How to improve dimensional accuracy on FDM parts

Two dimensions on the same part, printed in the same material at the same time, can land with noticeably different accuracy depending only on which axis they run along. This is one of the most common sources of confusion for engineers new to FDM, and it comes directly from how the process builds geometry: point by point in a plane, then stacked layer by layer in height.

Understanding the difference between XY and Z accuracy lets you place your most critical dimension in the direction that will actually hold it, rather than fighting the process after the first article measurement comes back off.

Where XY error comes from

In the XY plane, the printer traces the outline of each layer with a nozzle of finite width, typically 0.4 or 0.6 mm on our machines. The actual deposited line is always somewhat wider or narrower than the nominal value depending on flow rate, print speed and how well the slicer's extrusion multiplier is calibrated for that material. A small flow error of a few percent translates directly into a wall that is a few hundredths of a millimetre thicker or thinner than modelled, and that error accumulates around a perimeter.

External dimensions in XY, such as a boss diameter, tend to print slightly oversize because the outer wall's centreline sits a half line width outward from the theoretical edge unless the slicer compensates. Internal dimensions, such as a hole or a pocket, tend to print slightly undersize for the same geometric reason in reverse. This is predictable and repeatable enough that it can be compensated in the model once it has been characterised for a given material and nozzle.

Why Z heights land on layer multiples

Every printed layer has a fixed height set before the print starts, commonly 0.10 to 0.30 mm on our machines depending on the material and the surface finish required. The printer can only stop a vertical dimension at a whole number of layers. A modelled height of 10.05 mm at a 0.2 mm layer height will actually terminate at either 10.0 mm or 10.2 mm, whichever the slicer rounds to, not at the value in the CAD file.

This quantisation is the single biggest reason Z tolerances are wider than XY tolerances in practice. It also means that if a Z dimension is genuinely critical, it is worth asking us to pick a layer height that divides evenly into it, rather than leaving the layer height to be chosen purely for surface finish or print time.

First layer squish and the elephant foot effect

The first layer is intentionally printed slightly compressed against the bed to maximise adhesion, which is essential for the rest of the print to succeed. This squish makes the first layer wider than the layers above it, a phenomenon commonly called elephant foot. On a part standing on a flat face, this shows up as a slight outward bulge in the bottom few tenths of a millimetre of height, which matters if that face mates against another flat surface or slides into a slot.

• Add a small chamfer or radius on the bottom edge of parts that must sit flush against another surface
• Avoid tight sliding fits right at the base of a printed part, keep the first two to three millimetres slightly relieved
• Tell us if a bottom face is a mating surface so we can manage first layer settings and orientation accordingly

Hole and boss deviation direction

Because of the outline tracing behaviour described above, holes printed horizontally through a wall (in XY) tend to come out slightly undersize and slightly out of round, often a fraction flatter on the top of the hole due to bridging over the opening as it closes. Holes printed vertically (drilled through in Z, i.e. the axis of the hole is the build axis) are rounder because each layer traces a full circle, but their exact diameter still depends on the same flow and compensation factors.

Bosses and pins behave the opposite way: they tend to print oversize in the plane they cross, so a shaft meant to run in a printed bore should be modelled undersize from nominal to leave working clearance, following the guidance we give in the fits and clearance guide.

Placing critical dimensions in the right direction

When a part has one dimension that truly drives function, such as a bearing bore diameter or a snug-fit slot width, it is worth choosing the print orientation specifically to put that dimension in XY rather than Z, because XY tolerances are tighter and more controllable through flow calibration. Orientation decisions for accuracy sometimes conflict with orientation decisions for strength or overhangs, and we weigh all three during quoting rather than defaulting to whichever orientation prints fastest.