Orientation vs Accuracy and Surface Finish

Two dimensionally identical parts can leave the printer with visibly different accuracy and finish, purely because they were oriented differently on the build plate. This is not a defect, it is a direct consequence of how FDM builds geometry layer by layer. Understanding it lets you decide which surfaces and dimensions matter most, and orient accordingly rather than leaving it to chance.

XY accuracy versus Z accuracy

Dimensions measured in the XY plane, the plane of each individual layer, are governed by the machine's positioning accuracy and the extrusion width, and are typically the tighter of the two. At Miloshev Machinery we hold roughly ±0.2 mm on PLA, PETG and carbon-filled grades, and roughly ±0.3 mm on ABS, ASA and PC, largely for dimensions in this plane.

Z dimensions are built from a stack of discrete layer heights, typically 0.10 to 0.30 mm each, and are additionally affected by how well each layer bonded and by any slight variation in layer thickness across the height of the part. A feature that must hold a tight dimension is generally safer specified in the XY plane than stacked in Z, if you have a choice in how the part is oriented.

Stair stepping on curved surfaces

Any surface that is not vertical or horizontal, such as a shallow curve or a chamfer, is approximated by the printer as a series of flat layer steps. The shallower the angle relative to horizontal, the more visible the stair stepping, because each layer edge shows rather than blends smoothly into its neighbour. Orienting a curved surface to be as vertical as possible, or splitting a critical curve so it prints closer to the horizontal build plate plane, both reduce this effect, though never eliminate it at typical FDM layer heights.

A finer layer height, down to 0.10 mm, softens stair stepping considerably at the cost of print time. For cosmetic curved surfaces where finish matters more than speed, combining a finer layer height with the best available orientation is usually the practical answer.

Top, bottom and supported surfaces

• Top surfaces: printed last, generally smooth and flat, the best candidate for a cosmetic face if there is a free choice.
• Bottom surfaces: sit directly on the build plate or a raft, come out flat and dimensionally consistent, but can show a slight texture from the plate or adhesion aids.
• Supported surfaces: printed over removable support structures, always need some post-processing and generally have the roughest finish of the three, so avoid placing critical cosmetic or sealing faces here.

Elephant foot on the first layer

The first layer sits on a heated bed and can bulge slightly outward under its own weight and the pressure of the layers above, a well-known effect called elephant foot. Any surface at the very bottom of the print is the one most likely to show it, which matters directly for orientation choices: a face that needs to be a precise flat reference or mate tightly against another part is a poor candidate for the bottom-most layer unless a small chamfer or slight bed-temperature tuning is used to control it.

Hole roundness by axis

A hole printed with its axis vertical, so the slicer builds it up as a series of stacked rings, generally comes out rounder than a hole printed with its axis horizontal, where the top of the hole has to bridge across a gap and can sag slightly before it fully cools. If a bore needs to stay round without reaming, orient its axis vertically wherever the rest of the part allows it. If it must be horizontal for other reasons, plan for a small amount of post-processing such as reaming or drilling to final size.

How to nominate the cosmetic face

If your part has one face that matters more than the others for appearance, say so explicitly in the order notes or mark it on a drawing or screenshot, for example this face must be free of support marks. Without that instruction we will choose an orientation based on overall balance of accuracy, support and cost, which may not protect the specific face you care about.