Parts don't fit

CAD says the parts fit. The real ones do not. This is the most common surprise in making things, and almost always one of a handful of causes.

Symptom

• The lid is too tight, or will not go on at all.
• The lid is too loose and rattles or falls off.
• A screw will not enter a hole, or spins freely.
• A board or bought part does not fit its pocket.
• A hinge binds or a drawer sticks.
• Two printed parts meant to press together do not.

Likely causes

Diagnose with measurements

Do not guess. Take the printed part and the real mating part, and measure with calipers.

1. Measure the actual printed feature (for example the lid's width, or a hole's diameter).
2. Compare with the model value. Ask the agent:

What is the exact lid width and the box opening width in the model, and what is the clearance per side?

1. Measure the real mating part (the board, the screw head, the bearing).

Now you can see the gap between intent, model, and reality:

Fix it

Adjust the clearance, not the whole design

If you have a fit_clearance parameter, change it and reprint. If not, add one now:

Create a parameter called fit_clearance, set to 0.3 mm, in the Fit group, and use it for every gap between the lid and the box. Always keep it between 0.15 and 0.5 mm.

Use the right hole type

For screws, use clearance, tapped or heat-set holes instead of typing a diameter. Tell the agent: "Make these clearance holes for M3", and it handles the sizing. For printed holes, pick the larger end of the range (3.4 mm for M3).

Chamfer the bottom edge

If the base binds, add a 0.4 to 0.6 mm chamfer at the bottom edge to avoid the elephant's foot effect.

Tell the agent your real measurements

My actual board measures 52.3 by 27.9 mm, not the library's size. Update the pocket to fit with 0.3 mm clearance per side.

Calibrate the printer

If every part is off by the same amount, print a small calibration piece (a simple test with a few hole sizes and pegs), and measure it. Then compensate in the printer or slicer, or reflect the offset in fit_clearance.

Reprint small test sections

Instead of reprinting the whole box, print a 10 mm tall slice of the box wall and lid lip. It takes 10 minutes, and tells you the right number.

How to avoid it

1. Plan for 0.2 to 0.4 mm clearance per side for FDM slip fits. See Tolerances and fit.
2. Store it in one parameter so you can retune in seconds.
3. Measure real parts with calipers instead of trusting catalog sizes for critical fits.
4. Print a small test coupon before a long print.
5. Print in the orientation you plan to use, since orientation changes accuracy.
6. Ask the model: "Check every mating pair for clearance" in Make. The interference and tolerance check is tuned to your process.

Try it

On the Hinge box template:

Check the clearance between the lid and the box on all four sides and at the hinge. Tell me which pairs are below 0.2 mm, and raise them to 0.3 mm through a fit_clearance parameter.