Try Tolerance: 5 Basics Every Engineer Must Know

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  What a tolerance actually controls
• 2
  How to try tolerance grade levels without over-tightening
• 3
  Clearance, transition and interference: where tolerance meets assembly
• 4
  When a tight tolerance is the wrong answer
• 5
  How tolerance is held and verified on the shop floor
• 6
  Choosing a tolerance approach by part feature

A tolerance is the permitted amount of variation on a dimension. It is not a wish list. On a drawing it does two jobs: it defines the acceptable size band, and it tells the shop how much money and time to spend holding that band.

Four terms carry almost all of the meaning. Nominal size is the reference number on the drawing. Limit sizes are the two extremes the finished part may measure, the upper limit and the lower limit. Basic deviation is where that band sits relative to the nominal line. Tolerance grade is how wide the band is.

When you try tolerance on a real shaft, you are really asking one question: will this part still do its job at the worst allowable size? A Ø20 mm shaft that measures 19.98 mm and one that measures 20.02 mm can both pass, and they will behave differently in the same bore.

The band is not symmetric by default. A hole tolerance is usually pushed above nominal so a standard shaft slides in. A shaft tolerance is pulled below nominal for the same reason. That offset is the basic deviation, and it is chosen for function, not for machining convenience.

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  Nominal sizeThe reference dimension on the drawing, used for calculation only.
• 2
  Limit sizesThe upper and lower sizes a finished part may measure.
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  Basic deviationThe position of the tolerance band relative to nominal.
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  Tolerance gradeThe width of the band, from fine to coarse.

Tolerance grades run from very fine to very coarse. Each step roughly multiplies the band width, so grade selection matters far more than most people expect. Going one grade finer can double the inspection burden on a long bore.

The practical rule is to pick the coarsest grade that still makes the assembly work. If a cover plate bolts onto a casting with clearance holes, grade has almost no effect on function. If a bearing seat presses into a housing, the grade controls press force and runout.

For machined metal parts, we routinely hold ±0.005 mm on critical features, with surface finish from Ra 0.2–0.8 μm on sealing or sliding surfaces. That is a capability statement, not a default. Applying it to every dimension on a drawing raises cost and cycle time with no functional gain.

A useful habit: mark only the dimensions that touch another part. Everything else gets a general tolerance block. On a typical bracket, three or four dimensions carry the function and the remaining twenty do not.

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  Functional firstTighten only mating, locating and sealing features.
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  General block lastLet non-critical sizes fall under a title-block tolerance.
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  Cost follows gradeEach finer step adds gauging time and scrap risk.

Two tolerances combine into a fit. The hole band and the shaft band overlap in different ways, and that overlap decides whether the joint slides, locks or presses. The same nominal Ø20 mm pair can be a free-running fit or a light press depending only on the deviation chosen.

A clearance fit always leaves a gap at worst case. It suits sliding pins, dowel locations that must be assembled by hand, and anything that sees thermal growth. The trade-off is lost positional accuracy.

An interference fit always leaves material overlap at worst case. It carries torque without a key and holds a bearing race without adhesive. But it demands tighter control on both parts, and it makes field disassembly destructive.

A transition fit sits between them. Sometimes there is a small gap, sometimes a small overlap. It is useful for parts that are located but not heavily loaded, and it is the fit most often misapplied by designers who have not run a worst-case stack.

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  Always calculate worst caseStack the largest hole against the smallest shaft, then reverse it.
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  Watch temperatureAluminium grows about twice as fast as steel over the same rise.
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  Plan for gauge wearA plug gauge wears toward the lower limit over its life.

Tight tolerances are not free, and they are not always better. A dimension that no other part touches does not need ±0.01 mm. It needs to be manufacturable at a sensible cost, measured once, and shipped.

Geometry fights precision. A 500 mm long bore is far harder to hold than a 20 mm one. Thin walls deflect under clamping and cutting force. Deep pockets need long tools that bend. Designers who try tolerance on a drawing without checking the aspect ratio usually get an avoidable quote revision.

Material matters too. Aluminium 6061 and 7075 machine cleanly and hold ±0.005 mm on well-supported features. Titanium TC4 and Inconel move more under heat and take longer to cut, so the same callout costs more and needs more passes.

The honest test is functional: if the part works at the loose limit, use the loose limit. If it only works at one size, you have a design problem, not a tolerance problem. Adding a locating feature is usually cheaper than shrinking a band.

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  Long and slenderAspect ratio over about 5:1 pushes achievable precision out.
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  Thin wallsBelow roughly 1 mm, clamping force becomes a variable.
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  Hard alloysTitanium and nickel alloys need slower passes and more inspection.

A tolerance only exists if someone measures it. We check raw material on arrival, monitor dimensions during the run, and inspect finished parts before shipment. Reports can be supplied on request.

In-process monitoring is what keeps a 10,000-part run inside the band. The operator measures the first part, then at set intervals, and adjusts offset before the tool drifts out of the lower or upper limit. Waiting until the end of the run finds scrap, not deviation.

For critical features, the machine choice does part of the work. Simultaneous 5-axis machining holds position in one setup, which removes the re-fixturing error that stacks up across three or four separate operations. On a part with several coaxial bores, that single setup often decides whether the tolerance is reachable at all.

Surface finish and tolerance travel together. A Ra 0.8–1.6 μm finish is normal for a functional machined surface. A Ra 0.2–0.8 μm finish is used where a seal, a bearing or a sliding contact needs it, and it usually arrives with a tighter size band by default.

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  First articleMeasure and record before releasing the run.
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  Interval checksCatch tool wear before it reaches the limit.
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  Single setupFewer setups means less stacked position error.