Most People Design 3D Printed Holes Wrong

If your 3D printed parts never fit together the way they do on screen, the hole is the usual suspect. A new guide from Slant 3D breaks down the most common mistakes and the simple CAD fixes that make holes print true the first time.

Why Holes Shrink

Plastic shrinks as it cools. That is basic physics, but most designers model holes at their target size and hope the slicer compensates. It often does not. The result is a bearing that binds, a screw that strips, or a panel that sits proud of its frame.

On FDM printers, the outer perimeter is usually over-extruded slightly to improve layer adhesion. That makes the part grow outward. The hole, defined by inner perimeters, does not get the same compensation. It ends up smaller than the CAD model.

The Tear-Drop Fix for Vertical Holes

A vertical hole forces the printer to bridge a circle in mid-air. The top layers droop because the slope of a circle eventually exceeds the printer's overhang limit. Slant 3D recommends changing the profile to a teardrop shape. The flat top gives the printer a constant 45-degree slope it can handle without supports. The hole still functions the same way, but it prints cleanly.

Horizontal Holes Need Support Material

A hole printed sideways is even worse. Without support, the bridge sags and the diameter becomes irregular. The fix is not always to add support material, which leaves marks and wastes filament. Sometimes the answer is to rotate the part so the hole prints vertically, even if that means splitting the design into two pieces that bolt together later.

Oversize by Design

The most practical trick is to oversize the hole in CAD and then subtract material after printing. Drill out the exact size you need. It is not elegant, but it is reliable. For production parts where every micron counts, print a calibration piece first. Measure the actual hole size, adjust the slicer's horizontal expansion setting, and reprint.

Holes are one of those details that seem trivial until a customer tries to assemble your part. Get the geometry right in CAD and you save yourself a bin of scrap and a lot of frustration.