Thread callout

A thread callout is the short code beside a hole or a shaft that specifies the thread. It carries the type, the nominal size and the pitch, and often the thread depth and a fit class as well. Metric threads are marked with an M and the diameter, for example M8, with the pitch after it when it differs from the standard.

What it means for the calculation

Each thread is an operation of its own with its own tool: drilling, sometimes deburring, then tapping, thread milling or single-point threading. The number of threaded holes, the thread depth, and whether the hole is blind or through decide the time more than the diameter does. A fine pitch, a deep thread or a tough material raises the risk of tool breakage, and that risk belongs in the allowance, not in the cycle time. A threaded insert adds a whole extra handling step.

Where it appears on the drawing

Beside the hole or the shaft, on a leader line or in a hole table, for example M8 followed by the thread depth.

Common mistakes

Reading only the diameter and missing the thread depth, which prices a deep blind threaded hole as if it were a through hole. Also assuming tapping is always faster than thread milling: in hard materials or at large diameters the opposite is often true.

What does the M in M8 mean?

A metric thread with a nominal diameter of 8 mm.

Why is the pitch sometimes left out?

Because the standard pitch for that diameter then applies; a non-standard pitch is always stated.

Does thread depth change the price?

Yes, a deep blind threaded hole needs more passes, more chip clearing, and carries a higher risk of tool breakage.