 | 9* | 15 | 2.5* |
| 5.5 | d − 7.7 | 11* | 16.5 | 3.2* |
| 6 | d − 8.3 | 11 | 18 | 3.2 |
Dimensions in mm. Source: IS 1369-2:1993, identical to ISO 4755:1983 (official text openly available at law.resource.org), cross-checked with an independent database. * — for pitches 4.5–5.5 g1 and r come from the second source as they are unreadable in the scan. Internal thread undercuts and the DIN 76 series may differ — check against your standard.
The Groove cycle in Turn G-code writes the groove G-code; retaining ring grooves are covered in the article "Retaining ring groove".