3D printing holes: 5 simple tips

A nozzle deposits a bead of finite width along the hole perimeter. The inner wall of that bead eats into the opening, so a Ø5.00 mm hole in CAD often measures Ø4.80 mm off the bed. On FDM machines the loss is typically 0.10–0.20 mm on diameter, and it grows with layer height and nozzle diameter.

Compensate in the model, not by reaming every part. Add half the measured error to the radius: a Ø5.00 mm hole becomes Ø5.20 mm for a 0.4 mm nozzle at 0.2 mm layers. Print a coupon with Ø2, Ø4, Ø6, Ø8 and Ø10 mm test holes, measure with pin gauges, and you get a compensation curve for that machine and material.

This matters most below Ø3 mm. Tiny holes close up almost completely because the perimeter bead is a large fraction of a small circle. If the feature is a pilot for a self-tapping screw, oversize is harmless. If it is a bearing seat, it is not.

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  Starting pointAdd 0.15 mm to diameter, measure, then adjust.
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  Nozzle effectLarger nozzles need more compensation.
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  Under Ø3 mmExpect the largest relative error.

A hole printed with its axis horizontal is not a circle. The top of the opening sags into the bore, the bottom squashes onto the bed, and the cross-section becomes a teardrop. A Ø6 mm horizontal hole can easily measure 0.3 mm out of round. Pins will not pass through it cleanly.

Rotate the part so the hole axis is vertical and the layers stack as concentric rings. Each new layer is a smaller or larger circle, so the wall stays round. The trade-off is that vertical holes usually need support under any overhang elsewhere on the part, and support scars can land on a sealing face.

If the design forces a horizontal hole, do not expect a press fit. Either add a countersink, open the tolerance to a clearance fit, or plan to ream it after printing. Bridge quality also depends on cooling: set the fan high and slow the perimeter speed for those layers.

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  Best orientationHole axis parallel to the Z axis.
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  Worst caseAxis horizontal, no support, fast cooling off.
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  RecoveryReam or drill 0.2–0.3 mm oversize.

FDM parts shrink as they cool, and the shrink is not uniform. A 100 mm long ABS plate can lose 0.6–0.8% in X and Y while the Z direction moves far less. Holes near the center of a large flat plate often land smaller than holes near the edge, because the middle is restrained by the surrounding material.

SLA and other resin processes shrink during the post-cure, and the effect is different again. For a Ø8 mm hole, 0.7% shrink is roughly 0.06 mm. That is small on its own, but it stacks with the bead-width error and with any press-fit interference you designed in. The two errors can add or cancel.

For hole positions, use the drawing's true position and check it after printing. If a bolt pattern misses by more than 0.2 mm center to center, the part will not assemble even if every hole is the right size. Pattern error is usually a machine or calibration issue, not a slicer setting.

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  Shrink rangeRoughly 0.4–0.8% for common FDM materials.
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  Z directionShrinks less than X and Y on FDM.
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  PositionCheck bolt patterns as a set, not one hole.

Printing a hole 0.2 mm undersize and drilling it to final size is standard practice, and it works well for a handful of holes. The printed bore acts as a drill guide, so you keep the position that the printer gave you, and the drill only has to correct the diameter. Use a drill one size below the hole, then ream if the fit is critical.

Watch the heat. Most printed plastics have low thermal conductivity, so the drill bit rubs, the wall softens, and the hole comes out oversized or gummy. Run a slow spindle speed, clear chips often, and use a sharp bit. For a Ø5 mm hole in ABS, a pilot at Ø4.5 mm then a reamer at Ø5 mm H7 gives a repeatable slip fit.

This approach stops making sense once you have dozens of holes, or when the holes are in a pattern that must hold position to better than 0.1 mm. Hand drilling each hole adds labor and variance. At that point the part belongs on a mill, not a printer.

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  Typical stockLeave 0.2 mm on diameter for drilling.
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  ReamingUse for bearing seats and dowel fits.
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  Heat controlSlow speed, sharp tool, clear chips.

A printed hole is fine for a bolt that passes through with clearance. It is a poor choice for a bearing seat, a dowel pin, a hydraulic port, or anything that sets the position of two parts relative to each other. Those features need a round bore and a known diameter, and printing gives you neither with enough confidence.

The practical split is by function. Clearance holes, cable pass-throughs and lightening holes print well. Interference fits, threaded holes under load, and holes that act as datums go to CNC. A printed prototype with machined bores is a common and sensible combination: print the body to check form and fit, then machine the critical holes on a 3-axis or 5-axis mill.

GreatLight runs both. We print prototype housings on the 3D printing line and machine bores, bores with tight tolerances and mating faces on 127 CNC machines, holding ±0.005 mm where the drawing calls for it. Send the part and a note on which holes are fits; we will flag the ones that will not survive as printed.

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  Print itClearance holes, pass-throughs, cosmetic openings.
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  Machine itBearing seats, dowel holes, threaded load paths.
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  HybridPrinted body plus machined bores is common.