Dimensional Calibration

Print a small dimensional test, measure outer XY, Z height, and internal features (holes/slots) with calipers, then apply the smallest targeted correction that matches the error pattern. Calibrate flow first if outer dimensions are off; use XY size compensation for global XY bias; use hole/slot compensation (or CAD clearance) for internal features, because holes often print undersized even when outside sizes look perfect.

TL;DR

Print a dimensional test, let it cool, then measure outer XY, Z, and at least one hole and slot with calipers. Fix outside size with flow and/or XY compensation; fix holes/slots with hole/inner compensation or CAD clearance (don’t “chase” holes by changing flow if the outside is already correct).

Dimensional Calibration: what to adjustTopic-specific diagram for the concept, checks, and tradeoffs in this lesson.Measure XYouter width/lengthMeasure ZheightMeasure holesdia, 2 axesMeasure slotsclearanceFlow/EMfix global sizeXY compfine XY adjust.
A compact decision/matrix diagram helps learners map each measurement (XY, Z, holes, slots) to the most common adjustment without implying a single global fix.

Goal (what “dimensionally calibrated” means)

You want printed parts to match CAD closely enough that fits are predictable: outside width/length (XY), height (Z), and feature sizes like holes and slots. The key is to measure repeatably and change one thing at a time so you can tell which knob actually moved the result.

Tools and print setup

  • Digital calipers (0.01 mm resolution is fine)
  • A known filament and a stable profile with a reliable first layer
  • A dimensional test with: flat outer faces, at least one round hole, and one slot/clearance gap
  • Print with “normal” settings you use for real parts (avoid extreme draft speeds or unusual line widths)
  • Let the part cool to room temperature before measuring (plastic shrinks as it cools)

What to measure (and what each measurement tells you)

  • Outer XY (e.g., a 20.00 mm boss or cube face): reveals global XY bias from flow/line width and motion
  • Z height: reveals Z steps/mm problems, Z binding, or first-layer squish/elephant’s foot affecting the base
  • Hole diameter (e.g., 6–10 mm): reveals “FDM hole bias” (holes tend to print undersized even when the outside is correct)
  • Slot width / clearance gap: reveals how mating parts will actually fit, including corner effects and surface texture

How to measure so the numbers mean something

  1. Outer faces: measure each side in two spots and average. Avoid corners and avoid the first few layers if you see elephant’s foot.
  2. Z height: measure on a flat top surface; if the bottom is flared, don’t include that flare in your mental “Z error.”
  3. Holes: measure in two perpendicular directions and average; printed holes can be slightly oval depending on cooling and pathing.
  4. Record a small table: target, measured, and error for outer XY, Z, hole, and slot. (Example error = measured − target.)

Common patterns and the first fix to try

Outer XY is consistently oversized (e.g., +0.20 mm on a 20 mm feature)

Likely cause: Flow/line width effectively too large (over-extrusion), or slicer assumptions about line width/perimeter overlap don’t match reality

Fix: Calibrate flow/extrusion multiplier using your preferred method, then reprint/remeasure. If flow is already correct, apply a small negative XY size compensation/horizontal expansion.

Outer XY is consistently undersized

Likely cause: Under-extrusion, belts/pulleys slipping, or motion not reaching the commanded position

Fix: Verify extrusion is consistent and mechanics are solid (belt tension, pulley grub screws, no binding). If mechanics/flow are good, apply a small positive XY compensation.

Z height is off while outer XY is close

Likely cause: Incorrect Z steps/mm, Z binding/backlash, or you’re “measuring” first-layer squish/elephant’s foot as a Z error

Fix: If only the bottom is distorted, fix first-layer Z offset and elephant’s foot before touching steps/mm. If the entire height is proportionally wrong, verify Z steps/mm and check the Z axis for smooth travel.

Holes print undersized but outer dimensions look good

Likely cause: Normal FDM hole bias from toolpath geometry and shrink

Fix: Use slicer hole compensation (or an internal-only horizontal expansion if available). If you don’t have that control, increase hole diameter in CAD by the measured shortfall and validate with a hole test.

Slots/clearances are tight though dimensions look correct

Likely cause: Internal corners and surface texture reduce effective clearance; extrusion width limits how “sharp” internal geometry can be

Fix: Increase designed clearance (often the most reliable). If your slicer supports separate inner/outer compensation, adjust internal features without changing the outside. Validate with a clearance gauge/test piece.