Half the RFQs I see have the same problem: every dimension carries ±0.01mm, including the ones nobody will ever measure again. Last month it was a 120 × 80mm aluminum enclosure, four M4 holes, a couple of slots. The designer had copied a tolerance block from a molding project and left it untouched. Quoting that drawing as drawn added roughly 40% to the unit price, and the part would work exactly the same without it.
Here is how I talk customers through it.
What you are really paying for when you tighten a tolerance
Tolerances do not get expensive in a straight line — they get expensive in steps. On a 3-axis mill, holding ±0.1mm is basically free; the machine does it while the operator drinks coffee. Drop to ±0.02mm and we are still fine, but the machinist adds a finishing pass and checks the first article with a micrometer instead of calipers.
At ±0.01mm the finishing pass becomes mandatory, tool wear gets monitored part by part, and inspection moves to a CMM in a room where we actually care about the temperature. Below ±0.005mm you are into gauge-and-grind territory, and the quote reflects it.
There is also scrap. Every tightening step raises the odds that a perfectly usable part fails final inspection on a feature that has no function.
Start with ISO 2768
For dimensions that do not mate with anything — outer envelope, wall thickness, slot positions on a cover — the honest answer is usually ISO 2768-m (medium). That class allows ±0.2mm on features between 6 and 30mm, ±0.3mm up to 120mm. For a bracket or an enclosure, nobody on the assembly line will notice.
Coarse (c) works for weldments and large fabricated frames. Fine (f) — ±0.1mm on most mid-range sizes — is where I start when a cover has to sit flush with no visible step. One line in the title block (“General tolerances ISO 2768-mK”) replaces forty individual notes and makes life easier for the shop: the K adds the geometric portion of the standard (perpendicularity, runout) at a sensible level.
When parts mate, use the fit system, not vibes
Mating features deserve real tolerances, picked from ISO 286. The classic case: a Ø25mm shaft sliding into a housing bore. H7 on the bore gives 25.000–25.021mm; a g6 shaft lands at 24.980–24.993mm. That is a running clearance between 7 and 41 µm — it slides freely with no slop you can feel by hand. Press fits follow the same logic with H7/p6 or similar pairings.
What I see too often is a hole pattern with ±0.01mm between the holes and no datums anywhere. The whole pattern can still drift 2mm from the part edge and the drawing check will not catch it. If hole positions matter — and they usually do once bolts are involved — a position tolerance tied to datums (Ø0.25mm to A|B|C) controls what actually matters, and it is cheaper to hit than a blanket ±0.01.
Surface roughness: the silent budget killer
Ra 3.2 µm is the default for milled and turned surfaces, and it is what most faces need. Ra 1.6 makes sense for static sealing faces and anything a person touches regularly. Ra 0.8 for dynamic seals or bearing seats. Ra 0.4 and below generally means grinding or polishing as a secondary operation, and the clock runs the whole time.
I have received drawings calling out Ra 0.4 on an internal pocket face that seals nothing, touches nothing, and is visible to nobody. That is not attention to detail. That is a default somebody never changed.
My working checklist
Before a drawing goes out the door, I ask one question per dimension: does it mate, seal, align, or get measured by the customer? If yes, give it a real tolerance from the fit tables. If no, let ISO 2768-m handle it. Tighten only the datum features. And when in doubt, send the drawing to the machinist before release — a two-minute question about one decimal place beats a 40% price surprise after the fact.
If you want the reference version of all this, our tolerance quick reference sheet covers the ISO 2768 classes, typical achievable tolerances per process, and the fit callouts we see most often — free download. Material choice shifts these numbers too: aluminum, stainless and titanium each have their own practical floor, which we cover on the materials page. To see what we run day to day, start from our capabilities.
And one honest note: we would rather talk you out of a spec you do not need than quote it as drawn. Repeat orders come either way.