Machining
Tap drill size
Thread diameter and pitch against the hole to drill first.
Readings
Drag the red line, or type into any two boxes — the third is solved. The marked box is the one being solved for.
Print at 100% scale, then check the 100 mm bar with a rule before trusting a reading.
Why this one is an instrument
The only linear instrument in the collection, and worth having for the contrast: evenly spaced ticks all the way along, no crowding at either end, because subtraction has no leverage anywhere. Set against the reciprocal and logarithmic scales elsewhere it makes plain that the shape of a scale is a picture of the equation it came from. Note that this is the shop rule rather than the standard: full thread depth is d = D − 1.0825p, so subtracting the pitch alone leaves about 92% engagement. That is deliberate — 92% is what commercial tap drill charts actually give you, holds most of the strength, and is markedly easier to tap.
- Equation
- d = D − p
- Constants
- ISO metric coarse · ≈92% thread engagement
- Ranges
- D 3–24 mm · p 0.35–3 mm
- Reading
- Any two values fix the third. Outside the printed ranges the instrument stops being valid — that is what the end marks are for.
These are drawn from the stated equation and nothing else. Where an equation is itself an approximation — hyperfocal distance and tap drill sizes both are — the instrument inherits that approximation exactly, no better and no worse.