3D printing fits, clearances & threads
Design parts that assemble: distinguish clearance from tolerance, budget the mating pair, choose screw retention and validate snap fits in the final material and finish.
Start with what the joint must do
A lid that opens by hand, a locating pin and a bearing seat need different fits. Define the movement, load and service access before choosing a gap.
| Joint | Design priority | Acceptance check |
|---|---|---|
| Removable cover | Free assembly without excessive play | Check the complete perimeter after finishing, with the actual gasket and fasteners. |
| Locating pin or dowel | Position and repeatability | Budget both diameter and feature position; a loose hole can assemble but locate poorly. |
| Sliding or rotating part | Movement across the full travel | Check texture, straightness, debris, lubrication and the loaded condition. |
| Interference fit | Controlled interference and retention | Check assembly force, wall stress and long-term relaxation. Consider a machined seat or another retention method. |
| Printed-in-place mechanism | Separation plus cleaning access | Check that resin, powder or support can leave every moving interface. |
Budget both parts, then include the finished surface
Tolerance describes permitted size variation. Clearance is the space between mating features. Layer height and pixel size describe the process; neither is a guaranteed tolerance.
Design the mating pair
Clearance is the hole diameter minus the shaft diameter. Both parts contribute to the final fit.
Allow for the worst case
Minimum clearance = smallest hole − largest shaft. A nominal gap alone does not guarantee free movement.
Schematic illustrations; not to scale. Confirm the design for the chosen material, process and final use.
Explore the size stack
Change the example dimensions to see the worst-case diametral size difference. The starting values are an illustration, not a manufacturing capability.
Positive clearance throughout this size range
0.25 to 0.75 mm diametral size difference
Per-side difference: 0.125 to 0.375 mm. Positive means clearance; negative means interference.
Minimum = (D − hole tolerance) − (d + shaft tolerance). Maximum = (D + hole tolerance) − (d − shaft tolerance).
This checks size limits only. Position, form, roughness, temperature, moisture and the final finish can still prevent assembly. Use finished-part limits and validate the actual mating parts.
For a circular hole and shaft, diametral clearance is hole diameter minus shaft diameter. The worst-case minimum uses the smallest allowed hole and largest allowed shaft. Radial clearance is half the diametral clearance only for the ideal concentric circular pair.
The calculator handles size limits. Add separate checks for hole position, roundness, straightness, surface texture and temperature. A pair can satisfy its diameter limits and still bind because the axes are misaligned.
Specify inspection after the operations that control the final fit. For a uniform coating of thickness t on each mating surface, an internal diameter decreases by 2t and an external diameter increases by 2t. If both mating surfaces receive that coating, diametral clearance decreases by 4t. This is geometric bookkeeping; real coating thickness and distribution need confirmation.
Do not apply a universal clearance to every technology. Use the process-specific guides below, then confirm the allowance for the actual interface and finishing route.
Choose the fastener before drawing its hole
Leave material around the hole
Choose the insert or fastener first. Its geometry and installation method set the pilot hole, boss wall and edge distance.
Match the installation method
Heat-set, ultrasonic, press-fit and bonded inserts are different processes. Do not apply a thermoplastic heat-set rule to cured thermoset resin.
Schematic illustrations; not to scale. Confirm the design for the chosen material, process and final use.
| Retention method | Good reason to consider it | Design and review |
|---|---|---|
| Through bolt and nut | A serviceable joint with replaceable hardware | Provide tool access, a seating face and enough bearing area; avoid crushing a thin printed wall. |
| Captive nut | Repeated opening when the back is inaccessible | Provide a retention pocket, installation path and anti-rotation geometry. Check that the nut cannot pull through. |
| Threaded insert | Repeated assembly in a polymer body | Use the insert maker’s pilot hole, boss and installation guidance. Qualify pull-out and tightening torque in the actual printed orientation. |
| Tapped or thread-forming hole | A compact joint with a defined assembly procedure | Select compatible material and hardware. Provide engagement and debris access; test stripping torque and repeated use. |
| Printed thread | A coarse, accessible thread or prototype interface | Check flank detail, start, orientation and removal of internal support. Fine threads may be better machined or made with hardware. |
Heat-set inserts rely on a compatible thermoplastic installation process. Cured SLA and PolyJet photopolymers do not melt like thermoplastics; consider a qualified bonded insert, captive nut or through bolt instead.
A snap needs a strain budget, not just a gap
Separate the flexible arm, engagement feature and hard stop. The snap should deflect during assembly without relying on the whole enclosure to distort.
Use a representative snap coupon to compare geometry, then validate the complete assembly. A coupon cannot reproduce all enclosure distortion, adjacent ribs, sealing loads or user handling.
- Spread bending. Review arm length, thickness, taper and root radius together. More thickness can increase insertion force and strain rather than simply make the snap better.
- Specify the use cycle. One-time assembly, occasional battery access and daily opening are different requirements. Record insertion force, retention force, removal method and a test cycle count.
- Check the actual material. Ductility, conditioning, orientation, creep and temperature influence performance. A material-family name or tensile-strength value does not predict snap life.
- Make failure repairable. Consider a replaceable latch or screw closure when a broken integrated hook would discard the entire housing.
Send an assembly brief with the CAD
For scoped feature guidance, see the FDM, SLA, SLS, MJF, PolyJet and metal printing guides.
| Include | Useful detail |
|---|---|
| Mating parts | Both CAD models, their materials and the exact revision; include purchased hardware specifications. |
| Functional requirement | Sliding, locating, sealing or retention; permitted play, load and operating environment. |
| Drawing requirements | Critical dimensions and datums, allowable variation, surface condition and inspection stage. |
| Production route | Named material, build direction where functional, finishing, insert installation and any machining. |
| Validation plan | Fit sample, torque or retention check, repeated assembly and inspection of the complete mechanism. |
Frequently asked questions
What clearance should I use for 3D printed parts?
There is no universal value. Start with the specific process and material guidance, then account for both mating parts, interface size, orientation, surface texture and finishing. Printed-in-place joints also need a cleaning path. Validate the real assembly before repeating it.
Is 3D printer layer height the same as tolerance?
No. Layer height is the build increment. Final dimensional variation also depends on machine calibration, material behaviour, orientation, geometry and finishing. Define the required finished-part tolerance on the drawing and confirm it for the quote.
Can I print screw threads directly?
Some accessible, sufficiently coarse threads can be printed, but fine or heavily loaded threads may benefit from tapping, inserts or standard hardware. Check thread geometry, cleaning, torque and repeated use rather than assuming a printable thread meets a mechanical thread specification.
Can I use heat-set inserts in resin prints?
Do not apply a thermoplastic heat-set procedure to cured photopolymer resin. Review an adhesive-bonded insert, captive nut or through bolt, with the hardware supplier’s hole geometry and testing appropriate to the resin.
Sources and further reading
- Formlabs: adding screw threads to printed partsRetention methods and the distinction between thermoplastic and thermoset insert installation.
- Formlabs: snap-fit enclosure designGeometry and material considerations; sample dimensions are not universal fit rules.
- Formlabs Fuse 1 design specificationsProcess-specific fit and moving-assembly examples for Nylon 12.
- HP Multi Jet Fusion handbookMating-part and printed-assembly considerations; use the correct material and machine context.
Specify the details that make the part work.
Upload your CAD and drawing with the material, quantity, finishing and critical dimensions. Confirm the manufacturing route before production.