Tolerances That Cost Money: A Buyer's Guide to What Tight Is Really For

Every machinist has seen it: a drawing with ±0.01 mm on every dimension, even on the faces that never touch anything. The buyer did it to be safe. The shop priced it as if every feature was a bearing bore. The result is a part that costs twice what it should, built to a precision the design never needed. Tolerance is not free, and the most expensive drawings are usually the ones that were meant to be careful.

What Each Tolerance Band Actually Costs

Roughly speaking, the bands behave like this. ±0.1 mm and looser is near-free: it is what the machine does by default. ±0.05 mm is normal precision, standard machining, standard inspection. ±0.01 mm is where finish cuts, sharp tools and careful fixturing enter, and where features move to a CMM. ±0.005 mm and tighter is a premium: temperature-controlled measurement, extra operations, and a process tuned around one or two features. Each step up is not a small price bump; it is a step change in process.

The rule of thumb: hold tight tolerance only where the part mates, seals, rotates or carries a load. Everywhere else, loosen it. The drawing that does this is cheaper and often more accurate, because the shop is not fighting the machine on features that do not matter.

General Tolerances Are a Default, Not a Free Pass

Most drawings leave the majority of dimensions unspecified, and a standard fills the gap: ISO 2768-1, the general tolerance standard for linear and angular dimensions, assigns default classes from fine (f) through medium (m) to coarse (c). The class the shop assumes decides what happens to all the un-toleranced numbers. A buyer who never discusses it has silently chosen a precision class by default, and usually the default is tighter than needed. Agree the class with the shop on the drawing, and a quiet cost disappears.

Where Tight Tolerance Is Effectively Free

• Features machined in the same setup. If two holes are drilled in one chucking, their relative position is nearly as accurate as the datum system allows, and a true position callout there costs little extra.
• Short, stiff tools. Bores and faces reached with a short endmill hold close tolerance without special effort. Deep features are where tight calls turn expensive.
• Matched datums. When the part rests on surfaces it already has, the shop is not inventing location. Datum choice often buys tolerance for free.

Where Tight Tolerance Breaks the Budget

Long reaches, thin walls, large faces, and stacked tolerances are where the shop struggles. A flatness callout on a 200 mm face is a different job from flatness on a 30 mm boss. True position on a bolt circle measured across a large diameter accumulates error. These are the calls to review in a DFM meeting: relax the ones that cannot be helped, and push them onto the features that survive.

What We Do at KGL

KGL holds ±0.005 to 0.01 mm on critical features and standard general tolerance elsewhere, and the drawing review points out over-toleranced dimensions before quoting. The buyer gets a price that matches the real precision required, and the CMM data proves the features that matter. Tight where it counts, normal where it does not, is how the drawing should read before it ever reaches the machine.

Conclusion

Tolerance is a design decision, not a safety margin. Give the shop a drawing that says which features are critical, and you buy precision only where the assembly needs it. The part that arrives cheaper and fits better is the one that was tolerated on purpose.

FAQ

Q: Is ±0.01 mm on every dimension the safest way to draw a part?

A: No. It is the most expensive way, and it often produces parts that drift because the shop spends effort on the wrong features. Mark the few critical dimensions tight and leave the rest to the general tolerance class.

Q: What is a general tolerance class?

A: It is the default tolerance applied to dimensions the drawing does not specify, defined by ISO 2768-1. Choosing medium or coarse instead of fine on non-critical features is one of the cheapest cost reductions available.

Q: Can I loosen a tolerance after quoting to save money?

A: Yes, before production. Ask the shop to re-quote the relaxed dimension. Most of the cost drop comes back to you, and the part still works, because the relaxed feature was never mating anything.