Tolerances and Fits
A tolerance is the amount a dimension may vary from its nominal size and still be acceptable, such as 10 mm ±0.1 mm. A fit is the designed relationship between two mating parts: a clearance fit lets them move, an interference fit makes them grip, and a transition fit sits between the two.
Updated 28 Sep 2026 · 5 min read · By Wyntek
What a tolerance is
No process makes a part exactly to its CAD size. A tolerance states how far a dimension may stray and still pass. A shaft drawn as 10 mm ±0.1 mm is acceptable anywhere from 9.9 to 10.1 mm. The 10 mm is the nominal size, and 9.9 and 10.1 mm are the limits.
Accuracy is how close a part comes to the CAD size. Precision is how consistently a process repeats. Tolerance is the range you decide is acceptable, and it should come from what the part has to do.
Tight tolerances cost money. In machining they need extra and slower steps, and more accurate fits are more expensive to make. Good drawings put tight limits only on the features that need them and let a general tolerance note cover everything else.
Typical accuracy by process
| Process | Typical accuracy | Notes |
|---|---|---|
| FDM printing | About ±0.2 to ±0.5 mm | Prusa says its printers are accurate to at least 0.2 mm. Formlabs quotes ±0.5%, with a lower limit of ±0.5 mm, for FDM in general. Warping and shrinkage vary by material. |
| SLA resin printing | ±0.15% on features of 1 to 30 mm, ±0.2% on 31 to 80 mm, ±0.3% on 81 to 150 mm | Formlabs figures for its current printers, with a lower limit of ±0.02 mm. Post-curing shrinks resin slightly, and Formlabs' print software compensates for it. |
| CNC machining at Wyntek | ±0.2 mm standard, ±0.1 mm tight, ±0.05 mm precision | The tighter classes suit only some features and cost more. |
Treat these as typical ranges, not guarantees. Machine, material, feature size, and orientation all shift them, so test critical fits before ordering in quantity.
Clearance, transition, and interference fits
A fit describes how two mating parts go together, usually a shaft in a hole. The ISO and ANSI systems both group fits into three families.
- Clearance fit: the hole is always larger than the shaft, so the parts slide or turn. Pivots, latches, and sliding guides use clearance fits.
- Transition fit: the hole and shaft are so close in size that a pair may have slight clearance or slight interference. The parts locate accurately and go together with light force, such as a rubber mallet.
- Interference fit: the shaft is always larger than the hole, so the parts are pressed together or fitted using heat, and they stay put. Bushings and bearings pressed into housings use interference fits.
| Fit | Family | Description and typical use |
|---|---|---|
| H11/c11 | Clearance: loose running | Large clearance where accuracy is not essential, such as pivots and latches |
| H8/f7 | Clearance: close running | Small clearance with moderate accuracy, such as shafts and sliding rods |
| H7/g6 | Clearance: sliding | Minimal clearance for accurate guiding; turns and slides freely |
| H7/h6 | Clearance: location | Very close clearance for precise location; goes together without force |
| H7/k6 | Transition | Negligible clearance or interference; fitted with a rubber mallet |
| H7/p6 | Interference: press | Light interference; pressed together cold |
| H7/s6 | Interference: driving | Medium interference; pressed hot or with large force, for permanent mounting |
In the hole-basis system, the hole keeps a standard tolerance, H, and the shaft tolerance changes to set the fit. The numbers are small: on a 50 mm diameter, an H7 hole may be up to 0.025 mm oversize. That is machining territory. Printed parts cannot reliably hold ISO fits, so printed fits are set with clearance and a test print instead.
Practical clearances for printed parts
Two printed parts modelled at exactly the same size will usually not go together, so the gap has to be designed in. Prusa suggests at least 0.3 mm (0.012 in) as a first value for parts that move against each other, such as a hinge. For a pin in a hole, that means 0.3 mm per side. There is no universal number: size, orientation, the shape of the mating features, calibration, settings, and material all change the result.
- Moving FDM fits, such as hinges and sliders: start at 0.3 mm per side and adjust after a test print.
- SLA fits: separately printed resin parts hold size more closely than FDM, so fits can be tighter. Parts printed together in one piece still need a real gap so they do not fuse.
- Parts that must grip permanently: a printed press fit is hard to predict. Test several sizes, or design a clip or a screw joint instead.
- Orientation: a fit that works printed upright may not work printed on its side, so test in the orientation you will order.
How to call out critical dimensions
- Decide which features matter. A bearing bore, a locating pin, or a sealing face may need a tight tolerance. A cosmetic edge does not.
- Tolerance only those features, and let a general tolerance note, such as one to ISO 2768, cover the rest.
- Dimension critical features from datums: the faces or axes the part locates on in use. Measuring from one reference stops tolerances adding up along a chain of dimensions.
- Write fits as ISO codes, such as 10 H7, or as numeric limits. Words like 'tight' or 'snug' are open to interpretation.
- Use GD&T where form or position matters more than size, such as the flatness of a sealing face or the position of a bolt pattern.
- Keep the drawing and the 3D model in agreement, and state which one controls if they differ.
At Wyntek
Wyntek machines aluminium and steel parts to three tolerance classes: ±0.2 mm standard, ±0.1 mm tight, and ±0.05 mm precision for suitable features. Tighter classes cost more, so call out only the features that need them. You see the price in US dollars before you submit, and every part is scanned and inspected before it ships.
Machine a part to tolerance