Tolerances and fit: clearance, slip and press

In CAD, a 20.0 mm shaft goes perfectly into a 20.0 mm hole. In the real world, neither the shaft nor the hole is exactly 20.0 mm, and "perfectly" would mean they could never be assembled or moved. A fit is the deliberate gap (or overlap) you design between mating parts so they behave the way you want.

Three words

• Tolerance. How much a dimension is allowed to vary from its nominal value.
• Clearance. The gap between two mating surfaces. Positive means there is a gap.
• Interference. The overlap when one part is larger than the space it goes in. It is negative clearance.

Per side, not total

Most fit advice is given per side: the gap on each surface. A shaft that is 0.3 mm smaller in diameter than its hole has 0.15 mm of clearance per side. A box with a lid that has 0.25 mm clearance per side needs the lid to be 0.5 mm narrower than the box opening.

Rules of thumb for FDM printing

These assume a 0.4 mm nozzle and 0.2 mm layers on a reasonably tuned printer. They are starting points, not laws.

The overall FDM range for most functional fits is roughly 0.2 to 0.4 mm per side. Smaller than that and layer lines and first-layer squish start to fuse parts together.

Why FDM needs so much

• Holes print undersize. Round holes often come out 0.1 to 0.2 mm smaller than designed, because the nozzle's path rounds in on small circles.
• The first layer squishes outward. The bottom edge flares slightly ("elephant's foot"), so a part that fits at the top can bind at the base.
• Layer lines add roughness. Surfaces are not mirror smooth.
• Materials shrink. ABS and ASA shrink more than PLA and PETG as they cool.
• Direction matters. Accuracy in XY differs from accuracy in Z, which is set by layer height.

Other processes

Make fit a parameter

Create one parameter for your printer's behavior, and reference it everywhere two parts meet.

fit_clearance = 0.25
lid_width     = box_inner_width - 2 * fit_clearance

Change fit_clearance after your first test print and every mating feature updates together.

Test before you commit

1. Print a small test coupon with the same fit you plan to use: a short section of the lid lip and the box wall, or a peg and a hole.
2. Try the same orientation and material as the real print, since both change the result.
3. Adjust fit_clearance by 0.05 mm steps.

Ten minutes of testing saves hours of reprinting.

Ask, don't measure

You do not need to measure fits by hand in the viewport. Ask:

The agent measures it on your model and answers. The interference and tolerance check in Make is tuned to the manufacturing process you chose, so it flags gaps that are too tight for FDM but fine for CNC.

Bought pieces

Bought pieces come with their real, nominal dimensions. A screw is 3.0 mm in diameter, but a hole for it should be bigger. A clearance hole for M3 is typically 3.2 to 3.4 mm, and a printed hole is safest at 3.4 mm. When you say "fits an M3", the agent picks sensible sizes. A heat-set insert, on the other hand, wants a hole slightly smaller than its outer diameter. Follow the insert's datasheet, or choose the heat-set hole type and let the library size it.

Try it

On the Hinge box template:

If your first print is too tight, raise the parameter to 0.3 and reprint. That is the whole workflow.