Bolt a printed part to something

Screw holes, nut traps and heat-set inserts — cut the space the hardware needs, at the size the standard says.

Almost anything useful eventually has to bolt to something else, and the move is always the same: you do not model the screw, you model the space the screw needs. OwlCAD ships that space as ready-made shapes, so the numbers come from a standards table instead of from your memory of what an M3 is.

The red ones are cutters. Screw hole, Insert pocket, Nut trap and Slot arrive tinted translucent red — that is the palette telling you they are holes waiting to happen rather than parts. Washer, Hex nut and Standoff arrive as ordinary solids, because those you print or fuse on. Nothing else about the red is special: move it, resize it, subtract it like any other shape.

Pick the size, not the diameter. The Standard size row is the whole reason these shapes exist. For one M3 screw there are three different holes — 3.4 mm to pass through freely, 3.2 for a close fit, 2.5 for a hole it taps its own thread into — and which one you want depends on which side of the joint you are on. A bolted joint is nearly always a clearance hole on the near part and a nut or an insert on the far one; a self-tapping screw into printed plastic wants the tap size. Choose the size and the fit and the fields fill themselves.

Do not add clearance twice. A nut trap is already 0.2 mm wider across the flats than the nut it holds, because a hex cut to exactly 5.5 mm will not accept a 5.5 mm nut. That allowance is inside the shape. Stacking a Clearance modifier on top of it is the quickest way to a nut that rattles and rounds out its own pocket — and clearance is the right tool everywhere else, which is precisely why people reach for it here.

A trap has to be reachable. A hex pocket buried in the middle of a solid can only be filled by pausing the print half way through and dropping the nut in. A pocket that opens onto a side face can be filled whenever you like: slide the nut in, and it is captive the moment the screw goes through. If you do bury one, remember its ceiling is a bridge the printer spans over thin air, so keep it small and flat.

Check that the two features actually meet. A nut trap stopping a hair short of the bore leaves a skin the screw has to punch through, and you cannot see it, because it is inside the part. X-ray makes the solids translucent so you can look. Overlap the trap into the hole rather than butting it up against it — the same overshoot a channel needs where it breaks the surface.

Then look at what is left. A counterbore takes a surprising amount of meat out of a thin plate, and the print check is what tells you the wall behind it is thinner than your nozzle before the print does.

1. Open the Shapes panel and look in the Fasteners group. Screw hole, Insert pocket, Nut trap and Slot drop in translucent red because they are cutters; Washer, Hex nut and Standoff drop as ordinary solids.
2. Place a Screw hole and set its Standard size in the Properties panel — M3, M4, 1/4-20 — then a Fit: Clearance for a screw that passes through, Close for a snug one, Tap for a hole a screw cuts its own thread into.
3. Pick a head. Counterbore sinks a socket-cap head flush, Countersink takes a flat head. The recess is at the top of the tool, so drop it in pointing the way the screw goes in.
4. For a heat-set insert, place an Insert pocket instead and give it the same standard size. Its Lead-in flares the mouth at the top, which is what lets the insert start square rather than leaning. A disc magnet takes the same Insert pocket: pick its size from the Disc magnets group in the same list, which also switches the lead-in off so the magnet sits flat. An alignment pin takes a Screw hole with no head at the Close fit — M4 for a 4 mm pin.
5. For a captive nut, place a Nut trap, set the size, and move it so the hex crosses the screw hole. Switch on X-ray to check that the two really meet.
6. Select the part first and the tools after it, then Subtract the lot in one go — difference keeps the first body and cuts everything else out of it.
7. Run the print check to see how much wall is left around each pocket, then export.