3D printing tolerances and fits — a practical guide

Two parts that touch perfectly in CAD will usually not move freely after printing. Actual dimensions depend on process, material, orientation, shrinkage and calibration. Mating geometry therefore needs an intentional tolerance and clearance strategy.

Nominal size, tolerance and clearance are different

• Nominal size is the value in the model.
• Tolerance is the permitted variation in the finished feature.
• Clearance is the deliberate difference between mating parts.

If a shaft and hole share the same nominal diameter, the digital assembly may look perfect but the physical components are likely to bind. First decide whether the joint should be loose, sliding, rotating, press-fit or permanent.

Define the type of fit first

Loose assembly

Parts should assemble and separate easily. This is useful for covers, low-precision guides and hand-assembled enclosures.

Sliding or rotating fit

The component should move without obvious binding. Surface texture, contact length and layer direction matter alongside the nominal gap. A long bushing is less forgiving than a short opening.

Press fit

Parts are retained by controlled interference. Printed press fits are sensitive to process variation, so an entry chamfer, short engagement zone and test sample are valuable.

Snap fit

A clip must deform during assembly and recover. Root radius, layer direction, material and expected cycle count matter more than the hook dimension alone.

Why holes often need special attention

A deposited path has finite width and its edge is not mathematically perfect. The relative effect is larger in small holes. A horizontal hole may also have an uneven upper region where material bridges open space.

A practical workflow is to mark the hole as critical, add an entry chamfer, choose a favourable orientation, allow for drilling or reaming when needed, and print a small gauge before the full part.

Agree how the feature will be measured

“A 10 mm hole” might mean a two-axis caliper measurement, a go/no-go pin check or the final diameter after reaming. State:

• which dimensions are critical,
• what component they mate with,
• whether post-machining is allowed,
• whether easy assembly, low play or smooth movement is the priority,
• how often the joint will be assembled or cycled.

The safest method is a test coupon

A coupon contains several hole, slot or clip variants in a small model. It is printed in the same material, orientation and comparable settings as the final part.

It is especially worthwhile when the full part is large, the fit is press-based, several components form a mechanism, or the design will be repeated as a series. The selected result can then be transferred back into CAD.

Threads, screws and inserts

Large and lightly used threads can be printed directly. Fine or repeatedly used threads may be better produced with a tap, threaded insert or retained nut. Provide enough material around the joint; high infill alone does not replace a properly designed boss, wall paths and stress-relieving radii.

Pre-submission checklist

• Is the connection fixed, sliding, rotating or press-fit?
• Are critical surfaces and dimensions identified?
• Is assembly one-time or repeated?
• Can the hole be drilled, tapped or fitted with an insert?
• Would a coupon or partial test reduce risk?
• Does orientation weaken the clip or boss?

Tolerance is not a correction added after manufacturing. It is part of the design. The earlier the intended fit is described, the easier it is to choose a process and avoid an expensive iteration.