Tolerances that cost money — and the ones that don’t
A tight tolerance on the right feature is worth paying for; the same tolerance everywhere is just a bigger bill. Where precision is free, where it stacks up, and how to spec only what the part needs.
Tolerance is a price, not a wish
It’s tempting to put ±0.1 mm on every dimension "to be safe". But a tolerance is an instruction to the shop to work harder — to slow down, to check more, sometimes to add an operation — and every one you tighten adds cost. The skill in a good drawing isn’t holding everything tight; it’s knowing the two or three features that must fit, and letting the rest breathe.
Cut features are tight — and nearly free
A laser holds roughly ±0.1 mm on a cut feature, and it comes at no extra charge — the beam is that precise by nature. So hole sizes, hole-to-hole spacing and outside profile on a flat part are the cheapest place to be accurate. Datum your part off cut features wherever you can, and you get precision for free.
Bends stack up
Every bend adds two small uncertainties: the angle (typically ±0.5° to ±1°) and where the bend line lands (a few tenths). One bend is no problem. But those errors accumulate — three bends in a row can drift a feature half a millimetre or more from nominal, because each bend is measured from the last. A dimension that crosses several bends is the expensive one to hold tight.
| FEATURE | TYPICAL TOLERANCE | COST TO TIGHTEN |
|---|---|---|
| Cut hole / edge | ±0.1 mm | Free |
| Single bend angle | ±0.5–1° | Low |
| Dimension across 1 bend | ±0.2–0.3 mm | Low |
| Dimension across 3 bends | ±0.5 mm+ | High — needs a fixture |
| Welded assembly | ±0.5–1 mm+ | High — heat moves it |
To hold a tight dimension across several bends, we fixture the part — worth it when it matters, wasteful when it doesn’t.
Welded parts move
Welding pours heat into a part and heat moves metal — so a welded assembly is the loosest thing in the shop unless we plan against it. We hold tolerance on weldments with fixtures and a heat-balanced weld sequence, but "tight" on a weldment costs real money. If a bore or a mounting face on a welded frame has to be precise, the cheapest answer is often to weld it slightly oversize and machine that one feature after.
Tell us which three dimensions have to be right, and we’ll build the job around them. Tell us all forty are critical and we’ll build the job around your budget.
Spec tolerance where it earns it
- 01 One clear datum Dimension from a single reference, not a chain.
- 02 Tight only where it fits something Mating holes, bearing bores, mounting faces — not every edge.
- 03 A sensible general tolerance A blanket ±0.5 mm on everything else keeps the drawing honest and the price down.
- 04 Flag the critical few Call out the two or three that must fit — we’ll build to protect them.
Not sure what to tolerance?