FDM Tolerances: What Accuracy You Can Actually Design For

Buyers regularly ask what tolerance we hold. The honest answer is that FDM tolerance depends on material, part size, geometry and orientation, not on a single machine specification. What we can do is tell you the ranges we plan production around and the design decisions that keep a part inside them.

The ranges we plan around

• Dimensionally stable materials such as PLA, PETG and the carbon filled grades: roughly ±0.2 mm on small features, or ±0.2 percent of the nominal dimension on parts above 100 mm, whichever is larger
• Shrinking materials such as ABS, ASA and PC: closer to ±0.3 mm or ±0.3 percent, with more deviation on long flat sections
• Unfilled PA12 and PA6: the widest range, because the material absorbs moisture and continues to move dimensionally after printing
• Z height is usually the most repeatable direction, in-plane features are affected more by material shrinkage and cooling

Where deviation actually comes from

Four effects dominate. Shrinkage as the polymer cools, which scales with part length. Extrusion width, which makes a nozzle path slightly wider than the nominal wall and pulls holes undersize. Thermal warping on large flat areas, which lifts corners and changes flatness rather than a single dimension. And elephant foot on the first layers, where squeeze creates a slightly wider base. None of these are random, which is why the same file printed with the same profile repeats well from batch to batch.

Holes print small, and by a predictable amount

A vertical hole in FDM comes out undersize because the extruded path is approximated with straight segments and pushed slightly inward. On a 5 mm hole the loss is often 0.1 to 0.3 mm depending on material. For anything that has to take a bolt, a bearing or a pin, design the hole nominal and let us either compensate in the profile or leave a machining allowance and ream it after printing. Reaming a printed hole is fast and gives a far better result than guessing at an oversize.

Design fits with clearance, not with tolerance stacks

• Free running clearance between printed parts: 0.3 to 0.5 mm total
• Sliding fit that should feel guided: 0.15 to 0.25 mm
• Press fit for a printed pin in a printed hole: 0.05 to 0.1 mm interference, and expect to adjust once
• Threaded joints: use a heat set insert or a machine screw into a brass insert instead of a printed thread wherever the joint is opened more than a few times

Tell us the critical dimensions

The most useful thing you can send with a file is a drawing or a short note marking the two or three dimensions that must be right. That changes how we orient the part, which surfaces face the build plate, whether we add a machining allowance and what we measure on the first article. A part with three critical dimensions is straightforward. A part where every dimension is declared critical usually means the geometry belongs in machining, and we will say so.

How we keep tolerances stable across a batch

• One documented parameter set per part, locked after the first article is approved
• Dried filament from a controlled batch, because moisture changes extrusion and therefore dimensions
• The same orientation and plate position on every repeat order
• Dimensional inspection of the marked critical features, with results recorded per batch

If you have a part where a fit has failed before, send the geometry and the dimension that caused the problem. In most cases the fix is orientation, a clearance change or a reamed hole, not a different supplier.