The model is perfect and the print is not
On screen, a 10 mm peg slides neatly into a 10 mm hole. Print both and the peg often will not go in. Printing is a physical process, and several small effects add up to differences of a fraction of a millimeter, which is plenty to ruin a fit. Learning to allow for this is one of the most practical skills in desktop printing.
Safety note: when you test-fit parts, do not force them with tools in a way that can slip or snap plastic. Wear eye protection when trimming or reaming holes, and keep fingers out of the way of knives and drills.
Why sizes drift
A few causes are common:
- Extruded lines have width and shape. The nozzle lays down rounded lines of plastic that spread slightly when pressed onto the layer below. Outer walls tend to bulge outward a little, which makes pegs larger and holes smaller.
- Small holes suffer most. A round hole is drawn from a series of short straight moves, and the plastic sags inward at corners. Small circles often come out noticeably under size.
- The first layer is squashed. Pressing the first layer firmly onto the bed often produces a slight flare at the base, sometimes called an elephant’s foot. This affects any part that mates at the bottom.
- Plastic shrinks as it cools. The amount differs between filament types. Some shrink more than others, which matters for large parts.
- Machine calibration. Belts, steps per millimeter and flow settings all matter. A printer that is slightly off in one direction will make all parts slightly off.
Because these causes depend on your machine, material and settings, the right allowance for one printer is only a starting point for another.
Measure before you guess
Use calipers if you can. Print a simple calibration object, such as a block with holes and pegs of known sizes, and measure what actually came out. Record how much each dimension differs from the model. Then you can compensate in the design by a consistent amount. Keep notes by material, because a change of plastic or nozzle can move the numbers.
Calipers themselves have limits, and the way you measure matters. Measure at several points and avoid squeezing soft plastic.
Clearance: designing the gap
The gap between mating parts is called clearance. Different jobs need different amounts:
- Press fit. The parts are meant to hold by friction. The gap is very small or the peg is slightly larger than the hole.
- Snug or location fit. The parts go together by hand and stay without wobbling.
- Sliding or running fit. One part moves freely against another, such as a hinge pin or a lid. This needs more room.
- Loose fit. Easy assembly, with play. Fine for parts that are fastened another way.
As a very rough starting point, many people begin with something like 0.2 to 0.4 mm of total clearance for parts that should slide, and test from there. The best number depends on your printer, so print a small test set at several clearances and pick the one that feels right. Do not treat the above as a rule.
Practical design habits
- Make holes slightly larger in the model. Add a small offset, such as an extra tenth of a millimeter or two, and adjust after testing.
- Drill or ream critical holes afterward. A drill bit or reamer gives a more accurate hole than printing. Do this with care and eye protection, and hold the part securely.
- Use sacrificial test pieces. Print just the joint, not the whole object, to check a fit in minutes.
- Orient fits well. Holes that run vertically print rounder than horizontal ones, which may need a teardrop shape to avoid sagging, a topic related to avoiding support structures.
- Keep tight fits thick enough. A press-fit boss with very thin walls may crack when pushed. Designing parts with enough wall thickness avoids this.
- Plan for the screw, not just the hole. A threaded fastener cutting into plastic needs a pilot hole sized to the screw type. Check the guidance for your fasteners and test.
- Avoid fusing moving parts. For print-in-place hinges and moving joints, extra clearance prevents layers from welding together.
Fits that change over time
Printed parts are not always stable. Some plastics soften with warmth, creep under constant load or absorb moisture. A press fit that is perfect today may loosen later, and a tight lid may swell in humid air with certain materials. If a part must stay precise, think about stronger materials and mechanical fasteners instead of relying on friction alone.
Tolerance as part of the design process
Good fit is not a single setting but an iteration. Designers building anything, from small gadgets to products made in volume, use the same loop: guess, test, measure, adjust. This is why learning CAD modeling pays off, as you can change a dimension in the model and reprint quickly. It also connects to design for manufacture, since a part that depends on very tight tolerances is harder to repeat reliably, especially if you later make many copies.
If you are building enclosures for electronics, remember that board and component dimensions vary between sellers, so measure the parts you actually have before finalizing the design. Use the real component, not a datasheet drawing alone, whenever you can.
A short checklist
- Print a test with several clearances.
- Measure with calipers and record the results.
- Add offsets to your design based on those results.
- Retest when you change material, nozzle or settings.
- Use drilling or sanding for the most critical surfaces.