How do you match the tolerances of a replacement part?
Design in clearance rather than copying the measurement. The University of Florida makerspace guide gives 0.15 to 0.20 mm for a snug locating fit, 0.20 to 0.25 mm for a sliding fit and 0.30 to 0.40 mm for a loose one. Print a tolerance test in the part’s filament first, since Prusa quotes 0.3 mm accuracy in X and Y.
The clearances
| Fit | Clearance | Used for | Source |
|---|---|---|---|
| Snug / alignment | 0.15 to 0.20 mm | Locating features | Marston Makerspace, University of Florida |
| Standard sliding | 0.20 to 0.25 mm | Tabs, rails, enclosures | Marston Makerspace, University of Florida |
| Loose | 0.30 to 0.40 mm | Covers, adjustable components | Marston Makerspace, University of Florida |
| Print-in-place | The larger end of the sliding and loose ranges | Hinges printed assembled | Marston Makerspace, University of Florida |
| Snap-fit clip | 0.5 mm | The moving arm of a cantilever clip | Protolabs Network |
The clearance is the gap between the two parts. Which part gives it up depends on which one you print. A bought shaft keeps its size, so the printed hole carries the whole gap.
- Bought shaft
- Clearance gap
- Printed part
Why you cannot copy the measurement
The printer’s error is larger than the caliper’s. Prusa Research puts a well-assembled Original Prusa at 0.1 mm on Z and 0.3 mm on X and Y, improvable to 0.05 mm on all axes after extrusion multiplier and extruder linearity calibration. The measuring procedure reads far finer than the printer’s band. The printer then places the surface anywhere inside that band.
A bore modeled at exactly the measured diameter therefore prints too small half the time. Bambu Lab’s wiki names two settings that pull holes back to size: filament shrinkage and XY hole and contour compensation. They are corrections to calibrate, not values to assume. Once a test part is measured, a shrinkage scale-factor calculator turns the error into the value each slicer expects. Clearance leaves the fit loose when the errors stack the wrong way, instead of jammed.
No cited source gives an interference value for a true press fit, where the peg is larger than the hole. Start from the snug band and close it on a test coupon.
The procedure
- Decide what each fit does. A locating peg wants snug, a lid wants sliding, a cover wants loose.
- Add the clearance from the table on the printed part. Holes get it as extra diameter, pegs as reduced diameter.
- Print the fit before the part. Bambu Studio ships a built-in model, added as the primitive ksmFDMTest, whose nested cylinders are there to inspect fit tightness and whose pins must all come out. On any other printer, a downloaded tolerance test model with stepped clearances does the same job.
- If the cylinders stick, work through Bambu Lab’s checks in order: cleaning and lubrication, then belt tension, then machine accuracy calibration, then filament shrinkage, then hole and contour compensation. Only after those does the clearance itself need to change.
- Print a coupon of the real feature in the real filament and orientation, then measure it against the mating part. A coupon costs minutes; a full part costs hours.
- Design in compliance where you can. The University of Florida guide’s tools are lead-ins on pegs and tabs, mouse ears at internal corners and flexible arms, all of which forgive a tenth of a millimeter that a rigid fit will not.
Holes and orientation
Round holes print best with their axis vertical. The University of Florida guide’s fallback for a horizontal hole is the teardrop shape, so the roof does not sag. A hole that must be exact can also be drilled or reamed after printing: model it a step small and finish it with the tool, which takes the printer’s XY band out of the fit altogether.
Orientation also moves the numbers. Prusa’s 0.1 mm Z figure and 0.3 mm XY figure mean a fit-critical dimension is held tighter when it runs along Z. That competes with strength, which wants the load along the layers, so read the orientation rule before committing.
Why do 3D printed parts not fit together?
Six causes stop printed parts fitting: no clearance designed in, elephant’s foot flaring the first layer, over-extrusion, holes printing undersized, shrinkage as ABS, ASA or nylon cools, and warped corners. Each leaves its own mark, so a few caliper readings on a test cube or hole show which one to fix first.
Causes, checks and fixes
| Cause | What you see | Check | Fix |
|---|---|---|---|
| No clearance designed in | Every fit binds, even off a tuned printer | Compare the model’s hole and peg sizes | Add the gap from the clearance table above |
| Elephant’s foot | The bottom layer sticks out, so parts jam only at the base | Calipers across the first layer, then mid-height | Elephant foot compensation, under Print settings, Advanced in PrusaSlicer; Prusa says around 0.2 mm suits a 0.4 mm nozzle |
| Over-extrusion | Rough top surfaces and oversized walls | Measure the wall of Prusa’s 40 mm single-wall cube | Set the extrusion multiplier to 0.45 divided by the measured wall; Prusa’s adjusted values usually run 0.9 to 1.1 |
| Undersized holes | Outer sizes are right, holes are small | Measure a printed hole against the model | X-Y hole compensation (positive enlarges) or polyhole conversion |
| Shrinkage | The whole part reads small in acrylonitrile butadiene styrene (ABS), acrylonitrile styrene acrylate (ASA) or nylon | Measure a known length against the model | Scale by the filament shrinkage setting; Prusa puts ABS shrinkage at about 1 to 2% |
| Warping | Lifted corners, so mating faces rock | Lay it on a flat surface | An enclosure, a brim, and a bed wiped with 90% or stronger isopropyl alcohol |
Values come from the Prusa Knowledge Base and the OrcaSlicer wiki; Bambu Studio has the same compensation settings. A part lifted at one corner only has its own diagnosis.
Why printed holes come out small
Exported CAD turns a cylinder into a polygon whose flats sit inside the true circle, and the nozzle cuts corners as it drags filament round the curve, per nophead’s HydraRaptor polyhole write-up. OrcaSlicer’s polyhole conversion prints the hole with fewer sides set on the circle.
Which cause to check first
Print a small cube with a round hole in the part’s filament, then take three caliper readings:
- First layer against mid-height. A wider base means elephant’s foot.
- Mid-height against the model. Oversized points to flow; undersized in ABS, ASA or nylon points to shrinkage.
- The hole against the model. Small with true outer sizes means hole compensation.
If all three read true and parts still bind, the design needs clearance.
Sources
- Designing for 3D Printing Tolerances on the MK4 and XL (Marston Makerspace, University of Florida Libraries) (accessed September 23, 2026)
- FAQ - Frequently Asked Questions, printing precision/tolerance of the Original Prusa (Prusa Knowledge Base) (accessed September 23, 2026)
- Quality Test Print and Calibration in Bambu Studio (Bambu Lab Wiki) (accessed September 23, 2026)
- How do you design snap-fit joints for 3D printing? (Protolabs Network knowledge base) (accessed September 23, 2026)
- Elephant foot compensation, Prusa Knowledge Base (accessed October 13, 2026)
- Elephant foot, Bambu Studio parameter (Bambu Lab Wiki) (accessed October 13, 2026)
- Extrusion multiplier calibration, Prusa Knowledge Base (accessed October 13, 2026)
- Flow Ratio Calibration (OrcaSlicer Wiki) (accessed October 13, 2026)
- Precision: elephant foot, X-Y hole compensation and polyholes (OrcaSlicer Wiki) (accessed October 13, 2026)
- Polyholes, HydraRaptor (nophead) (accessed October 13, 2026)
- ABS, Prusa Knowledge Base (accessed October 13, 2026)
- Warping, Prusa Knowledge Base (accessed October 13, 2026)