3D print press fit tolerance: how much interference to model

A press fit on a filament (FDM) printer starts at 0.05 mm of interference per side: the pin drawn 0.10 mm larger than the hole across the diameter, in PLA (polylactic acid), on a printer whose holes print true. Published values run from 0.15 mm of interference to 0.2 mm of clearance per side. Where you land depends on how small your printer prints holes, so settle it with one test print before the real part.

How much interference does a 3D printed press fit need?

A 3D printed press fit needs 0.05 mm of interference per side, or 0.10 mm (0.004 in) on the diameter, in PLA on a printer whose holes print true. Printer maker QIDI Tech publishes 0.05 to 0.15 mm per side for a press-fit pin and warns that past about 0.20 mm the boss splits along a layer line.

Interference is negative clearance: the pin is drawn larger than the hole. QIDI’s fit guide assumes a 0.4 mm nozzle, 0.2 mm layers and PLA or PETG (glycol-modified polyethylene terephthalate) on a well-calibrated machine. The tolerance calculator defaults its press fit to -0.05 mm per side, the light end of that range.

C = 2 × c     c = interference per side, C = across the diameter

steel pin into a printed hole:     hole = pin - C
printed pin into a printed hole:   hole = D        pin = D + C

Press fit tolerance for PLA, PETG, ABS and nylon

Negative values are interference and positive values are clearance.

Material	Start at, per side	Total, on the diameter	Inches, per side / total	QIDI allowance, per side
PLA	-0.05 mm	-0.10 mm	-0.0020 / -0.0039 in	0 (baseline)
PETG	-0.05 to 0 mm	-0.10 to 0 mm	-0.0020 to 0 / -0.0039 to 0 in	0 to +0.05 mm
ABS, ASA, PC	0 to +0.05 mm	0 to +0.10 mm	0 to +0.0020 / 0 to +0.0039 in	+0.05 to +0.10 mm
PA-CF, PET-CF	0 mm	0 mm	0 / 0 in	+0.05 mm
Nylon (PA), unfilled	+0.05 to +0.15 mm	+0.10 to +0.30 mm	+0.0020 to +0.0059 / +0.0039 to +0.0118 in	+0.10 to +0.20 mm

Each row is the calculator’s -0.05 mm plus the allowance QIDI adds per side for that material. QIDI applies that allowance to every fit in its table, the press fit included. PA-CF and PET-CF are the carbon-fiber-filled grades of nylon and of PET (polyethylene terephthalate). Why the materials move:

• PLA is QIDI’s dimensional baseline, but brittle in flexure: it snaps rather than bends. Go easy on interference in a thin PLA boss.
• ABS and ASA (acrylonitrile butadiene styrene and acrylonitrile styrene acrylate) contract more as they cool. Sovol, another printer maker, warns that in ASA “a nominal 0.1 mm press can end up far tighter than intended.”
• Unfilled nylon absorbs moisture and grows for days after printing. QIDI says to test the fit again after 24 hours.
• Polycarbonate (PC) holds surface stress. Stratasys sends PC parts to heat-set or glued inserts and names Nylon 12 as the material best suited to interference inserts. Covestro’s joining guide says press fits are used “only rarely” in its Makrolon polycarbonate. ABS and nylon tolerate excess interference better, it adds, but can relax and loosen over time.

Worked examples, in mm and inches

1. 6 mm steel dowel pin in PLA: hole = 6.00 - 0.10 = 5.90 mm (0.2323 in).
2. 1/4 in dowel pin in PETG, at the tight end: hole = 0.2500 - 0.0039 = 0.2461 in (6.25 mm).
3. Printed 8 mm pin into a printed hole, PLA: keep the hole at 8.00 mm (0.3150 in) and draw the pin at 8.10 mm (0.3189 in).

All three assume holes that print to size. If yours print small, measure how much with the hole compensation calculator and either fix it in the slicer or add it to the hole.

Steel dowel pin into a printed hole

A steel pin can’t give, so the printed hole takes all of the interference. The pin is the accurate part: hardened ISO 8734 dowels are ground to tolerance class m6, which the ISO 286-2 tables put at 6.004 to 6.012 mm for a 6 mm pin. Covestro’s press-fit chart also allows less interference for a steel shaft than for a plastic shaft in the same hub. If your test lands between two holes, take the looser one for steel.

Bearings and magnets are hard parts too, so the same rule holds: the bore takes all the interference, sized from the part’s measured outside diameter. A captive nut is not a press fit. QIDI sizes a hex nut pocket 0.20 to 0.30 mm over the nut’s width across flats and 0.10 mm deeper than the nut.

Printed pin into a printed hole

QIDI draws the pin larger and keeps the hole at size. Both parts carry print error here. A printed pin comes out oversized from extrusion overlap, plus a bulge at its base from the squashed first layer (elephant’s foot). QIDI’s fix is a 0.4 mm × 45° chamfer on the pin’s base and 0.15 to 0.25 mm of elephant’s foot compensation in the slicer.

Print the hole with its axis vertical. Stratasys calls the X/Y plane the most accurate, so a hole drawn in it comes out round. The press also stretches the hole around its circumference, along the layer lines rather than across them. QIDI notes that the bond between layers is a print’s weakest direction.

How pin diameter and hole depth change the press fit

The diameter sets how much interference the boss can take, and the depth sets how hard the pin holds.

Diameter. Covestro sizes its maximum interference as a fraction of the shaft diameter, not as a fixed amount. Its example is a solid polycarbonate shaft of 0.250 in in a hub of 0.450 in outside diameter: 0.0093 in per inch, or about 0.002 in on the diameter. The chart allows less for a steel shaft and more for a thicker hub.

Printed-part guides give a fixed amount instead, and 0.10 mm is 3.3% of a 3 mm pin but only 0.5% of a 20 mm pin. Small pins belong at the light end of the range. Large holes have the opposite problem: QIDI cites a PETG study in which 20 mm holes printed 0.28 to 0.52 mm small on the diameter. Run the test print at the real diameter, not a scaled-down one.

Depth. Press-fit force is the friction coefficient times the contact pressure over the contact area (Nwe and Pimsarn, Assembly Magazine). For a round pin that area is π × diameter × engaged length. At the same interference, twice the depth holds about twice as hard, and the extra length doesn’t add to the stress that splits the boss. When a pin pulls out, lengthen the engagement before you add interference.

Wall around the hole. Covestro’s chart allows more interference as the hub wall gets thicker. On a print, that wall is perimeters and infill, so give a press-fit boss enough walls to be solid plastic around the hole.

Press fit clearance or interference: why the numbers disagree

The numbers disagree because printers print holes small, and each source handles that differently. Sovol blames arc compression, which pulls the filament inward as the nozzle traces a circle. QIDI puts the error on internal holes at roughly four times the error on outside dimensions. The number you model is the fit plus your printer’s hole error:

Source	Press fit as published	Per side or total	Conditions
QIDI Tech	0.05 to 0.15 mm interference	per side	0.4 mm nozzle, 0.2 mm layers, PLA or PETG; its slicer settings include 0.10 to 0.20 mm XY hole compensation
Sovol	0.05 to 0.15 mm clearance	per side	0.4 mm nozzle, well-tuned flow
UAL lab, Bambu Lab H2D Pro	0 to 0.05 mm clearance	per side	“Always test first”
UAL lab, UltiMaker S7	0.2 mm clearance	per side	“Safer starting point on Ultimaker”
ISO 286, H7/p6 at 10 mm (machined)	0 to 0.024 mm interference	total	hole 10.000 to 10.015 mm, shaft 10.015 to 10.024 mm

If your slicer opens holes back to size, the model needs real interference, as QIDI’s does. If it doesn’t, the printer’s shrink supplies the interference. Then the model needs clearance, as Sovol’s does. Layer ridges add a little more: QIDI notes that they act as small interferences. The UAL lab is the Creative Computing Institute’s digital fabrication lab at University of the Arts London.

Compare the last row. ISO 286 is the international standard for machined limits and fits, and its H7/p6, the machinist’s press fit that can still be taken apart, allows at most 0.024 mm of total interference at 10 mm. A printed 20 mm hole can miss by more than ten times that. So the printed number corrects for the printer as much as it sets the fit.

Push fit vs press fit

A push fit, one you press home by hand, is what Sovol’s 0.05 to 0.15 mm per side describes: “Needs firm push; friction holds it.” For locating pegs that must come apart again, use the snug fit in the tolerance calculator, 0.10 mm of clearance per side.

How to find your press fit tolerance with one test print

Print one bar with a row of holes, one per offset, and press your real pin into each. For a 6 mm pin, step the hole 0.05 mm per side at a time:

Offset per side	-0.15	-0.10	-0.05	0	+0.05	+0.10	+0.15
Hole to model, mm	5.70	5.80	5.90	6.00	6.10	6.20	6.30

1. Match the real part. Use the same filament, slicer profile, orientation and layer height. Make each hole as deep as the real engagement, with the same wall around it. Emboss each hole’s offset next to it, and print them all in one job so they share one thermal history (QIDI).
2. Let it cool. Sovol says warm parts are softer and “lie about the fit.” Test at room temperature, and test nylon again after 24 hours.
3. Press the real pin into each hole with a vise, from the loosest hole down.
4. Read the result. Sovol’s rule: the largest gap that still feels like a press fit is your value. In the tighter holes, the first crack or white stress mark shows the boss’s limit.

The winning offset holds for that printer, filament and profile. QIDI says to write it in your slicer profile notes, since a new nozzle size or filament brand changes the value.

How do you press a metal pin into a 3D printed hole?

Press a metal pin into a 3D printed hole with an arbor press or a bench vise. Keep the pin square and push it in one steady stroke until it sits flush. Chamfer the hole’s entrance so the pin starts straight. If the pin needs more force than the part can take, heat it and press lightly.

1. Clean both parts. Covestro says press-fit parts should be free of lubricants and degreasers.
2. Give the hole a lead-in. Sovol’s fix for a tight fit includes chamfering the lead-in edge. Covestro adds a counterbore lead-in to screw bosses because it aligns the screw and lowers the stress at the top of the boss, where cracks start. A 0.4 mm × 45° chamfer, QIDI’s size for elephant’s foot, also clears the squashed first layer when the hole opens on the bed side.
3. Back the part and press square. Stratasys sets the pin on the hole, aligns it under an arbor press and brings the ram down until the pin sits flush. Support the part on a flat block right under the hole so it doesn’t flex. A vise works the same way with the pin square to the jaws.
4. If the hole is too tight, open it with a drill or reamer. Stratasys drills or reams press-fit holes first for the right diameter and roundness. Printing the hole slightly small and reaming it to size gives a truer bore than the printer alone.

Heat-assisted pressing

Stratasys calls heat setting the preferred method for thermally conductive hardware such as a steel pin: the plastic flows around the part, and heat relieves the stress at the interface as it goes in. It heats the soldering iron to about 170% of the plastic’s glass transition temperature. For polycarbonate, whose glass transition is 161 °C (322 °F), that is 288 °C (550 °F). Rest the iron on the pin and let the pin heat up. Then apply light, constant pressure until it is flush. Stratasys notes that minimal pressure is required.

Heat changes what holds the pin. The plastic flows to the pin’s size, so the modeled interference matters less. For a heat-pressed pin, start from the steel-pin hole size in the worked examples.

3D print press fit design: crush ribs and polygon pins

A round pin in a plain round hole is the least forgiving press fit. Two shapes take up the printer’s error for you.

Crush ribs. These come from injection molding. Protolabs describes three or more ribs running the length of the hole. Their narrow tips touch the mating part and deform during the press, so the fit stays tight without much stress on the part. Hackaday’s write-up of the technique for printed parts calls the fit “far easier (and more forgiving)” because the pin touches only the ribs.

One starting geometry from this page’s numbers: draw the bore with the calculator’s loose fit, 0.40 mm per side, so the wall never touches the pin. Then add three ribs whose tips sit on the press-fit circle, pin minus 0.10 mm. For a 6 mm pin, that is a 6.80 mm bore with rib tips on a 5.90 mm circle, so each rib stands 0.45 mm off the wall. Run the stepped test on the rib circle instead of the bore.

Polygon pins for printed parts. In Make: Magazine, Kurt Hamel makes the pin an octagon or hexagon whose corner-to-corner size equals the hole. His rule: “If the mating piece has a 1.00” hole, the cross-corner dimension of the pin also wants to be 1.00”.”

That is zero clearance. Only the corners touch, which concentrates the stress there and lets both parts flex. A hole through the pin’s center adds flexibility, and the flat bottom and 45° faces print well lying down.

If the press fit cracks, binds or works loose

• The boss cracked: reduce the interference one 0.05 mm step, thicken the wall around the hole, or print the hole upright. QIDI’s limit is about 0.20 mm of interference. Crush ribs or heat pressing avoid the problem.
• It binds only at the bed face: that is elephant’s foot. Set 0.15 to 0.25 mm of elephant’s foot compensation, or chamfer the bottom edge (both QIDI).
• Every hole is tight: QIDI checks flow before touching the model. If a 20 mm calibration cube measures over 20.1 mm, fix flow first. If the cube is right but holes are tight, raise XY hole compensation by 0.10 mm. The dimensional accuracy guide puts these corrections in order.
• The pin is loose: go one 0.05 mm step tighter (QIDI), or lengthen the engagement. To glue it instead, Stratasys leaves 0.127 mm (0.005 in) of clearance for the adhesive and uses cyanoacrylate (Loctite 495) or epoxy. It does not say whether that clearance is per side or total.
• It loosened after weeks: Covestro warns that ABS and nylon can relax under the load of a press fit. It also advises against press fits between two different materials that see heating and cooling cycles. For a steel pin in a part that runs warm, glue or heat-set the pin.