Tolerances and Fit: Printing Parts That Click Together

Danielle A.

The Gap Between Design and Reality

Designing a part in CAD with perfect dimensions doesn't guarantee that those dimensions will be the same when printed. FDM printing introduces systematic deviations from the designed geometry, some predictable and compensatable, others variable and harder to account for. Understanding these deviations and designing appropriate clearances and tolerances into your parts is what separates functional assemblies that work the first time from those that require three iterations and significant filing before fitting together.

The foundational insight: don't design mating parts to fit perfectly with zero clearance. They never will in FDM. Design in the clearance you need from the start, verify it against your specific printer's actual dimensional accuracy, and iterate quickly on calibration pieces before committing to the full assembly.

Systematic FDM Dimensional Deviations

FDM printing has several characteristic and generally consistent dimensional deviations. 

Hole undersizing: 

• Circular holes print systematically smaller than designed, typically 0.1–0.2mm undersized for a 0.4mm nozzle.
• A 10mm hole in your CAD model may print as 9.8–9.85mm. Design holes 0.1–0.2mm oversize, or account for this in your clearance values. 

First layer expansion (elephant's foot): 

• The first few layers often spread slightly wider than the design due to the nozzle pressing the plastic against the bed.
• This affects the accuracy of the base perimeter.
• Fix it with elephant's foot compensation in the slicer or Z offset adjustment. 

Thermal contraction: 

• Larger prints contract as they cool, generally proportionally to print temperature and material.
• ABS and ASA contract significantly; PLA contracts less.

Fit Types and Recommended Clearances

Working clearance values for PLA on a properly calibrated printer: 

• Press fit (requires force to assemble and stays assembled permanently): 0.0–0.1 mm clearance on each mating surface. 
• Tight/slip fit (smooth, hand pressable assembly, no slop): 0.15–0.25 mm.
• Running/sliding fit (moves freely, light hand clearance visible): 0.25–0.35 mm. 
• Loose fit (easily assembled and disassembled, visible gap): 0.4–0.6 mm.
  • Adjust upward by 0.05–0.1 mm for PETG (which tends to print slightly larger) and more significantly for ABS (which contracts during cooling, making clearances tighter after cooling than they appear at print temperature).

The Calibration Piece Workflow

For any new assembly design, print a calibration set first: a simple pin of the target diameter, a block with a matching hole, and a mating interface of the target clearance. Test them, measure the gap, and adjust your CAD model before printing the full assembly. This 20 minute investment prevents 3-hour reprints and saves expensive filament on large multi part assemblies. Use the flow rate calibration process to ensure your printer's overall dimensional accuracy is dialled in before testing fit tolerances, as an uncalibrated flow rate introduces its own systematic dimensional errors that make tolerance design impossible to get right.