M1 thread data
| Major diameter D | 1 mm |
| Pitch P | 0.25 mm · coarse pitch |
| Pitch diameter D2 | 0.838 mm |
| Minor diameter D1 (basic profile) | 0.729 mm |
| Thread engagement H1 | 0.135 mm |
| Cutting tap drill | 0.75 mm |
| Forming tap drill, indicative range | 0.87 – 0.92 mm |
| OSG 6HX | 0.87 – 0.89 mm |
| Yamawa ISO2X | 0.89 – 0.92 mm |
Why a forming tap needs a larger hole
A cutting tap removes material as chips, so the hole starts close to the minor diameter: 0.75 mm here. A forming tap does not cut: it displaces the material and pushes it up to form the thread crest. That is why the hole is larger, from 0.87 mm up: 0.12 mm more than for cutting. With the cutting drill, a forming tap would jam or break.
Where to sit within the range
Towards the minimum (0.87 mm) the thread is fuller, but the torque on the tap rises. Towards the maximum (0.92 mm) the tap runs lighter, but the crest stays more open. The right point depends on the tap and the material and comes from the manufacturer's catalogue. With forming taps every hundredth counts: use a good drill and check the hole.
Small sizes
Below M3 the tap is slender and breaks easily: a tight or crooked hole is enough. Forming helps here, because there are no chips to pack, but the drill size must be held precisely.
What the makers say
For M1: OSG 0.87 – 0.89 mm, Yamawa 0.89 – 0.92 mm. Each maker designs the tap profile its own way, so the range shifts by a few hundredths. The range at the top runs from the lowest to the highest of these values: if you use a tap from one brand, follow that brand's catalogue.
Forming tap drills: indicative range, from the lowest to the highest value given by the tap makers (DIN 13 / ISO 965-1): OSG, forming taps guide to tap drill sizes (class 6HX); Yamawa, Quick Guide, bored hole size for thread forming (class ISO2X). Basic profile to ISO 68-1. Indicative values: the catalogue of the tap you use is what counts.