Most machined parts that fail on assembly do not fail because the shop cut them wrong. They fail because the drawing asked for the wrong thing, or asked for nothing at all. This guide shows you how to specify machining tolerances and read a drawing so your parts fit the first time, without paying for precision you do not need.
Why tolerance is a cost decision, not just a number
Every tolerance you tighten adds machining time, better tooling, more inspection, and sometimes scrap. A hole held to a hundredth of a millimeter can cost several times more than the same hole held to a tenth. The goal is not the tightest tolerance possible; it is the loosest tolerance that still lets the part work. Tolerance is where design meets budget.
The building blocks of a clear drawing
Datums: where measurements start
A datum is the reference face, edge, or hole that everything else is measured from. If you do not define datums, the shop guesses, and two parts measured from different references will not match. Pick the surfaces that matter for function, usually the ones that locate the part in the assembly, and label them.
Fits: how two parts go together
When a shaft goes into a hole, the gap between them is the fit. A clearance fit slides freely, an interference fit is pressed together, and a transition fit is in between. Decide the fit first, then set the tolerances that produce it. Do not dimension the shaft and hole independently and hope.
Surface finish
A smooth sealing face, a bearing seat, or a sliding surface needs a finish callout. Leaving it blank means the shop delivers whatever the cutting process happens to produce, which may be too rough for a seal or needlessly polished for a hidden face.
General versus specific tolerances
| Type | Use it for | Effect on cost |
| General title block tolerance | Non critical dimensions | Low, keeps the drawing clean |
| Specific tight tolerance | Mating and locating features | Higher, apply only where needed |
| Surface finish callout | Seals, bearings, sliding faces | Higher, only on the faces that need it |
Put a sensible general tolerance in the title block, then tighten only the few dimensions that control function. This is the single biggest lever on machining cost.
A real example
A customer sent a drawing where every dimension carried the same tight tolerance, applied by default across the whole part. The quote came back high and the lead time long, because the shop had to hold that tolerance everywhere, including on faces that never touched anything. We reviewed the part and found only two features mattered: the bore that held a bearing and the two mounting holes. We tightened those, relaxed everything else to a general tolerance, and specified a finish on the bore only. The part fit exactly the same, but cost dropped by roughly a third and machining time fell. Nothing about the function changed; only the drawing got smarter.
Common mistakes and how to fix them
Applying one tight tolerance everywhere. This multiplies cost for no benefit. Tighten only mating and locating features; use a general tolerance for the rest.
No datums. Without a defined reference, measurements drift. Label the functional faces as datums so everyone measures from the same place.
Dimensioning shaft and hole separately. Decide the fit first, then derive both tolerances from it, so they are guaranteed to work together.
Forgetting surface finish where it matters. A seal face with no finish callout can leak even when every dimension is in tolerance.
Over dimensioning. Giving the same feature two conflicting dimensions creates ambiguity. Dimension each feature once, from a clear reference.
Checklist before you release a drawing
- Are the functional reference faces labeled as datums?
- Is there a general tolerance in the title block for ordinary dimensions?
- Are tight tolerances applied only to mating and locating features?
- For each mating pair, is the fit decided and the tolerances derived from it?
- Do sealing, bearing, and sliding faces have a surface finish callout?
- Is every feature dimensioned once, with no conflicts?
- Would a machinist who never saw the part know exactly what matters?
Conclusion and next step
A good drawing tells the shop what matters and lets them relax everything else. That is how you get parts that fit the first time without overpaying. As a next step, take one of your current drawings and mark which two or three dimensions actually control function; you will likely find you can loosen the rest and cut cost with zero loss of quality.
FAQ
What tolerance should I put if I am unsure?
Start with a general title block tolerance suitable for the process, then tighten only the features that mate or locate. When in doubt, ask the machinist what the standard process holds naturally.
Why did my parts pass inspection but not fit together?
Usually because the fit between mating features was not defined, or the two parts were measured from different datums. Define the fit and share a common reference.
Does a tighter tolerance always mean a better part?
No. It means a more expensive part. A tolerance tighter than the function needs adds cost without adding value.
What is a datum and do I always need one?
A datum is the reference feature measurements start from. Any part with related features that must line up benefits from clearly defined datums.
When do I need to specify surface finish?
Whenever a face seals, carries a bearing, slides against another part, or is visible and must look a certain way. Otherwise the general process finish is usually fine.
References
For readers who want to go deeper, the ISO 2768 general tolerance standard and the ASME Y14.5 standard on geometric dimensioning and tolerancing are the widely recognized authorities on this topic.
