Scaling and Compensation

TL;DR

If holes are tight but the outside size is right, use hole compensation (or add clearance in CAD), not global scaling. If everything is off by the same percentage, use XY/uniform scale; if it’s off by a fixed amount, use horizontal expansion, then verify with a small test coupon and calipers.

What these settings actually change

Scaling multiplies coordinates (a percentage change), so every feature grows/shrinks together. Horizontal expansion (often called XY compensation) offsets perimeters in the XY plane by a fixed distance, like “pushing” the outline in or out. Hole compensation targets internal circular features by enlarging them, which is useful because FDM holes tend to print undersized even when the outside of the part measures correctly.

Choose the right tool (by what is wrong)

• Many dimensions wrong by the same percentage (example: 100 mm prints as 99.2 mm): use XY scale or uniform scale.
• Outside dimensions consistently too big/small by about the same mm amount (example: +0.15 mm on most outside faces): use horizontal expansion / XY compensation.
• Only holes, slots, or internal diameters are tight (example: pins don’t enter, screws won’t pass): use hole compensation or add clearance in CAD for those features.
• Only height is off (example: 20 mm tall prints as 19.6 mm): use Z scale only after checking Z steps/mechanics; Z fit problems are often first-layer or extrusion related, not true Z scaling error.

Why holes print small (even on a calibrated printer)

In FDM, a “hole” is made from bead width and squish. The nozzle lays down a line with real width; at curves the bead tends to crowd inward, and small segments approximate circles. Internal perimeters also cool and shrink in a way that biases the diameter smaller. This is why hole compensation (or designing in clearance) is usually the right fix when only internal fit is wrong.

Why outside dimensions can print large

Common causes are first-layer elephant foot (the first layer spreads), too much flow, an effectively wider line width than expected, or excessive heat that makes corners round and edges bulge. If the oversize is mostly at the bottom, fix elephant foot directly (elephant-foot compensation, less first-layer squish, or a small chamfer) instead of shrinking the whole model.

Fast verification workflow (10–20 minutes)

1. Pick one critical feature to tune (a specific hole size, a snap gap, a sliding rail, a pin).
2. Print a coupon that isolates that feature (hole strip with multiple diameters; pin-and-socket pair; a short sliding dovetail).
3. Measure with calipers and write down: target, printed value, error, filament, nozzle size, line width, layer height, and orientation.
4. Change one setting only (scale OR horizontal expansion OR hole compensation) by a small step, then reprint the same coupon.
5. When the coupon hits your fit goal, apply that exact profile to the full model and keep the notes with the project.

Rule-of-thumb starting values (then confirm with a coupon)

Hole clearance (M3–M5 pass-through)

Add 0.1–0.3 mm on diameter (or use hole compensation roughly +0.05–0.15 mm on radius).

Sliding fit (plastic on plastic)

Plan 0.2–0.4 mm total XY gap; increase if surfaces are rough, cooling is poor, or parts are long and can warp.

Press-fit pins

Try only 0.05–0.15 mm interference if your printer is consistent and the material is tough; otherwise design a lead-in chamfer and test.

Elephant foot

Prefer elephant-foot compensation or a 0.3–0.6 mm bottom chamfer; avoid shrinking the whole part to hide it.

When compensation makes things worse