Holes Print Small and Pegs Print Large
FDM prints are biased: internal features (holes, slots) tend to come out smaller, while external features (pegs, bosses, outer diameters) tend to come out larger. This comes from finite bead width, perimeter overlap on inside curves, and “puffy” perimeters from excess flow/heat or weak cooling. Plan clearance intentionally, then confirm your printer+filament combo with a small stepped hole/peg test before committing to an assembly.
TL;DR
Expect holes to print undersized and pegs to print oversized, especially near your line width. Build in clearance (or plan to drill/ream), then validate with a small hole/peg test before printing the full part.
What you will observe on real prints
Round vertical holes usually measure smaller than the CAD diameter, and pegs/outer diameters usually measure slightly larger. The bias is most noticeable on small features, tight fits, and any geometry close to your nozzle line width (for example, a 3–6 mm hole with a 0.4 mm nozzle).
Why holes print small (internal features)
A printed “line” is actually a squished bead with width. On an inside curve, the toolpath can’t place that width without some overlap toward the center, so the effective hole diameter shrinks. Small circles are also approximated by many short segments, and the corners between segments get rounded by molten plastic, which further steals area from the hole. Slight over-extrusion, too-hot plastic, or weak cooling makes the inner perimeter bulge inward even more.
Why pegs print large (external features)
On an outside curve, the same bead width and rounding tend to push material outward, increasing the effective outer diameter. If flow is a bit high, temperature is high, or cooling is weak, the outer perimeter stays soft longer and “puffs” outward. Small pegs are extra sensitive because one or two perimeters make up most of the feature, so a tiny perimeter error becomes a big percentage of the diameter.
What changes the size bias the most
- Line width
- Wider lines magnify both effects on small features.
- Flow / extrusion mult.
- Too high: holes shrink, pegs grow; too low: gaps/weak walls.
- Temperature
- Too hot makes perimeters round and fat; too cool risks poor bonding.
- Cooling
- Weak cooling leaves edges soft and oversize; strong cooling helps hold shape.
- Layer height
- Coarser layers worsen stair-stepping on bores/shafts and hurt roundness.
- Orientation
- Vertical holes go polygonal and small; horizontal holes sag and go oval.
- Material shrink
- Higher-shrink plastics can change fit more across ambient/part temps.
Design choices that avoid “mystery fits”
- Add intentional clearance for moving/sliding fits instead of aiming for perfect nominal dimensions.
- Treat printed holes as near-net shape unless tested; for precision, size for drilling/reaming or use heat-set inserts where appropriate.
- For press-fits, keep the engagement length short, add a lead-in chamfer, and avoid long straight interference that will split parts.
- Prefer flats/hex pockets for torque transmission (nuts, shafts) instead of relying on a tight round hole.
- When a hole is near nozzle size, expect a step-change in behavior; test where your printer stops producing usable circular holes.
Quick validation workflow (10–20 minutes of printing that saves hours)
- Print a small test coupon with the hole/peg sizes you care about, stepping in 0.1–0.2 mm increments (and in the same orientation as your real part).
- Measure with calipers: record CAD size, printed size, and whether it was a hole or a peg.
- Pick one adjustment path and stick to it for the test: CAD clearance, slicer XY/horizontal compensation, or flow calibration.
- Reprint the same coupon until the fit you need is repeatable, not just “close once.”
- Save the results with the filament type, nozzle size, line width, and profile name so you can reuse the numbers later.