Tolerances Explained

Last updated September 26, 2026

A tolerance tells the manufacturer how much a dimension is allowed to vary from the number on your drawing or model. No machine cuts a perfect dimension, so every real part lands within a range. The tolerance defines how wide that range may be.

What ±0.05 mm means

A dimension of 20.00 mm ±0.05 mm may measure anywhere from 19.95 mm to 20.05 mm and still be in spec. The stated value is the target; the ± figure is the allowed deviation in each direction.

Tighter is not automatically better. A tighter tolerance means slower machining, more careful setups, more inspection time, and a higher scrap risk, all of which raise the price. The right tolerance is the loosest one that still lets the part do its job.

Standard tolerances on OpusFab

At checkout, tolerances are shown per part. The standard options are:

  • CNC machining: Standard (±0.05 mm). This covers most functional parts, including fits, enclosures, brackets, and fixtures.
  • SLA resin 3D printing: Standard (±0.5 mm). Printing builds parts layer by layer, so accuracy is looser and varies slightly with part size and orientation.

The tolerance shown in your order summary is the tolerance the part is inspected against. If your design genuinely needs something tighter than standard on critical features, request a custom quote so the team can review it.

Machining tolerance vs final tolerance

This is the distinction that catches people out, and it matters whenever you add a finish.

  • Machining tolerance is the accuracy held by the machine before any post-processing. It is what the machinist measures right after the last cut.
  • Final tolerance is the accuracy of the finished part, after anodizing, powder coating, plating, or any other surface process. This is what matters for assembly.

Finishes change dimensions. Anodizing converts the aluminium surface into oxide, and the part grows, roughly 5 to 25 micrometres per surface for standard Type II and more for hardcoat. Powder coating adds about 0.05 to 0.10 mm per surface, so an external dimension grows by 0.10 to 0.20 mm and a hole or slot shrinks by the same amount. Bead blasting removes very little but can matter on the tightest fits.

A worked example: you need a 10.00 mm bore that stays a slip fit after hardcoat anodizing. If the shop machines the bore to exactly 10.00 mm and then anodizes it, the oxide growth closes the bore and the pin no longer fits. The correct approach is to machine the bore slightly oversize so it lands on 10.00 mm after the coating.

When you select a finish on OpusFab, treat the dimensions in your model as the final, after-finish requirement, and keep the standard tolerance selection unless a custom review is needed. If you have press fits, sliding fits, or mating threaded features on a finished part, it is worth calling them out when you order. See Surface finishes overview for what each finish does to a part.

When tighter tolerances drive cost

Cost rises steeply as tolerances tighten, for practical reasons:

  • Slower cutting. Holding ±0.01 mm means lighter cuts, slower feeds, and often extra finishing passes.
  • More setups. Critical features may need to be machined in a single setup, or finished with grinding or reaming.
  • More inspection. Tighter tolerances require measurement with higher precision equipment, on more features, sometimes on every part.
  • More scrap. The tighter the window, the more parts fall outside it.

A tolerance of ±0.05 mm is routine. Halving it does not double the cost; on difficult features it can multiply the price of the part several times over.

How to keep tolerance costs down

  • Apply tight tolerances only where the function demands them, typically mating surfaces, bearing seats, and press fits. Leave everything else at standard.
  • Avoid tight tolerances on thin walls or flexible features; the part itself moves too much to hold them. See Minimum radius and wall thickness.
  • Remember the finish. If a dimension is critical after anodizing or powder coating, design for the final condition.
  • For 3D printed parts, design assemblies with clearance in mind: ±0.5 mm standard means printed parts are not suited to precision press fits without post-machining.