Blog

3D Printing Tolerances: Design Parts That Fit

By Dynafabs

3D printing tolerances explained: clearances for press, sliding and loose fits, plus holes, threads and orientation tips so your parts fit first time.

Why tolerances matter for 3D printed parts

You've designed a lid, a hinge or a bracket that bolts onto something else. On screen it fits perfectly. Then the print arrives and the lid won't close, the pin won't go in, or the bolt rattles around in its hole. Nine times out of ten the culprit isn't the printer or the material. It's that the design didn't leave enough room for how 3D printing actually behaves.

Every manufacturing process has tolerances, and 3D printing is no different. Plastic shrinks slightly as it cools, holes tend to print a touch smaller than drawn, and the first layers can spread a little wider than the rest. None of that is a problem once you design for it. This guide covers the practical side of tolerances and clearances, so the parts you order from Dynafabs fit first time.

If you're still choosing a material, our PLA vs PETG vs ABS/ASA vs TPU guide is a good place to start.

Tolerance versus clearance

These two words get mixed up a lot, so it's worth separating them:

• Tolerance is how far a printed dimension might differ from the one in your file.
• Clearance is the gap you deliberately design between two parts that need to fit together.

You can't remove tolerance completely, but you can choose your clearance. Good clearance is what turns a "nearly fits" part into one that slides, snaps or presses together exactly as intended.

Choosing the right kind of fit

Start by deciding how the two parts should behave together. On FDM prints from a standard 0.4 mm nozzle, these are sensible starting points for the gap per side:

• Press fit (parts held by friction): a very small gap, often around 0.1 mm or less. Good for pegs, inserts and parts you don't plan to take apart.
• Snug or sliding fit (parts that move but don't wobble): roughly 0.2 mm. Ideal for lids, drawers and sliding covers.
• Loose or free fit (parts that spin or move easily): around 0.3 mm or more. Best for hinges, print-in-place joints and rotating parts.

Treat these as starting points rather than rules. The exact numbers vary with material, part size and geometry, which is why we're always happy to look over critical fits before printing. Resin printing can usually hold finer detail and tighter gaps than FDM, which makes it a better match for small, precise parts.

Holes, pins and threads

Holes print small. Round holes, especially small ones printed vertically, often come out slightly undersized. If a bolt or pin has to pass through, add a little extra clearance. For a really accurate hole, a common approach is to design it slightly undersized and drill or ream it to size afterwards.

Pins and shafts behave the other way round, so it's best to size the hole to the pin rather than the other way round.

Threads are possible to print, but fine threads are fragile and fiddly. For anything that gets screwed in and out repeatedly, heat-set threaded inserts or captive nuts are much tougher. Just tell us if you'd like inserts fitted.

How orientation affects accuracy

The way a part sits on the build plate changes how accurate each feature is. Circles printed flat on the bed (holes running up and down) come out rounder than holes printed on their side, which can droop slightly at the top. The first layer can also flare out a fraction, so a very tight fit right at the base may need a small chamfer.

Orientation affects strength too. We covered that in our layer height, infill and walls guide, so it's worth balancing both when a part has to fit and take a load.

Material makes a difference

Different materials shrink and flex in different ways:

• PLA is stiff and fairly predictable, which makes it great for test fits and prototypes.
• PETG is a little more forgiving and slightly flexible, so snap-fits are less likely to crack.
• ABS and ASA tend to shrink more as they cool, so larger parts may need a touch more clearance.
• TPU is flexible, so it can often take a tighter fit and simply squeeze into place.
• Nylon and carbon fibre blends are excellent for tough mechanical parts, though nylon absorbs moisture, which can change its fit slightly over time.

You can see what we print with on our materials page.

Our top tips for parts that fit

1. Print a test piece first. A small coupon with a few hole and pin sizes is cheap and quick, and it answers the question for your exact material. 2. Add chamfers and fillets to edges that slide together. They guide parts into place and hide that first-layer flare. 3. Design one part to adjust. If only one side of a fit needs tweaking, a reprint costs less. 4. Tell us which dimensions are critical. A quick note with your file means we know what to check.

FAQ

Can Dynafabs match a dimension in my CAD file exactly? We'll get as close as the process allows, but every printed part has some tolerance. Let us know which dimensions matter most and we'll advise on clearances.

Should I adjust my design or let you do it? If you're comfortable, build the clearance into your file. If not, tell us how the parts should fit and we'll talk it through with your quote.

Is resin better for tight fits? Often, yes, for small and detailed parts. For larger or tougher functional parts, FDM in the right material is usually the better choice. Our services page and FAQ cover both.

Ready to print parts that fit?

Upload your file, mention any parts it needs to fit with, and we'll suggest the right material and settings. Get a quote from Dynafabs whenever you're ready.

--- #3DPrinting #3DPrintingTolerances #DesignForPrinting #PrintSettings #FunctionalParts #3DPrintingTips #Dynafabs