3D printing tolerances: plan fits, holes and threads

A small layer height describes vertical detail, but it does not guarantee that every finished dimension matches CAD. Material shrinkage, extrusion, orientation, support contact and post-processing can all affect the final geometry.

Resolution describes the detail a process can represent. Repeatability describes how consistently it repeats a result. Accuracy describes closeness to the target, while tolerance defines the acceptable range for the function.

Treat each critical feature separately. A decorative surface may accept a visible deviation that would make a hinge, bearing seat or sliding connector unusable.

FDM dimensions are influenced by nozzle path, line width, cooling, shrinkage, first-layer compression and print orientation. Internal holes can finish smaller, outside contours can finish slightly larger, and an elephant foot can disturb a mating surface near the build plate.

A sliding fit, a loose locating joint and a press fit need different clearance. Part size, contact length, material and assembly direction also change how that clearance feels.

Do not give both mating parts the same nominal CAD dimension and assume the printer will create a working joint. When the fit matters, test several candidate clearances in a small representative feature.

SLA can reproduce fine features and smooth surfaces, but support placement, resin shrinkage, washing and post-curing can change dimensions. Fine detail does not remove the need to validate a functional fit.

SLS avoids conventional support scars, yet its surface, powder removal and process-specific dimensional behaviour still affect moving parts and internal channels. Assemblies printed in place require enough freedom for powder removal and movement.

Choose a process for the actual feature: a smooth visible shell, a durable sliding fit, an internal channel or repeatable batch production may lead to different decisions.

A printed hole is not automatically equivalent to a drilled hole. Decide whether a fastener only passes through, cuts into the plastic, engages a printed thread or uses a separate insert.

Large coarse printed threads can work when the profile, clearance and load are appropriate. Small threads or joints opened repeatedly are often more dependable with post-machining or a suitable threaded insert.

A screw boss needs enough surrounding material and a gradual connection to the main wall. A thin cylinder with a hole can split more easily than a reinforced boss with ribs and a considered layer orientation.

A tolerance coupon can combine holes, pins, slots, snap details or several clearances with little material. It reveals how the selected process, material, orientation and finishing method work together.

Use the real mating part, fastener and assembly tool during the test. A fit that feels correct by hand may behave differently under load or after repeated use.

For repeat orders, record the validated model revision and production assumptions. A change in material, process or orientation can require another coupon even when the nominal CAD dimensions stay unchanged.

A printing service cannot infer whether a gap is cosmetic, functional or irrelevant. Identify the dimensions that control motion, fastening, sealing or alignment and state which post-processing steps are acceptable.

Do not demand the same tolerance for every surface. A focused requirement is easier to review and manufacture than a blanket expectation that every CAD dimension must be equally exact.

If the required fit has not been validated for the chosen material and process, order the coupon or first article before the complete quantity.