Fit test coupon

One print that tells you your printer’s real clearance — then feed it back into every joint.

Every printed joint is decided by one number: how much slack the two halves need. It is not a property of the design — it is a property of your printer, your filament and your slicer — so no default can be right for everyone, and the usual way to find it is to print a joint, discover it binds, and print it again.

This coupon replaces those three prints with one. The gauge plate carries a row of holes (or slots) stepping through a clearance range, each engraved with its own value, and beside it stands one gauge pin (or tab) at the nominal size. Print both, then work the gauge along the row from the tight end. The first station it enters cleanly is your printer’s real clearance for that fit.

Then use it. Every joint generator — snap-fit, annular snap, pin hinge, ball joint — carries a Fit control with Tight, Normal, Loose and Custom. Pick Custom and type the number you just read off, and that joint is built to your printer instead of to an average one.

Do it twice. A round bore and a flat slot are not the same measurement: the perimeter order and the seam land differently, and most printers come out looser on one than the other. Run the coupon once in each mode and keep both numbers.

Nothing about the coupon is guessed for you except the layout: the plate sizes itself around the widest station in the row, so widening the step or adding stations grows the part rather than crowding it.

1. Open the Generate panel from the left sidebar and choose Fit test coupon (under Calibration).
2. The coupon plate and its gauge drop in as editable parts. The parameters open in the Properties panel, where you set them.
3. Pick the feature — Pin & hole for dowels, pins and bores; Tab & slot for flat joinery.
4. Set the nominal size to whatever you actually build with, and the plate thickness the gauge has to pass through.
5. Set the first clearance and the step — the read-out under the fields shows the range the row will sweep.
6. Print both parts and push the gauge into each hole from the tight end. The first one it enters is your number.