How to design 3D printing tolerances in CAD: screw holes, heat-set inserts, Fusion vs Onshape

In metalwork, a tolerance is often a bilateral band around a machined dimension. In FDM, the useful question is different: what diameter should I model so the printed hole does the job after shrinkage, extrusion, and orientation?

Treat clearances as design offsets you calibrate, not ISO fit classes copied into a sketch. A close sliding plastic-on-plastic fit, a free-running axle, and an interference for a heat-set insert are three different offsets. Copying one number across all of them is how kits fail on someone else’s bench.

Start from the mating part you can measure — screw major diameter, insert outer knurl, pin OD — then add the clearance your process needs. Verify on a coupon printed with the same material, walls, and orientation as the real boss.

A clearance hole lets a screw pass without cutting plastic threads. Model the through diameter, not the M-size of the screw. For common metric SHCS hardware on FDM:

M2 → about 2.2–2.4 mm through M3 → about 3.2–3.4 mm through M4 → about 4.3–4.5 mm through M5 → about 5.3–5.5 mm through

Use the tight end when the screw must locate the part; the loose end when stacking plates or when your printer undersizes holes. Countersinks and counterbores need their own diameters — do not enlarge only the through hole and hope the head seats.

If the joint will be opened often, do not rely on plastic threads. Heat-set inserts or a captured nut beat a hole you tapped with the screw once.

Always pick screw length from the full stack: mating thickness + washer + insert engagement + clearance under a blind insert so the tip does not bottom out.

M3 on the screw is not the CAD hole. The hole follows the insert’s outer geometry and your printer’s shrinkage.

For common ruthex / CNC Kitchen–style M3 inserts, the datasheet often calls for a 4.0 mm installation hole. Printed holes usually come out small, so many designers start around 4.2 mm in CAD so the cooled hole lands near that 4.0 mm feel — insert pre-seats on the tip, melts in cleanly, and does not push a plastic burr into the threads. CNC Kitchen’s pull testing around that range is the reason 4.2 mm shows up in so many maker notes; it is a starting point for that insert family, not a universal law.

Rules that travel across brands:

1. Use the manufacturer hole for your insert series (short vs long M3 are not interchangeable). 2. Make depth at least insert length + ~1 mm so displaced plastic has somewhere to go. 3. Keep enough wall around the boss (often ~1.6 mm radial minimum on small M3 bosses — check the drawing). 4. Print a diameter ladder (for M3: 4.0 → 4.4 mm in 0.1 steps) in the same orientation as the real part. Keep the smallest hole that installs straight without cracking or clogging threads.

Blind vs through matters: a through hole needs a stop so you do not drive the insert out the far side.

A correct CAD diameter still fails if the boss is a single perimeter or the hole prints as a bridge on a vertical face. Prefer:

Bosses with enough perimeters that the insert loads plastic, not sparse infill Hole axes printed vertically when strength matters (horizontal holes ovalize and droop) Fillets at the boss root so stress does not crack the skin on first torque

Installation is part of the tolerance: support the part, keep the iron square, seat flush (or slightly below), and let it cool before you run a screw. If plastic floods the internal thread, the hole was too small or the heat too aggressive — fix the coupon before you revise the whole assembly.

Both tools are fine for FDM hardware design. Pick based on how you work, then make clearances parameters, not buried sketch numbers.

Fusion: user parameters such as m3_clearance, m3_insert_hole, and insert_depth_extra; drive Hole or extrude-cut from those; retune one clearance across a kit after the first coupon. Strong when you also live in the same CAD for mesh cleanup or CAM habits.

Onshape: Variable Studio / variables for the same named clearances; Hole feature with diameter and depth tied to variables; browser-native collaboration when a team revises a multipart kit. Drawing tolerances exist, but for FDM the win is still parametric hole sizes, not ISO title-block theater.

Neither CAD replaces a coupon on your printer. What they replace is hunting twenty sketches when PETG needs +0.1 mm more than PLA.

Practical rule: name hardware parameters after the joint (heatset_m3_long_hole), not after a one-off part, so every boss that takes that insert stays consistent.

CAD gets the hole right for you. Builders still need the shopping list: insert series, screw length, qty, and which printed part gets the boss. That is where most MakerWorld / Printables releases go quiet — and where functional kits stall.

On Slicebuild, put vitamins on the step that uses them: 2× M3×5.7 heat-set inserts into Plunger Cap; 2× M3×12 SHCS. Print notes (walls, material, orientation) sit on the STL. When you revise the insert hole from 4.1 to 4.2 mm, restage the step; the same public URL updates instead of a stale PDF.

If you already have the STLs and a Discord BOM, the missing layer is the ordered manual with exact sizes — not another zip named final_v7.