Designing 3D-Printed Parts That Fit: Clearances and Tolerances

Nominal zero-clearance CAD geometry fails to assemble in fused deposition modeling (FDM) because extruded polymer tracks have semicircular side profiles, surface roughness, Z-seam protrusions, and thermal contraction. Designing parts that mate predictably requires applying an explicit radial or diametral offset in CAD—ranging from 0.00 mm for interference press fits to 0.40 mm (0.016 in) per side for print-in-place hinges—while controlling flow rate and seam placement in the slicer. You can compute target bore and shaft dimensions for specific joint classes and nozzle sizes using the Fit & Tolerance Calculator.

Radial vs. Diametral Clearance Mathematics

Clearance specifications in mechanical design fail most often due to ambiguity between radial (per-side) and diametral (total) gap definitions. When a cylindrical shaft of nominal diameter d_shaft mates inside a cylindrical bore of diameter D_hole, the two values are related by a factor of two:

Diametral clearance:  c_d = D_hole - d_shaft
Radial clearance:     c_r = (D_hole - d_shaft) / 2 = c_d / 2

In parametric CAD tools, the Offset Face or sketch offset command moves a cylindrical wall radially by c_r. Offsetting a 10.00 mm (0.3937 in) bore face outward by 0.15 mm (0.0059 in) increases the radius from 5.00 mm to 5.15 mm, yielding a final bore diameter of 10.00 + 2 * 0.15 = 10.30 mm (0.4055 in). Conversely, adding 0.15 mm directly to a diameter dimension produces D_hole = 10.15 mm, leaving only c_r = 0.075 mm (0.0030 in) of radial gap per side and turning an intended sliding fit into an interference joint.

For prismatic rectangular slots and tabs of width W_slot and w_tab, the same relationship applies along each mating axis:

Total width clearance:  c_total = W_slot - w_tab = 2 * c_per_side

Worked Example: Dowel Pin and Housing Bore

Suppose a steel dowel pin measures d_shaft = 6.00 mm (0.2362 in) and requires a close sliding fit with a target radial clearance of c_r = 0.12 mm (0.0047 in) per side, plus an allowance for uncorrected bore shrinkage of e_hole = 0.10 mm (0.0039 in) on the diameter:

1. Convert radial clearance to diametral clearance: c_d = 2 * 0.12 mm = 0.24 mm.
2. Add the diametral clearance and bore shrinkage allowance to the shaft diameter: D_CAD = 6.00 + 0.24 + 0.10 = 6.34 mm (0.2496 in).
3. If applying a radial face offset in CAD instead of editing the sketch diameter, offset the 6.00 mm cylindrical face outward by 6.34 / 2 - 3.00 = 0.17 mm (0.0067 in).

Functional Fit Classes for Desktop FDM

Unlike machined metal fits governed by ISO 286—where tolerances are measured in micrometers across ground surfaces—FDM parts contact along the crests of stacked polymer beads. Initial interference occurs at the peaks of the layer lines before the bulk wall experiences elastic compression.

Fit Class	Target Radial Clearance (c_r)	Target Diametral Clearance (c_d)	Typical Mechanical Application
Interference / Press Fit	0.00 to 0.05 mm (0.000 to 0.002 in)	0.00 to 0.10 mm (0.000 to 0.004 in)	Dowel pins, permanent housings, press-fit bearings
Close Location / Snug Fit	0.06 to 0.10 mm (0.0024 to 0.0039 in)	0.12 to 0.20 mm (0.0047 to 0.0079 in)	Removable lids, friction tool holders, alignment tenons
Close Sliding Fit	0.10 to 0.15 mm (0.0039 to 0.0059 in)	0.20 to 0.30 mm (0.0079 to 0.0118 in)	Linear drawers, telescoping tubes, pistons, linkages
Loose Running Fit	0.20 to 0.25 mm (0.0079 to 0.0098 in)	0.40 to 0.50 mm (0.0157 to 0.0197 in)	Free-spinning spools, coarse latches, pivot pins
Print-in-Place (Vertical)	0.25 to 0.35 mm (0.0098 to 0.0138 in)	0.50 to 0.70 mm (0.0197 to 0.0276 in)	Captive barrel hinges, articulated links printed upright
Print-in-Place (Overhang)	0.35 to 0.45 mm (0.0138 to 0.0177 in)	0.70 to 0.90 mm (0.0276 to 0.0354 in)	Captive ball joints, planetary gears, conical pins

Interference, Sliding, and Print-in-Place Joints

Because vertical FDM bores naturally print undersized due to polygon chord error and inward corner pulling (see Why Vertical and Horizontal 3D-Printed Holes Print Undersized), a nominal CAD clearance of c_r = 0.00 to 0.05 mm yields a physical interference of 0.05 to 0.15 mm on the diameter. When pressing a rigid metal shaft or bearing into a printed bore, hoop stress concentrates across the perimeter weld lines; using 3 to 4 perimeters and a 0.5 mm x 45° entry chamfer prevents top-layer shearing.

A close sliding fit (c_r = 0.10 to 0.15 mm) allows smooth motion by hand without radial rattle. When two printed surfaces slide against each other, parallel layer ridges can interlock like microscopic ratchets; orienting one part at 90° or increasing radial clearance to 0.20 mm eliminates layer-line locking.

Print-in-place mechanisms require larger gaps because radiant heat and extrusion squish can fuse adjacent walls while the polymer remains above its glass transition temperature. Vertical cylindrical gaps separate reliably at c_r = 0.25 to 0.35 mm, whereas conical or spherical joints contain overhangs that curl upward toward the nozzle and require c_r = 0.35 to 0.45 mm.

Hole-Basis vs. Shaft-Basis Design Systems

Standard engineering practice (ISO 286) defines two reference systems for assigning tolerances:

• Shaft-basis system (for commercial hardware): Off-the-shelf M3 bolts, 608 bearings (22.00 mm OD, 8.00 mm ID), and steel dowel pins have fixed dimensions. Apply all clearance adjustments to the printed bore or boss.
• Hole-basis system (for printed-to-printed assemblies): Keep the female printed socket at its nominal dimension (or calibrated with slicer XY hole compensation) and apply the clearance offset (-c_r per side) to the male printed plug. Keeping sockets at nominal dimensions simplifies assemblies where multiple plugs or test gauges mate with a shared housing.

How Slicer and Hardware Parameters Shift Clearances

Layer Height and Extruded Track Rounding

As documented in the Slic3r Flow Math manual, slicers model a perimeter track of width w and layer height h as a rectangle capped by two semicircular ends of radius h / 2. Increasing layer height from h = 0.12 mm to h = 0.28 mm on a 0.4 mm nozzle (see Nozzle Diameter vs. Layer Height, Line Width, and Overhangs) more than doubles the edge radius from 0.06 mm to 0.14 mm. Thicker layers also bulge outward more during corner decelerations, reducing effective mating clearance by 0.03 to 0.06 mm per side unless flow and pressure advance are tuned (see Flow Rate and Extrusion Multiplier Calibration).

Nozzle Diameter and Internal Corner Radii

A nozzle of orifice diameter d_n cannot deposit an internal or external corner sharper than half its extrusion width (w / 2). Switching from a 0.4 mm nozzle (w = 0.45 mm, minimum corner radius 0.225 mm) to a 0.6 mm nozzle (w = 0.65 mm, minimum corner radius 0.325 mm) rounds off the corners of square slots. Always chamfer male tenon corners by at least 0.75 * w or add dog-bone relief fillets in CAD so square tabs do not bind on rounded corner beads.

Z-Seam Bulge and First-Layer Elephant’s Foot

Every closed perimeter loop starts and stops at a Z-seam, documented in the Prusa Knowledge Base on Seam Position, creating a localized radial bump of 0.05 to 0.15 mm. When both a printed bore and a printed shaft use an aligned seam, colliding seams consume up to 0.30 mm of clearance. Hide the seam on a non-mating exterior corner, model a 0.3 mm deep vertical seam relief groove inside the bore, or enable Scarf Joint Seam in the slicer.

Similarly, excessive first-layer squish (see First-Layer Squish and Z-Offset Math) flares the bottom 1 to 3 layers outward by 0.10 to 0.25 mm per side. Always model a 0.4 to 0.8 mm x 45° bottom chamfer on build-plate edges and enable slicer Elephant Foot Compensation (0.15 to 0.20 mm).

Material Shrinkage (PLA vs. PETG vs. ASA/ABS)

As explained in the Slic3r Dimension Errors manual, thermoplastics contract when cooling from extrusion temperatures to 20 °C room temperature (see PLA vs. PETG vs. ASA Mechanical and Thermal Properties):

• PLA: ~0.2% to 0.3% (0.02 to 0.03 mm per 10 mm)
• PETG: ~0.3% to 0.5% (0.03 to 0.05 mm per 10 mm)
• ABS / ASA: ~0.6% to 0.9% (0.06 to 0.09 mm per 10 mm)

On a 120.00 mm (4.724 in) ASA enclosure lid, 0.7% linear shrinkage contracts the span by 120.00 * 0.007 = 0.84 mm (0.0331 in), completely consuming a 0.20 mm per-side sliding clearance. Separate scale-dependent thermal shrinkage (corrected via percentage Shrinkage Compensation in the filament profile) from scale-independent surface clearance (applied in CAD as a fixed millimeter offset), and verify Extrusion Multiplier Calibration and HydraRaptor Polyholes before altering CAD tolerances.