Tolerance & Fit Guide

Reference clearances for snap fits, press fits, and sliding fits based on printer accuracy and material.

How It Works

Fit Types Explained

Engineering fits describe how two mating parts relate in size. For 3D printing, standard ISO/ANSI tolerances are too tight, so wider clearances are used to account for printer inaccuracy and material behavior.

• Press Fit (Interference): The shaft is larger than the hole. Parts must be forced together and hold by friction. Used for permanent assemblies, bearing seats, and pins that should not move.
• Snap Fit: Designed with a slight interference plus a deflecting feature (cantilever, annular, or torsion snap). The part flexes during assembly and locks in place. Material flexibility is critical.
• Transition Fit: Parts are nearly the same size with minimal clearance or slight interference. May require light tapping. Used when parts need to be locatable but removable.
• Sliding Fit: A small gap allows parts to slide freely but with minimal play. Used for drawers, telescoping tubes, and linear guides.
• Clearance Fit: A generous gap ensures parts always fit together easily, even with printer inaccuracy. Used for loose assemblies, covers, and parts that must be easy to assemble and disassemble.

How Clearances Are Calculated

The recommended clearances combine three factors:

1. Printer accuracy: The base tolerance of the printer type (e.g., ±0.3 mm for standard FDM). Tighter printers allow tighter fits.
2. Material behavior: Each material has a shrinkage factor and stiffness that shifts the effective clearance. Flexible materials like TPU need more clearance for press fits; rigid materials like PLA need more clearance for snap fits.
3. Nominal dimension: Larger parts need proportionally more clearance as dimensional errors scale with size.

Material Considerations

• PLA: Low shrinkage, rigid. Good for press fits and tight tolerances. Brittle under stress — poor for snap fits that flex repeatedly.
• PETG: Moderate shrinkage, slight flexibility. Good all-around material for functional fits. Better snap fit durability than PLA.
• ABS: Higher shrinkage (0.5–0.8%), needs enclosed printer. Good for snap fits due to toughness. Needs larger tolerances to account for warping.
• Nylon: High shrinkage (1–2%), hygroscopic. Excellent for snap fits and bearing surfaces. Needs significantly larger tolerances.
• ASA: UV-resistant alternative to ABS with similar mechanical properties. Ideal for outdoor functional parts. Needs enclosed printer.
• Polycarbonate (PC): Very high strength and heat resistance (up to 110°C). Excellent for structural press fits. Requires high temps (260–300°C) and enclosure.
• Polypropylene (PP): Chemical resistant with excellent living hinge capability. Very high shrinkage (1.5–2.5%) makes tight tolerances difficult. Poor bed adhesion.
• TPU: Flexible, very low shrinkage. Unsuitable for press fits (deforms). Good for gaskets and seals where compliance is needed.
• Resin: Very accurate, minimal shrinkage. Brittle — avoid snap fits. Excellent for press fits and tight sliding fits.