3D-printing tolerances and clearance: make parts fit their purpose

Nominal size, tolerance and clearance mean different things

The nominal size is the design dimension. Tolerance defines the accepted lower and upper limits for the produced feature. Clearance is the space between adjoining parts, often designed deliberately for movement or assembly.

One dimension within tolerance does not guarantee that the combination works. Both parts matter. For a new lid on an existing enclosure, measure the actual opening as well as specifying the lid.

A calculation, not a printing recommendation

For this calculation only, suppose a pin may measure 9.9–10.1 mm and its opening 10.3–10.5 mm. The minimum diameter difference is 10.3 − 10.1 = 0.2 mm. The maximum is 10.5 − 9.9 = 0.6 mm. This is diametral clearance, not the gap on one side.

These invented numbers are neither recommended fits nor printer capability or an OmniTechs promise. They omit form error, surface texture and misalignment. They only demonstrate why two dimensional agreements must be considered together.

The same relationship matters for a phone holder around its case, a removable drawer organiser and a vase around an insert. Measure the real counterpart; do not copy these example allowances.

Describe the connection's action

For sliding, identify the direction and surfaces that must stay free. Fitting once does not assess repeated movement. For clamping, specify what stays fixed and how it assembles without damaging the counterpart. For removable connections, describe how often they open and whether hands or tools can reach them. Adhesive assembly answers a different need from a serviceable enclosure.

When splitting a large model, also choose seam location and alignment before printing.

There is no universal clearance for every print

Connection geometry, size, material, orientation, settings and calibration affect the result. Supports and finishing can leave marks on a mating surface. A number from another printer or shape is not a proven allowance for yours.

Mark contacting surfaces and the visible face. Discuss whether a small representative connection can test the uncertainty without printing the whole object. Its material and process conditions must represent the intended final execution.

Layer height is not dimensional tolerance

Layer height describes the thickness of individual printed layers. Smaller layers change vertical detail and layer appearance; they do not set the dimensional error of a hole or outside surface. A 0.12 mm layer setting does not mean every dimension will be delivered within ±0.12 mm.

The three-version workshop case shows visible changes around a horizontal hole and surface. Without measurements, those photographs do not establish a dimensional tolerance.

Check the connection before repeating production

Start with known measurements and a counterpart photo. The replacement-part measurement guide helps select reliable reference faces. Agree what the trial must demonstrate—for example, closing and reopening as well as dimensional limits.

Record revision, material, measurements and where the trial binds or feels loose. Separate the observation from a proposed fix. Use the prototype assessment to make that decision reproducible.

Choose the next fit assessment

For a replacement, send both mating-part measurements and the intended movement to 3D-printed parts. Mark wear or damage: the old measurement need not become the new design dimension. For one uncertain connection, request a bounded prototype and agree geometry, material and measurement. Keep the accepted revision and counterpart record afterwards. Achievable limits and inspection are agreed for the particular order.