Won't a teardrop hole look odd?

The point sits on top of the hole, often hidden inside the part. A bolt still sits on the round lower part. If you need the hole round, cut the tip flat; the flat top becomes a short bridge that prints cleanly.

My slicer has no sacrificial layer option. What now?

Model it yourself: a slot one layer deep, then a square two layers deep, above the counterbore, as described in this lesson. Any slicer prints those as normal bridges.

Is it ever better to just use supports?

Yes, for organic shapes like figures, or when the support-free version would be weaker. This lesson is for functional parts you design yourself, where a few CAD changes remove the need.

Design for no supports: chamfers, teardrop holes, bridges and sacrificial layers

Supports cost plastic, print time and a rough surface where they touch. For parts you model yourself, most supports can be designed away. The idea is simple: every surface that faces down must either be steep enough to build on the layer below, or be a short bridge anchored at both ends. This lesson is the list of CAD moves that make that true.

Numbers assume FDM with a 0.4 mm nozzle and 0.2 mm layers. Why 45° works and how to tune bridges is in Overhangs and bridging; when supports really are needed, see Supports: when and how.

The rule behind every trick

Prusa puts the clean overhang limit at 45–60° from vertical, depending on nozzle and settings, and Hubs uses 45° as the safe value. Design to 45° and it will print on any reasonably tuned machine.

Shape that needs supports	Support-free version
Fillet on a bottom edge	45° chamfer
Flat ceiling over a pocket	45° roof, or a short bridge
Round horizontal hole	Teardrop or diamond hole
Counterbore facing the bed	Sacrificial layer, or crossed bridge layers
Part with overhangs in several directions	Split into pieces that each sit flat

Chamfers instead of fillets on the bottom

A fillet on an edge that faces the bed starts almost flat: its first layers overhang at nearly 90°. Prusa’s guide says this hurts the surface and recommends a chamfer when finish matters. Hubs recommends a 45° chamfer on edges touching the build plate, which also hides elephant foot.

Bottom edges that sit on the bed: chamfer 0.5–1 mm at 45°.
Edges that hang in the air, such as the underside of a ledge: chamfer at 45°, or slope the whole underside.
Fillets are fine on top edges and vertical edges. Only the downward-facing ones cause trouble.

Teardrop and diamond holes

A horizontal hole, with its axis parallel to the bed, has a round top. Near the top, the wall turns flat and overhangs at close to 90°, so it droops and the bolt will not go in.

Teardrop: keep the bottom half round, and replace the top with two 45° lines tangent to the circle, meeting in a point. Nothing overhangs more than 45°.

Diamond: a square rotated 45°. Every side is at 45°. It suits small holes and slots where the exact shape does not matter. Make the side length equal to the needed diameter so a circle of that size fits inside, touching all four sides.

Worked example: a 6 mm teardrop

Hole diameter 6 mm, so radius r = 3 mm.

Two 45° tangent lines meet at r × √2 above the centre: 3 × 1.414 ≈ 4.24 mm.
The top of the circle is 3 mm above the centre, so the point sticks up 1.24 mm above where a round hole would end.
Need the hole to stay under 6 mm tall? Cut the tip flat at the circle’s top. The flat is 2 × (4.24 − 3) ≈ 2.5 mm wide, a short bridge that prints cleanly.

In CAD, sketch the circle, add two lines at 45° tangent to it, and trim the arc between them.

Bridges: make the gap short and anchored

A bridge is a horizontal span with both ends resting on something. Hubs puts the safe length at under about 5 mm; tuned printers go much further, and Overhangs and bridging shows how to find your limit.

Turn a flat ceiling into a bridge by making sure walls hold it on both sides, along the shortest direction.
An overhang anchored on one side only is not a bridge. Give it a 45° slope or a second wall.
Long ceilings: add a rib down the middle to halve the span.

Two crossed bridge layers give the screw hole a floor

Counterbores and sacrificial layers

A screw hole with a counterbore on the bed side is the classic problem. The counterbore prints fine, but the layer where the narrow hole begins sits on nothing: it is a ring floating over the wider bore.

Three fixes:

1. Sacrificial layer in the slicer. OrcaSlicer has a counterbore hole bridging setting with three choices: none, partially bridged, or sacrificial layer, which closes the hole with one layer so the next prints on it. You poke it through after printing.

2. Crossed bridges in CAD. Works in any slicer. Directly above the counterbore:

Cut a slot one layer deep (0.2 mm), as wide as the screw hole and as long as the counterbore. The printer bridges across the counterbore on both sides of the slot.
Cut a square two layers deep (0.4 mm), as wide as the screw hole. This layer bridges at 90° to the first, between the first layer’s bridges.
Above that, the round screw hole sits on solid plastic.

Example for M3 with 0.2 mm layers: screw hole 3.4 mm, counterbore 6 mm. Slot 3.4 × 6 mm at 0.2 mm deep, then square 3.4 × 3.4 mm at 0.4 mm deep, then the 3.4 mm hole. The hole comes out square for 0.4 mm, which the screw does not notice. If you change layer height, change the depths to match.

3. Flip the part, so the counterbore faces up. Then it is an ordinary hole stepping out to a wider one, which needs nothing.

Sacrificial and crossed-bridge layers depend on your layer height. Store them as parameters tied to it, or they break when you print at 0.12 mm.

Split parts that cannot sit flat

If a part has overhangs in several directions, no orientation fixes all of them. Split it so each piece has a flat face on the bed, then join with dowels, screws or glue. Prusa’s guide and Hubs both recommend this; Part orientation shows the cut tool and dowel sizes.

Splitting is also a strength decision: you choose which way the layers run in each piece.

Common mistakes

Fillets on every edge by habit. The bottom ones print rough or need supports.
Round horizontal holes for bolts. The top droops and the bolt will not pass.
A bridge anchored on one side only. That is an overhang, and it curls.
Counterbore on the bed side with no plan. The ring above floats and sags into the bore.
Sacrificial layer depths not matching the layer height. The slicer drops or merges them.
Fighting a shape that should be two parts. Hours of support removal for something a dowel would solve.

Sources

Circuits and code were tested on sample boards. Boards differ in pin layout, so check your board's pinout before you power anything.