CAD for making

Tolerances, clearances, and the fit test coupon

Why your holes come out small, and the twenty-minute calibration that ends the guessing.

The part is never the size you typed

You model a 5.00 mm hole. You print it. It measures 4.7. You model a 5.00 mm peg and it measures 5.1. Put them together and the peg is 0.4 mm too fat for a hole that was supposed to be exactly its size.

Nothing has gone wrong. Extruded plastic is pushed slightly wide of the toolpath, it shrinks as it cools, and a circle traced by a nozzle of finite width loses a little on the inside of every curve. The errors are systematic, they are repeatable, and that means they are correctable — once you know their size on your printer.

Three fits, and choosing on purpose

Three clearance values shown as gaps between a peg and a holePress fit0.1 mm per side — assemble once, stays putSlip fit0.3 mm per side — pushes together by handFree fit0.5 mm per side — moves and rotates freely
Clearance is per side. A hole that needs 0.3 mm of clearance around a 10 mm peg is modelled at 10.6 mm, not 10.3 — the gap exists on both sides of the part.
  • Press fit — no gap or a slight interference. Goes together with force and stays there. Use it for pins, dowels, and parts you never intend to separate. Get it wrong in the tight direction and you split the part.
  • Slip fit — assembles by hand, holds its position, comes apart when you want it to. This is the fit you want for lids, snap-together enclosures, and anything you will open again.
  • Free fit — moves and rotates freely. Hinges, axles, sliding drawers. Anything intended to keep moving after the plastic has had a chance to relax.

Published numbers disagree, and that is the lesson

Look at what three reputable sources recommend as a starting gap:

  • Hydra Research — about 0.1 mm for a tight fit, about 0.2 mm for a loose one.
  • Layer X — at least 0.3 mm per side as a practical starting point for parts that must fit together.
  • 3DPut — 0.05 to 0.10 mm press, 0.20 to 0.30 mm sliding, 0.40 to 0.50 mm loose.

None of them is wrong. They are describing different printers, nozzles, materials and slicer profiles. Copying a number off a web page is how you end up reprinting a lid four times. Measuring your own takes twenty minutes, once, and then you are done for as long as you keep that printer and profile.

The one calibration worth doing

Print a coupon: a plate with a row of holes stepping up in size against a peg of known nominal diameter. Push the peg into each hole in turn. The smallest hole it enters without force is your slip fit; the one before it is your press fit; a step or two up is your free fit. Then write those numbers into your variables and never guess again.

Where to put the gap

Clearance has to live somewhere, and you get to choose. Two rules make the choice easy.

  • Put it on the part that is cheaper to reprint. If a bracket mounts to a bought fastener, the fastener is fixed and every bit of adjustment goes into your part.
  • Put it on the part you did not measure. When mating to something real — a battery, a pipe, a phone — measure that object carefully and let your model carry the whole gap.

And keep it in one variable. #clearance from lesson five exists precisely so that the day you switch filament, one edit fixes every fit in the model.

Hands on

Measure your printer's real clearance

Done when: You can state your press, slip and free clearance in millimetres, and #clearance in your document is set from measurement rather than from a blog post.

  1. New Part Studio. Sketch a plate about 80 × 25 mm and extrude it 4 mm.
  2. Add a variable #peg = 10 mm. Sketch six circles in a row, each dimensioned as #peg + 0 mm, #peg + 0.1 mm and so on up to #peg + 0.5 mm. Cut them through all.
  3. Engrave the offset next to each hole with the Text tool — 0.5 mm deep is plenty. Unlabelled test parts are useless within a day.
  4. In the same Part Studio, make a separate peg: a #peg diameter cylinder, 15 mm tall, with a small chamfer on the leading edge.
  5. Print both flat on the plate, in your normal material and profile. No supports needed.
  6. Test every hole with the peg. Record three numbers: the tightest that goes in at all, the one that slips in by hand, and the one that spins freely.
  7. Open your bracket document and set #clearance to your measured slip-fit value. Then write the three numbers down somewhere permanent — the workspace has a learning/cad/learning-records/ folder for exactly this kind of hard-won fact.

The numbers you measure go on the design rules sheet, which has blanks for them next to the published defaults.

Check your recall

Answer from memory — no scrolling back.

  1. 1. A 10 mm peg needs 0.3 mm of clearance. Model the hole at:
  2. 2. Why do reputable sources quote different clearance values?
  3. 3. Two parts must fit and one is a bought fastener. Where goes the gap?
  4. 4. When should you reprint the clearance test coupon?

Read this next — primary source

Design Rules & Best Practices for FFF 3D Printing

Hydra Research

A single-page rule sheet from a printer manufacturer, with numbers expressed as multiples of extrusion width rather than as absolutes. Read it next to the Protolabs page from the last lesson and notice where the two disagree — that gap is the whole argument for measuring your own machine.

Stuck, curious, or think this lesson is wrong? Ask your teaching agent. That’s the point of the workspace — the lessons are the scaffold, the conversation is where the learning gets unstuck.