Your 3D printed part does not fit. Here is the five-minute test that ends the reprints
Too tight, too loose, reprint, repeat. Why printed fits miss by 0.2 mm, and how a five-minute fit coupon plus a printer calibration makes parts fit first time.Four hours of printing to learn one number
You designed (or described) a stand for your phone. The slicer says 3 h 50. You print it overnight, and in the morning the phone does not go into the slot. It is close. It is 0.2 mm from fitting. So you widen the slot, and print 3 h 50 again.
Almost everyone who prints functional parts has done this. The part that fails is rarely the shape: it is the fit, the few square centimetres where your print meets a real object. The rest of the plastic was never in doubt.
Why it misses, even when the model is exact
A printer squeezes molten plastic through a nozzle, and the bead comes out a little wider than the path the slicer planned. Outer walls grow by 0.1 to 0.2 mm. Holes and slots shrink by 0.2 to 0.3 mm. The exact amount depends on your machine, your nozzle, your filament and your profile. That is why a model that "fits perfectly" for the person who shared it jams on your printer.
Designers compensate with clearance, and good defaults exist (0.2 to 0.3 mm for a snug fit, 0.4 to 0.6 mm for a sliding one; the full table is in our tolerances guide). Defaults get you close. Close is the phone that almost goes in.
What experienced makers do
They do not print the part. They print a slice of it: three or four millimetres of the model, cut around the slot, the hole or the lid rim. It prints in minutes, it answers the only question that matters, and if it is wrong they change one number and print the slice again.
It is the right method. It also needs CAD: a boolean cut in the right place, the same orientation as the final print (a printer is not equally accurate along X/Y and along Z), and the patience to do it for every part. Most people skip it, which is how the 3 h 50 reprint happens.
Fit Check: the slice, without the CAD
SpeakCAD now does this for you, and then goes one step further.
- The AI marks the fit. Describe the part and what it mates with ("my phone, 78 x 11 mm in its case"). The AI gives that interface its own clearance and marks the small volume around it. A "Must fit" chip appears under the model.
- SpeakCAD cuts the coupon. Open Fit check and you see the slice, solid, inside the translucent full part. It is cut from your model itself, in the print orientation of the final part. Downloading it is free: no credits, on any plan. On Pro it can be G-code already sliced for your printer.
- You print it. About five minutes, a gram or two of filament.
- You answer one question. Try it on the real object and tap Too tight, Perfect or Too loose on your phone, standing at the printer. The model rebuilds with 0.1 mm more or less clearance. If your answers cross over (tight, then loose), the step drops to 0.05 mm.
- Your printer is remembered. This is the part no slicer and no CAD tool does. The verdicts teach SpeakCAD how *your* printer prints, and once a coupon is perfect that calibration is saved on your account. Every model after that (new ones, old ones you reopen, models you buy on the marketplace) is compiled with the clearance your printer needs. The second part you design tends to fit on the first print.
What it costs, compared
| Reprint the whole part | Fit coupon | |
|---|---|---|
| Print time per try | 2 to 6 hours | About 5 minutes |
| Filament per try | 40 to 150 g | 1 to 2 g |
| CAD skill needed | Edit the model each time | None: three buttons |
| What the next part learns from it | Nothing | Your printer's calibration |
Tips for a test you can trust
- Same filament, same nozzle, same print profile as the final part.
- Do not scale or rotate the coupon in the slicer.
- Let it cool for a minute before trying it.
- Test with the real object in its real state: the phone in its case, the magnet you will actually use.
- Changed nozzle or filament brand? Reset the calibration in the Fit check panel and run one coupon.
Try it on a part you have been avoiding
The parts people put off are the ones with a fit, because a miss is expensive. With a five-minute test and a printer that is remembered, they become the easy ones. The Fit Check guide has the step-by-step; the prompts below open the editor with a fit already in them.
Prompts to start from
Open one in the builder, change the numbers to yours, and the model renders in seconds. Keep adjusting in the chat before you print.A desk stand for my phone: the phone is 78 mm wide and 11 mm thick in its case, leaning back 20 degrees, with a cable slot underneath. The phone must slide into the slot easily.
An enclosure for a 70 x 50 mm PCB, 1.6 mm thick, with four M3 mounting posts on a 62 x 42 mm pattern, a USB-C cutout on one short side and a snap-on lid.
A fridge hook with two 6 x 3 mm round magnets pressed into the back, holding up to 1 kg.
Questions
Because the printer is not exact: extruded plastic spreads, so outer walls come out 0.1 to 0.2 mm larger and holes 0.2 to 0.3 mm smaller, by an amount that depends on the machine, nozzle, filament and profile. The model needs a clearance matched to your printer. SpeakCAD's Fit Check finds it with a five-minute test slice and three buttons (too tight, perfect, too loose), then saves it as your printer's calibration.
Print only a thin slice of the model around the feature that must fit, in the same orientation as the final print. In SpeakCAD this is automatic: the AI marks the fit, Fit check cuts the coupon from your model, and the coupon downloads free as STL or 3MF (or as sliced G-code on Pro). It prints in about five minutes.
Yes. Downloading a fit coupon as STL or 3MF costs no credits on any plan. A coupon sliced to G-code for your printer is part of Pro, like every G-code download.
No. Once a coupon has been reported perfect, your printer's calibration is saved on your account and applied to every model you build or buy afterwards. Run a coupon again for a fit that really matters, or after changing nozzle or filament brand.