Designing holes for FDM 3D printing

Of all the geometry features engineers put into printed parts, holes cause more rework, more failed fits and more support headaches than anything else. A hole is a negative feature: the printer has to trace its wall from nothing, it often needs a bridge across its own top, and its final diameter almost never matches the CAD model exactly. Understanding why that happens is the first step to designing holes that work on the first print.

This guide covers the fundamentals: how the hole axis relates to the build direction, why holes print undersized, what diameters are practical at all, and how much material you need around a bore so it does not crack or distort. The other guides in this cluster go deeper into horizontal versus vertical holes, dimensional compensation, bearing seats, counterbores and post-machining.

Why holes are the most common failure point

A hole in a CAD model is a perfect cylinder. On the printer it becomes a sequence of layer contours, and each contour is drawn by a nozzle with a finite width, typically 0.4 mm, following a toolpath that has to compensate for the fact that molten plastic does not stop exactly where the slicer tells it to. Corner rounding, die swell at the extruder, and the way the perimeter loop closes on itself all pull the final geometry slightly inward. On a vertical hole this shows up as a smaller diameter. On a horizontal hole it shows up as an oval cross-section, because the top of the hole has no support underneath it and sags before it solidifies.

On top of the geometric error, holes concentrate stress. A part that is otherwise solid gets a stress riser wherever material is removed, and because FDM parts are inherently anisotropic, a hole that crosses layer lines behaves differently under load than one that sits within a single layer. All of this means holes need dedicated design attention rather than being treated as an afterthought once the rest of the part is modelled.

Hole axis versus build direction

The single most important decision for any hole is whether its axis ends up parallel or perpendicular to the build platform. A vertical hole, printed with its axis along Z, is built up ring by ring exactly like a small tube. It comes out round, if slightly undersized, and it needs no support because each layer closes on the one below it. A horizontal hole, with its axis parallel to the bed, has to be printed as a stack of arcs of increasing then decreasing width, and the topmost few layers are effectively a bridge across open space.

That difference is not academic. A 6 mm hole printed vertically will typically come out round within normal tolerance and just needs the standard oversizing compensation. The same hole printed horizontally will often be visibly egg-shaped, with the top flattened or drooping by several tenths of a millimetre, unless bridging settings and cooling are tuned specifically for it. Whenever a hole carries a tight tolerance, a bearing, a shaft or a pin, orient the part so that hole prints vertically if the rest of the geometry allows it.

Undersizing: what actually happens

Almost every hole prints smaller than modelled, and the amount depends on diameter, material and orientation. On PLA, PETG and carbon-filled grades we typically see 0.1 to 0.3 mm of undersize on vertical holes in the 3 to 12 mm range, worse on smaller holes because the perimeter has less circumference to spread the error over. On ABS and ASA the number can run slightly higher because of additional shrinkage as the part cools. Horizontal holes can lose an additional few tenths at the top from sag, on top of the general undersizing.

The practical consequence is that a hole modelled at exactly nominal diameter, say exactly 5.00 mm for an M5 clearance hole, will usually come out too tight for the screw. This is why we compensate hole diameters in the model rather than relying on the slicer alone, a topic covered in detail in the compensation guide in this cluster.

Minimum practical diameters

With a 0.4 mm nozzle, holes below about 2 mm diameter become unreliable. The perimeter loop barely closes, corner rounding dominates the whole feature, and the actual opening can end up half the modelled size or blocked entirely by stringing. For functional through-holes we recommend 2.5 mm as a practical minimum, with 3 mm and above being comfortably repeatable. If a design needs something smaller, drilling after printing is more reliable than trying to print it to size, and is covered in the post-processing guide.

Edge distance and wall around a hole

A hole needs enough material around it to survive both printing and loading. As a rule of thumb keep at least one hole diameter, or a minimum of 2 mm, whichever is larger, between the edge of a hole and the outer edge of the part or the edge of another feature. Less than that and the perimeter loops around the hole and the outer wall can merge into a thin, weak section that is prone to cracking, particularly in ABS and ASA where cooling stresses are higher.

Where a hole sits near a load path, for example a mounting hole at the corner of a bracket, add a local boss or rib rather than just thickening the whole wall. This keeps material where it is structurally useful without adding print time and mass everywhere else. We regularly rework bracket designs this way during DFM review, moving a hole a couple of millimetres or adding a small boss instead of asking the customer to thicken the entire part.

How this fits into the wider design process

Holes rarely exist in isolation. Their orientation depends on how the whole part is placed on the bed, their diameter depends on the material and post-processing plan, and their proximity to walls depends on the overall wall thickness strategy. That is why we look at hole design as part of a full DFM review rather than as a checklist item on its own. Getting the orientation right early avoids redesign later when a bearing does not fit or a screw strips its clearance.