Direct answer: when a 3D-printed hole is too small, do not scale the model or add compensation first. Let the part cool, remove only loose strings or a seam nub, measure the opening in two directions, and compare it with an outside reference on the same coupon. A tight bottom edge points to first-layer bulge; a flat or sagged roof points to horizontal-hole overhang behavior; a repeatable hole-only offset points to hole compensation or designed clearance; and a whole-part error belongs in the printer, flow, material, or source-model baseline.
This guide covers one exact symptom: an FDM hole or bore that measures smaller than intended or rejects the intended screw, pin, shaft, or bearing. It does not treat a cracked bore, an oval hole caused by a loose motion system, or a globally wrong part as the same problem. Work through the checks in order and change one variable per coupon.
Short answer: measure the error before adding compensation
Print a coupon with several hole diameters using the same material, layer height, nozzle, wall settings, and orientation as the real part. Let it cool fully, then measure each opening in more than one direction. Also measure an outside dimension on the coupon. If outside dimensions are accurate but every hole is consistently tight, use a small hole-compensation or CAD-clearance change. If the entire coupon is oversized, undersized, or distorted, fix the printer or material baseline first.
A single correction value is not universal. A 3 mm screw hole, a 10 mm shaft bore, and a 22 mm bearing seat can miss by different amounts. Vertical-axis holes and horizontal holes also fail for different reasons, so test the same orientation the final part will use.
What the shape of the hole tells you
| What you see | Likely cause | First check |
|---|---|---|
| Round hole is uniformly too small | Normal inward curve bias, excess flow, or cooling shrinkage | Compare the bore with an accurate outside dimension |
| Horizontal hole has a flat or drooping roof | Unsupported overhang and bridging at the upper arc | Rotate the part or test a teardrop-shaped hole |
| Hole is oval | Axis calibration, belt motion, cooling distortion, or poor measurement angle | Measure X and Y separately and inspect a square feature |
| Only the bottom of a vertical hole is tight | Elephant foot or first-layer over-squish | Inspect the first 1 to 3 mm instead of changing the entire bore |
| One side has a seam bump | Restart pressure or seam placement inside the bore | Find the vertical witness line and measure away from it |
| Large bores are close but tiny holes close up | Feature-size limit, path overlap, or a hole smaller than the process can resolve cleanly | Test several diameters instead of applying global scale |
Why FDM holes print smaller than the CAD model
The nozzle follows a path, not a mathematically perfect edge
A slicer places the centerline of each extrusion so the deposited bead should form the requested boundary. The bead has width, pressure, and rounded edges, however. On a tight internal curve, small path and pressure errors consume more of the opening than the same error would affect a broad outside wall. Curves are also represented by many short moves, so low-resolution source geometry can add visible facets.
Hot plastic pulls inward around a curve
Fresh extrusion is stretched and turned around the perimeter of the bore. Surface tension and cooling contraction tend to pull that line toward the open center. The effect is modest on a well-tuned machine, but it becomes noticeable when the requested fit has little clearance. Materials with greater shrinkage can amplify it, although material choice is only one part of the result.
The roof of a horizontal hole is an overhang
A hole whose axis runs parallel to the build plate is not printed as a stack of supported rings. As the nozzle reaches the upper arc, each new line has less material beneath it. The line can curl or sag inward, making the vertical diameter smaller even if the side-to-side diameter is close. More CAD clearance alone does not always create a clean roof; orientation, cooling, layer height, speed, or a self-supporting hole shape may matter more.
Too much plastic consumes the opening
If flow is genuinely high, every perimeter becomes wider than the slicer expects. Internal features close up while outside dimensions can grow. But do not reduce flow solely because one screw will not pass through one hole. Flow should be based on a controlled wall or mass test and on surface evidence, not used as a convenient hole-size knob.
How to check the problem without chasing random settings
- Let the part cool. A warm bore and a warm caliper can give a moving measurement, especially in larger parts.
- Remove loose strings and the seam nub. Do not grind or drill the test feature before recording its as-printed size.
- Measure in two directions. Check the largest and smallest diameter you can find without forcing the caliper jaws into the plastic.
- Measure an outside reference. A 20 mm outside width beside a 10 mm bore helps separate global scale error from internal-feature bias.
- Compare several hole sizes. Use at least three relevant diameters. A constant 0.2 mm error and a percentage-based error suggest different corrections.
- Repeat the real orientation. A flat coupon with vertical holes does not prove how a horizontal shaft bore will print in the final bracket.
Calipers are useful for a first diagnosis, but their inside jaws contact only small points and are easy to tilt. For a critical round bore, gauge pins, known drill shanks, a bearing, or the actual mating shaft can provide a more representative go/no-go check. Never assume a displayed hundredth of a millimeter is the same as real process capability.
Fix undersized holes in this order
1. Correct obvious first-layer and extrusion faults
If the bore is tight only at the base, solve first-layer bulge rather than enlarging the complete feature. The elephant-foot troubleshooting guide covers nozzle gap, bed heat, and first-layer compensation. If walls look swollen everywhere, verify the normal extrusion baseline before touching dimensional compensation.
2. Improve horizontal-hole support
When the upper arc droops, rotate the part so the hole axis is vertical if strength and build layout allow it. Otherwise try a smaller layer height, slower overhang speed, appropriate part cooling for the material, or a teardrop/diamond relief at the roof. A removable support interface may help a large opening, but support scars can make a fit-critical bore less predictable than designing a self-supporting top.
3. Use slicer hole compensation only for a repeatable hole-only error
Bambu Lab's current XY compensation guide separates XY Hole Compensation, which changes closed internal holes without changing the model's outside size, from XY Contour Compensation, which changes the outside contour while leaving hole sizes unchanged. That distinction is the decision gate: if an outside reference is already correct and several same-orientation holes are consistently tight, test the hole control. If outside geometry is also wrong, a hole-only offset hides the larger problem.
| Evidence after cooling | Next controlled fix | Do not conclude yet |
|---|---|---|
| Outside reference is correct; several vertical holes are undersized by a similar amount | Test a small XY hole-compensation change on the same coupon | One corrected coupon does not qualify every diameter, material, or orientation |
| Outside reference and holes are both wrong | Check source dimensions, extrusion, material behavior, and the machine baseline | Hole compensation is not a global calibration tool |
| Only the bottom of the hole is tight | Correct first-layer bulge or elephant-foot compensation | Enlarging the whole bore can make the upper region loose |
| A horizontal hole has a flat, drooped, or inward roof | Change orientation, roof geometry, layer height, cooling, or supported finishing | An XY offset cannot rebuild a failed overhang |
Before entering a value, confirm the slicer's sign convention and inspect the resliced preview. Record the software version, field name, value, material, nozzle, layer height, orientation, and measured result. Reprint at least two relevant diameters; a single 10 mm hole is not proof that a 3 mm screw clearance and a 22 mm bearing seat need the same correction.
4. Add clearance in CAD when the fit belongs to one design
For a screw clearance hole, connector opening, or sliding shaft, the model should include manufacturing clearance. Nominal shaft diameter is not the same as a free-moving bore. Prusa's current design guidance treats moving-part clearance, elephant-foot compensation, fine-detail limits, and filament behavior as design inputs rather than one universal tolerance. Use that as the boundary: add clearance from coupons made with the actual printer, material, orientation, and finish plan, then document the nominal interface and accepted file revision.
5. Finish critical bores after printing
Drilling, reaming, or boring can create a cleaner final size when the design allows it. Leave enough printed material for the operation, keep the tool aligned, and avoid overheating the plastic. A twist drill is good for clearance, but it may not hold a bearing-grade diameter or coaxiality by itself. Critical bearing seats may need a reamer, a machined insert, or a different production method.
Do not scale the whole model to fix one tight hole
Global scaling enlarges wall thicknesses, center distances, outside envelopes, snap fits, and every other feature. It can make one bore usable while breaking the rest of the assembly. Use global scale only when measurement proves the entire part is proportionally wrong and the source geometry is meant to be scaled. Hole compensation or local CAD clearance is usually the safer fix for an internal-feature bias.
Clearance holes, sliding fits, and press fits need different targets
A screw that only needs to pass through should not be designed like a locating dowel. A shaft that must rotate needs running clearance. A bearing seat may need controlled interference, roundness, and sufficient wall support. Those are separate fit classes, not one universal "make holes bigger" rule. The GoodPrints fit and tolerance guide explains how to document mating features, acceptance checks, and file revisions when a part is being quoted or repeated.
For production parts, define how the bore will be checked: as-printed diameter, pass/fail with the actual mating part, pin-gauge range, or final post-machined dimension. That check belongs in the release criteria, especially when different materials or print orientations may be used.
A controlled test sequence
- Print a compact coupon with several relevant hole sizes and one accurate outside reference.
- Measure after cooling and record both X and Y diameters.
- If only the bottom is tight, correct elephant foot.
- If horizontal roofs sag, fix orientation, cooling, layer height, or hole shape.
- If all geometry is swollen, verify extrusion and motion calibration.
- If outside dimensions are accurate and hole error is repeatable, test a small hole-compensation offset.
- Reprint the coupon before committing to the full part.
- For a critical fit, test the actual screw, shaft, pin, or bearing and record the accepted revision.
Use limits and the honest next step
- Ordinary screw-clearance hole: drilling after printing can be reasonable when the wall is thick enough, the hole does not locate another feature, and the finished opening is inspected.
- Dowel, bearing, press-fit, sealing, or paired bore: a caliper reading alone does not prove roundness, coaxiality, surface finish, or retained interference. Use the actual mating part, suitable gauges, or a planned reaming or machining operation.
- Thin wall or edge-near hole: enlarging the opening removes load-bearing material. Redesign the boss or wall before applying a large compensation value.
- Safety-, pressure-, lifting-, electrical-, medical-, or other consequence-heavy use: this coupon workflow is diagnostic, not qualification. Follow the applicable design, material, inspection, and test requirements.
If the error changes by diameter or orientation, keep separate evidence rather than averaging it into one offset. If a controlled coupon passes but the full part fails, investigate local heat, warping, support, tool access, and feature location on the real geometry. The next step is a geometry- and process-matched proof, not a larger global correction.
Common questions
How much bigger should I model a 3D printed hole?
There is no safe universal value. The needed clearance depends on diameter, material, printer, nozzle, layer height, orientation, and the type of fit. Measure a coupon made with the final process and use the smallest correction that repeatedly meets the fit.
Why are vertical holes accurate but horizontal holes too small?
Vertical holes are built as supported perimeters on each layer. Horizontal holes develop increasingly steep overhangs and a short bridge at the roof, so the upper arc can curl or sag inward. Rotate the feature or use a self-supporting roof before assuming the machine's XY calibration is wrong.
Should I calibrate X and Y steps for undersized holes?
Only if accurate measurement shows outside dimensions and multiple features are wrong along an axis. Modern motion systems usually do not need steps-per-millimeter changed for a hole-only error. Belt looseness, part cooling, flow, and feature compensation are more common causes.
Can wet filament make holes print small?
Moisture can roughen walls, increase stringing, and make measurement inconsistent, but it is rarely the sole cause of a clean, repeatable hole-size offset. Drying is justified when the extrusion shows moisture symptoms, not as the default response to every tight bore.
Can I just drill every printed hole?
For ordinary screw clearance, often yes if the design leaves enough wall and alignment is not critical. For bearing seats, paired bores, or location features, drilling may remove too much material or wander off-axis. Define the finish operation during design rather than treating it as a rescue step.
What to read next
- How to Specify Tolerances, Fit, and File Versions for Custom 3D Printed Parts
- Why Do 3D Prints Get Elephant Foot?
- Common 3D Print Quality Problems and What Usually Causes Them
If a fit-critical part needs repeatable bores, documented inspection, or a short production run, JC Print Farm can help move it from test coupon to controlled output. If the file and fit requirements are ready, request a quote at quote.jcsfy.com.