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The reliable way to design 3D-printed parts is to choose the required fit first, then validate a small feature-specific test coupon. There is no universal “3D-printing tolerance.” The result depends on the process, material, printer settings, orientation, geometry, post-processing, and measurement method.
For a practical workflow: identify the interface, separate manufacturing error from functional clearance, add the fit parametrically in CAD, print the relevant features, measure them, test the actual assembly, and revise only after seeing the result.
Tolerance, clearance, and allowance are different
These terms describe different parts of the design problem:
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- Nominal dimension: The CAD value, such as a 10.00 mm hole.
- Actual dimension: The size produced by the printer.
- Dimensional tolerance: The acceptable variation around the nominal dimension.
- Clearance: Intentional space between mating surfaces so parts can move or assemble.
- Interference: Intentional overlap between mating dimensions for a press fit.
- Allowance: A deliberate offset used to achieve a particular fit.
- Accuracy: How close the printed feature is to its nominal CAD dimension.
- Repeatability: How consistently the process reproduces that dimension.
- Resolution: A process increment, such as layer height, pixel size, or nominal nozzle movement. It is not the same as usable accuracy.
- Compensation: A CAD or slicer adjustment made to counter a predictable process error.
A 0.4 mm nozzle or 0.1 mm layer height does not prove that a printer can reliably hold ±0.1 mm. Extrusion width, curing, shrinkage, warping, layer orientation, feature geometry, and measurement uncertainty all affect the finished part.
Prusa describes 0.2 mm as a general dimensional-accuracy reference for an Original Prusa and recommends at least 0.3 mm as an initial value for movable parts, while warning that material shrinkage and warping matter. That is useful starting guidance for the stated printer family, not a universal specification. Prusa’s modeling guidance explains the qualification.
Choose the fit before choosing a number
“Add 0.2 mm” is incomplete advice. The correct value depends on whether the parts must move, locate, retain one another, or seal.
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|---|---|---|
| Loose clearance | Moves freely with visible play | Use a generous gap for covers, removable parts, and dirty environments. |
| Sliding | Moves while remaining guided | Use a validated small gap, short bearing surfaces, good alignment, and smooth functional faces. |
| Locating or snug | Aligns accurately but can still be separated | Use modest clearance and a lead-in chamfer. |
| Press or interference | Stays together through friction and elastic deformation | Make the pin larger than the hole, or the boss larger than the socket, then test insertion force. |
| Snap fit | Flexes over a retaining feature and returns to hold | Design for deflection, strain, root radius, and cycle life—not just interference. |
| Print-in-place hinge | Moves without assembly | Allow for fused surfaces, support residue, trapped powder, and cleaning access. |
| Threaded | Engages and disengages repeatedly | Use robust, relatively coarse geometry and validate flank and root clearance. |
| Sealing | Controls air or fluid leakage | Use a gasket, O-ring, sealant, insert, or machined surface when the printed surface is not dependable enough. |
Formlabs’ engineering-fit guidance similarly treats clearance as a relationship determined by the intended use, not as a universal printer constant.
Clearance: per side or total?
Always state which convention you are using. For a round pin inside a round hole:
hole diameter = pin diameter + total diametral clearance
If the pin is 10.00 mm and the desired total diametral clearance is 0.20 mm, model a 10.20 mm hole. If “0.20 mm clearance per side” is intended, the hole must be 10.40 mm because the gap is added on both sides.
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For a rectangular sliding fit, enlarge the receiving slot in both relevant axes. Do not assume that error is equal in X, Y, and Z. Add a chamfer or taper to the insertion edge so a small amount of warping or dimensional error does not prevent assembly.
Starting values by printing process
Use the following numbers to choose the first test coupon—not to skip validation.
FDM and FFF
For a general-purpose movable FDM feature, a test series of 0.20, 0.30, 0.40, and 0.50 mm total clearance is a practical starting matrix. Prusa specifically suggests at least 0.3 mm for movable parts. A different nozzle, material, printer, orientation, or slicer profile may require more or less.
FDM holes commonly print undersized because of line placement, corner bulging, extrusion calibration, and the difficulty of approximating curves with deposited lines. Account for nozzle diameter and line width, but do not treat nozzle diameter as tolerance.
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Smaller gaps may be possible than on many FDM systems, but narrow clearances can close through overexposure, overcure, washing residue, support marks, or post-cure shrinkage. Resin type also matters: a stiff resin, tough resin, and flexible resin can produce very different assembly behavior. Use the printer and resin manufacturer’s design guide, orient the functional faces deliberately, and validate the washed and cured part.
Forge Labs’ SLA guidance highlights the influence of orientation, layer height, and machine tolerances and recommends an initial test print.
SLS and MJF
Powder-bed processes avoid many FDM support problems, but they introduce powder removal, thermal effects, surface texture, part spacing, and build-location variables. Integrated assemblies and separate nested parts need different clearances.
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For the Formlabs Fuse 1 generation, the published guide lists minimum assembly tolerances of 0.2 mm for features below 20 mm² and 0.4 mm for larger features. It lists integrated assembly clearances of 0.3 mm and 0.6 mm respectively, and recommends 5 mm spacing between separate parts, with 1 mm as a minimum. These figures apply to that process guidance and should not be transferred to every SLS or MJF machine. See the Fuse 1 design specifications.
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Use the quoted tolerance and design guide for the exact service, machine, material, build orientation, and finishing option. Commercial references from Protolabs, Makelab, and Fathom publish process-specific values, but their figures remain design guidance rather than guarantees for every geometry.
Variables that change the result
FDM and FFF variables
- Nozzle diameter, selected line width, extrusion flow, and extrusion calibration.
- Layer height, print temperature, cooling, and material moisture.
- Warping, bed adhesion, and first-layer squish.
- Elephant’s foot at the build plate, which can close a hole or tighten a sliding interface near the first layer.
- XY versus Z behavior and the orientation of holes, pins, and mating faces.
- Wall count, thin-wall handling, bridges, overhangs, and support residue.
- Layer seam placement and the direction of the expected load.
Prusa notes that materials can warp or shrink and that thin walls may require special slicer treatment such as “Detect thin walls.” Stratasys Direct’s FDM guidance likewise emphasizes that the extrusion process creates FDM-specific design constraints.
Resin variables
Exposure compensation, resin formulation, washing, drying, post-curing time and temperature, support placement, peel forces, orientation, trapped resin, and sanding all affect the interface. A support scar on a mating face may matter more than the nominal resolution of the printer. Hollow components also need drain holes and a reliable way to remove residual resin.
SLS and MJF variables
Consider powder escape paths, thermal distortion, build position, part spacing, surface texture, and the size of large flat areas. A print-in-place joint must be both printable and cleanable; an enclosed cavity with no powder-removal route is a design failure regardless of its nominal clearance. Forge Labs’ SLS guidance discusses these process-dependent assembly considerations.
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Put the fit into CAD parametrically
Keep nominal geometry separate from manufacturing compensation. A simple parameter table might contain:
pin_nominal = 10.00 mm
clearance_total = 0.30 mm
hole_diameter = pin_nominal + clearance_total
press_fit_allowance = 0.10 mm
press_hole_diameter = pin_nominal - press_fit_allowanceThe press-fit value above is only an example parameter for testing. It is not a universal interference specification.
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Named parameters let you create loose, sliding, snug, and press-fit variants without manually editing individual faces. Parametric CAD also makes it easier to document which dimensions express design intent. Autodesk describes Fusion’s parametric and print-preparation workflow.
- Offset only functional faces; do not scale the entire body to fix one hole.
- Keep a nominal model and a separate manufacturing-compensation parameter.
- Add lead-in chamfers or tapers to insertion edges.
- Add relief grooves at the end of long sliding fits so debris does not jam the parts.
- Use fillets at snap-fit roots and loaded corners.
- Use slots, shims, or adjustable features where the interface must tolerate variation.
- Reserve material intentionally if a hole will be reamed, drilled, sanded, or machined.
Print a feature-specific tolerance coupon
A calibration cube can reveal gross scaling problems, but it cannot predict the fit of a hole, snap, thread, or hinge. The most useful test contains the actual interface at the actual scale and orientation.
- Model variants. Include holes and pins, slot widths, gap sizes, or the actual snap, thread, dovetail, or hinge geometry.
- Label every variant. Emboss or engrave the clearance value so it remains identifiable after printing.
- Match production conditions. Use the final material, nozzle or layer height, slicer settings, orientation, and approximate wall thickness.
- Post-process identically. Remove supports, wash and cure resin, remove powder, sand, deburr, or apply the same finish planned for the final part.
- Measure the coupon. Record actual pin, hole, slot, wall, and gap dimensions.
- Test the real mating part. Assess insertion force, play, sliding behavior, retention, and repeat assembly—not merely whether the pieces enter once.
- Choose the best variant. Record the value and the process conditions that produced it.
- Update the parameter. Print the full part only after the interface has passed the small test.
This process is recommended in different forms by Prusa, Formlabs, and Forge Labs.
Measure holes and pins correctly
Calipers are useful for ordinary fits, but they have limitations. Outside jaws can measure a pin reasonably well; inside jaws are less reliable for small printed holes, especially when the mouth is chamfered, rounded, elephant-footed, or support-marked.
- Measure at the functional depth, not only at the opening.
- Measure a circular feature at several angles to detect ovality.
- Use gauge pins, a bore gauge, a micrometer, or carefully selected drill bits as go/no-go gauges for tighter holes.
- Measure several parts to separate systematic error from random variation.
- Record the instrument, location, orientation, and post-processing state.
Fusion’s manual-inspection workflow supports comparing measurements from calipers, micrometers, and height gauges with nominal dimensions and tolerance limits.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Correct the process before distorting the design
Use the pattern of the error to decide what to change:
- All dimensions are wrong in a similar way: Check printer scaling, extrusion flow, resin exposure, material condition, temperature, and calibration.
- Only holes are wrong: Check hole orientation, line width, exposure, and feature-specific hole compensation.
- Only the first layer is tight: Correct elephant’s foot or first-layer squish.
- Only supported faces fail: Move supports, change orientation, or add a controlled finishing allowance.
- The fit works once but later jams: Check thermal expansion, contamination, incomplete curing, moisture, powder, or material creep.
- A press fit cracks the part: Reduce interference, increase wall thickness, add a lead-in, change orientation, or redesign the load path.
Do not enlarge every hole in CAD when the printer is globally over-extruding. Fix a global process error globally. Use local CAD compensation when the printer is reasonably calibrated but a particular geometry repeatedly prints undersized.
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Special cases
Press fits
Press-fit behavior depends on material elasticity, wall thickness, layer direction, pin texture, insertion force, temperature, and long-term creep. Use a gradual interference series rather than one aggressive value. A brittle resin or a part whose layers split under radial stress may be unsuitable for a printed press fit. Decide whether the fit is permanent or serviceable and specify the acceptable insertion force.
Snap fits
A snap fit is a flexure, not merely an interference fit. Design its hinge length, deflection, root radius, retention force, cycle life, and print-layer direction. Avoid sharp roots and materials that are brittle or fatigue quickly.
Printed threads
Coarse threads are more forgiving than fine machined-style threads. Internal threads can retain resin or powder, and external and internal threads may need different compensation. Print a sacrificial test thread first. For repeated assembly, a metal nut, threaded insert, or heat-set insert is often more durable than relying on printed material alone.
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Bearings, shafts, and hinges
Use the actual shaft or bearing when testing. Add lead-ins, avoid long tight sliding surfaces, and consider a bushing or replaceable wear surface. For print-in-place hinges, include cleaning access and escape paths for support material or powder.
Seals
A dimensionally accurate printed interface may still leak because of layer lines, porosity, surface texture, or warping. Use an O-ring, gasket, sealant, or machined sealing face when leakage matters.
When printing is not the right finishing process
If the required interface is tighter or more repeatable than the chosen process can deliver, print oversize and ream or machine it. Other options include metal shafts, bushings, washers, nuts, heat-set inserts, printed molds, shims, compliant mechanisms, or a different process such as SLA, MJF, SLS, PolyJet, or conventional machining. A more generous mechanism redesign may be cheaper and more reliable than forcing a desktop printer to reproduce a miniature precision fit.
Document the tolerance as a process recipe
A useful specification names more than a dimension:
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- Material and material class.
- Nozzle diameter or resin layer height.
- Slicer or exposure profile.
- Part orientation and build location when relevant.
- Support strategy and post-processing.
- Nominal mating dimensions.
- Clearance convention: radial, per-side, or total diametral.
- Target fit and acceptable play or insertion force.
- Measured acceptance range and inspection method.
This matters especially when a design is shared between printers or produced in batches. A fit that works on one calibrated machine is not automatically interchangeable across a different material, process, or service bureau.
Practical tolerance checklist
- What must the interface do: slide, locate, retain, flex, rotate, seal, or transfer load?
- Is the value a manufacturing tolerance, functional clearance, or interference allowance?
- Have you stated whether clearance is per side or total?
- Which process, material, orientation, and settings will produce the part?
- Are first-layer, support, powder, resin, warping, and post-curing effects controlled?
- Are the mating dimensions driven by named CAD parameters?
- Does the coupon reproduce the final geometry and orientation?
- Was it post-processed like the final part?
- Were both the printed feature and mating component measured?
- Would a bushing, insert, machining operation, or different process be more reliable?
The key principle is simple: design the function, not a generic tolerance number. Use published figures as process-specific starting guidance, then let a small, measured test coupon determine the fit for your actual part.
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