What ±0.005mm Actually Costs

Tightening a tolerance is never free. Here is what the cost curve really looks like between ±0.05 mm and ±0.005 mm — and how to tell which tolerances your part actually needs.

The cost curve nobody shows you

Moving a cutting tool from ±0.05 mm to ±0.005 mm typically raises unit cost by 40–120%. It is not mark-up. Four things change at once: the grinding machine class, the rod grade, the inspection regime, and the scrap rate.

Typical industry ranges look roughly like this. Treat them as shape, not gospel — your geometry and material shift the absolute numbers.

One detail that catches people out: at ±0.005 mm, thermal expansion matters. A tool measured at 18 °C and used at 35 °C has already moved. If your drawing says ±0.005 mm and your shop is not temperature-controlled, you may be paying for precision you cannot use.

Three tolerances worth paying for

Cutting diameter. It sets your hole or slot size directly. If it drifts, every part drifts.

Runout. The largest single driver of surface finish and tool life. A 0.01 mm runout increase can halve edge life on a finishing operation.

Helix angle consistency. It governs cutting force stability. Inconsistent helix angle is a common hidden cause of chatter that gets blamed on the machine.

Three you can usually relax

Chamfer and non-functional corner sizes. If they only break an edge, ±0.1 mm does the job and costs far less than ±0.02 mm.

Surface roughness on non-critical faces. Ra is often specified out of habit. If no functional requirement cites it, it is cost without benefit.

Form tolerance on non-cutting features. Shank flats, marking positions, and packaging dimensions rarely need precision bands.

A practical rule

Before you pay for a tight tolerance, answer one question: what happens to the part if this dimension drifts to the loose end of the band? If the answer is "nothing measurable", you just found your cost reduction.