Designing parts for print

The cheapest fix for a failed print is a better design. A part that needs supports, prints in the weak direction, or has holes that come out the wrong size can be rescued by settings only up to a point, while a part drawn with the process in mind prints first time. This lesson collects the design rules that matter most for robot parts: overhangs, bridges, orientation, tolerances, holes and fasteners.

Overhangs and the 45 degree rule

Each layer is laid partly on top of the one below. If a wall leans outward, every layer sticks out a little further than the last, and only part of the new line rests on solid plastic. The offset per layer is:

offset = layer height x tan(angle from vertical)

With 0.2 mm layers and a 0.4 mm line, a 45 degree lean shifts each layer by 0.2 mm, so half of the new line is supported, which prints cleanly. At 60 degrees from vertical the shift is 0.2 x 1.73 = 0.35 mm, which leaves close to 90 percent of the line hanging in the air. It droops, curls and looks rough. That is why 45 degrees is the standard rule: keep overhanging surfaces at or below 45 degrees from vertical and they print without supports.

Practical consequences for your designs:

• Replace a horizontal underside with a chamfer at 45 degrees. A bracket that rests on a flange can have a triangular gusset instead of a flat shelf.
• Horizontal holes (for example a shaft going through a vertical wall) have an unsupported top. Draw them as a teardrop with a 45 degree point on top, or add a thin sacrificial layer.
• Bridges, flat spans between two supports, print well because the plastic is pulled taut between its anchor points and cooled by the fan. Spans up to about 10 mm are reliable and 20 to 30 mm are possible with good cooling, but the underside will sag slightly. Split longer spans with a pillar.

Orientation versus strength

You choose how the part sits on the bed, and that decides where the layer lines run. Plastic is strong along a line and weak between layers, so the rule is simple: do not let the load pull layers apart.

Consider an L-shaped bracket holding a servo. If it is printed standing up, with the L in the vertical plane, the corner is loaded in bending and tries to peel the layers apart exactly where the stress is greatest. Printed lying on its side, the layers run around the corner as continuous lines, and it can be several times stronger. In general:

1. Orient the part so the main load runs along the layer lines, not across them.
2. Place the flattest, largest face on the bed for adhesion and accuracy.
3. Look for the orientation that needs the fewest supports.
4. When these conflict, test with a hanging weight before trusting the part on a robot.

Orientation also affects accuracy: circles printed vertically as hole axes are rounder than holes printed on their side.

Tolerances and clearance

A printed part is never exactly the size you modelled. Plastic squashes outward, corners round off, and different printers vary by 0.1 to 0.3 mm. So mating parts need deliberate clearance added to the hole or pocket. The values below are per side, meaning that a 0.2 mm clearance makes a hole 0.4 mm bigger in diameter. They assume a 0.4 mm nozzle and 0.2 mm layers.

A servo is the classic case. Cheap hobby servos differ by a few tenths of a millimetre between brands, so measure yours with calipers instead of trusting a datasheet. If your servo body measures 22.6 mm by 12.1 mm, a 0.2 mm clearance per side gives a pocket of 23.0 mm by 12.5 mm.

Drag to orbithorn 90°

myservo.write()90°

Print a small tolerance coupon once per printer and filament: a block with holes at 3.0, 3.2, 3.4 and 3.6 mm and a series of slots. The first size that a screw slides into with light resistance tells you your offset more reliably than any rule.

Hole shrinkage and the drilling trick

Holes print smaller than designed, often by 0.1 to 0.3 mm. Two things cause it: the extruded line is wider than its path, so it bulges into the hole, and the slicer builds the circle from short straight segments that cut inside the true circle. A hole drawn at 3.0 mm may measure 2.8 mm.

For loose fits you add clearance and move on. For precise holes, such as a motor shaft, a pin or a bearing seat, print undersize and finish with a drill or reamer: draw the hole 0.3 to 0.5 mm smaller and drill it to size by hand. A drill bit cuts a true round hole where a printer cannot. Drill slowly to avoid melting PLA, and hold the part firmly.

Heat-set inserts and screw bosses

Screws threaded directly into plastic strip after a few uses. A heat-set insert is a small brass nut with knurled outside that you melt into the part with a soldering iron at about 200 to 230 degrees C for PLA or PETG. The plastic flows into the knurling and sets, giving a metal thread that survives repeated assembly and loads of tens of newtons. An M3 insert of 5.7 mm length typically needs a hole of about 4.0 mm, and the hole should be 1 to 2 mm deeper than the insert so that displaced plastic has somewhere to go. Check your supplier's datasheet, since insert dimensions vary.

A screw boss is the raised cylinder that holds an insert or a screw. Design it like this:

• Make the outside diameter at least twice the hole diameter, or 1.5 to 2 mm of wall around an insert. An M3 insert in a 4 mm hole wants an 8 mm boss.
• Add a fillet or ribs where the boss meets the floor, so the load spreads into the wall instead of snapping the base.
• Keep the boss axis vertical when you can: the layers then form complete rings around the hole, which resist splitting.
• Use at least 3 walls around the boss, and raise infill locally.
• Do not put a screw in a place where tension pulls along the layer stack, as the layers will peel apart.