Horizontal versus vertical holes in FDM printing

Two holes with the same nominal diameter, cut into the same material, can come out looking completely different once printed, purely because of which way they point on the build plate. This guide explains the mechanics behind that difference and gives concrete thresholds for when to accept a horizontal hole and when to redesign the part so it prints vertically.

Vertical holes: round but undersized

A vertical hole has its axis aligned with the Z axis of the printer. Every layer of the hole wall is a closed circular contour printed on top of the identical contour below it. Because there is no unsupported span anywhere in the hole, gravity and sag play essentially no role. The main source of error is the geometric one described in the pillar guide: nozzle width, corner handling and small amounts of shrinkage pull the diameter in by a fairly consistent 0.1 to 0.3 mm depending on material and size.

This makes vertical holes highly predictable. Once you know the undersizing behaviour for a given material and diameter range, you can compensate for it reliably and get repeatable results across a production run. This is the orientation we recommend for any hole with a functional tolerance: bearing seats, dowel pin holes, and any bore that has to accept a shaft.

Horizontal holes: oval and bridged at the top

A horizontal hole has its axis parallel to the bed. Print it in cross-section and you get an arc of material growing from the bottom of the hole, widening to the equator, then narrowing again toward the top, where the last few layers have to span open air with almost no support underneath. Those top layers are, in effect, a small bridge. If cooling and bridging settings are not tuned for the span, they sag inward under their own weight before solidifying, and the top of the hole ends up flattened, sometimes visibly drooping into the bore.

The result is a hole that is measurably oval: close to nominal width across the horizontal axis, but narrower in the vertical axis where the sag has pinched it. On small holes, under about 5 mm, this effect is often masked by the general undersizing. On larger holes, 8 mm and up, it becomes clearly visible and can be enough to prevent a shaft or bushing from seating properly.

When to split a part so a hole prints vertically

If a single critical hole is the only reason a part cannot be printed in its best orientation, splitting the part along a plane through that hole and printing it as two halves, joined afterward, is often the better answer. This is common for housings with a bearing bore that has to run perpendicular to the main body, or long parts where a cross-hole would otherwise always end up horizontal.

1. Identify which hole in the part carries the tightest tolerance or the highest load.
2. Check whether reorienting the whole part to make that hole vertical creates a worse problem elsewhere, such as large overhangs or a much taller print.
3. If reorientation is not viable, plan a split line that passes through or near the hole axis so each half can be printed with the hole vertical or as a shallow slot.
4. Add alignment features, such as pins or a stepped joint, so the two halves register accurately during bonding or fastening.
5. Specify the joining method, adhesive bonding, screws, or a combination, as part of the part definition, not as an afterthought.

Support inside horizontal holes

For horizontal holes above roughly 8 to 10 mm, or wherever the top surface sags visibly, internal support is an option, but it comes with a cost. Support inside a small bore is difficult to remove cleanly and can leave scarring on a surface that needs to be smooth for a bearing or shaft. In most cases we prefer to either reorient the part, add a teardrop profile to the top of the hole so it self-supports, or accept the oval shape and ream the bore afterward if the tolerance demands it.

The teardrop and other self-supporting hole shapes are covered in detail in the counterbore and teardrop guide in this cluster, along with slots and clearance holes that face similar sag issues.