Every extra decimal of precision costs money. Tolerances are where careful engineers quietly overspend — here's how to specify only the precision your part actually needs, and where tightening it is just burning budget.
A tolerance is the allowable wiggle room on a dimension — if a hole is 10 mm ±0.1, anything from 9.9 to 10.1 passes. It sounds like a detail, but it's one of the biggest levers on the price of a machined part. Tighten it without a reason and you pay for precision you'll never use.
What a tolerance actually buys you
Tighter tolerances cost more because they demand more: slower cutting, better tooling, more frequent inspection, higher scrap rates, and sometimes secondary operations like grinding. A shop can hit a loose tolerance on the first pass; a tight one might need three. You're not paying for a smaller number — you're paying for all the work that guarantees it.
The cost curve: standard, precision, tight
Cost doesn't rise gently with precision — it ramps steeply. Loosening a single critical dimension from "tight" to "standard" can meaningfully drop the price of the whole part.
±0.005" (±0.13 mm)
What machining naturally holds. Effectively free — this is where most dimensions should live.
±0.001–0.002" (±0.025–0.05 mm)
Requires care, better setups, and inspection. Fine on the few features that need it — wasteful everywhere else.
Under ±0.0005" (±0.013 mm)
Slow cuts, grinding, high scrap. Reserve for true mating or sealing surfaces, and expect to pay for it.
Where to spend precision — and where to save
The rule is simple: tighten a tolerance only where two things have to fit, seal, or locate against each other. Everything else can float.
Spend it on: bores that take a bearing or shaft, mating faces, sealing surfaces, dowel-pin holes, features that set the position of something else.
Save it on: outer edges, cosmetic surfaces, clearance holes, overall length of a bracket, anything that doesn't touch another part with a job to do. If a dimension has no functional reason to be tight, leaving it at standard is free money.
How to call it out on a drawing
Don't blanket the whole part in one tight title-block tolerance — that silently makes every dimension expensive. Instead:
Set a sensible general tolerance in the title block for all the "doesn't-really-matter" dimensions, then call out the few critical features explicitly with their own tighter numbers. For anything about relationships between features — position, perpendicularity, flatness — use GD&T, which controls the function directly instead of stacking up linear tolerances that fight each other.
The rule of thumb
Default everything to standard. Then go find the handful of features that actually mate, seal, or locate, and tighten only those. If you can't name the functional reason a dimension is tight, it shouldn't be.
Not sure which of your tolerances are worth paying for? Send us the drawing. A founder-reviewed DFM check comes with every quote — we'll flag the tolerances that are costing you money without buying you anything.