How to Measure a Part So the Print Fits
The hole to draw for a pin, bearing, bushing, sheet or bolt, from what you measured, with the allowances our hardware catalogue uses.
A printed part that has to fit something, a shaft, a bearing, a sheet of acrylic, a bolt, is decided before it is printed: by the number you measure and the number you draw. They are not the same number. A hole drawn at exactly the size of the pin will not take the pin. Measure what goes in, choose how it should fit, and draw the hole below. The allowances come from the hardware catalogue we fit parts from, where every pin, bearing and bolt lists the hole it wants.
What goes into the print
How it should fit
Slip fit: slides or turns with almost no play.
Draw the hole
Ø8.20 mm
+0.20 mm on what you measured. The same allowance our catalogue gives all 12 pins, shafts and rods it lists for this fit.
The same rule in our catalogue
- D8×40 dowel pin, stainlessØ8.20
- D6×30 dowel pin, stainlessØ6.20
- Aluminum rod, 6 mm, per 100 mmØ6.20
Our printed dimensions hold to ±0.5 mm or ±1%, whichever is greater: ±0.50 mm at this size. A press or slip fit asks for less than that, so when a fit matters, order one, try it, and adjust the drawing before you order more.
Allowances from the spec lines of the pins, shafts, bearings, bushings, rods, sheets and threaded rod in our hardware catalogue, read 2026-10-18; prices read live. The drawing shows the difference ten times bigger than it is.
The rule of thumb
Draw the hole from the fit you want, not from the size of the part.
From our catalogueFor pins and shafts: 0.1 mm under to press in, 0.2 mm over to slide, 0.3 mm over to drop in. A 3 mm dowel presses into Ø2.9 and slides in Ø3.2.
How to measure
- Use calipers
- Digital calipers read to a hundredth of a millimetre, which is the scale a fit lives at. A ruler is fine for the overall size of a part, not for a hole.
- Measure the real thing
- Measure the shaft, bearing or sheet you have in your hand, not the size on its label. A rod sold as 1/4 inch can be a little under, and a sheet sold as 3 mm is often not exactly 3.00.
- Measure three times
- Turn the part and measure across a different diameter each time. If the readings differ, it is slightly out of round; use the largest for anything going into a hole.
- Work in millimetres
- Convert inches before you draw (1 inch = 25.4 mm). The panel does it for you, and an STL is read as millimetres when it is printed.
- Write down the depth
- A pocket, a through-hole and a slot each need a depth as well as a width. Note it while the calipers are out.
The fits, from our catalogue
- Press fit
- Pushed in once and stays: pins and shafts 0.10 mm under, bronze bushings 0.05 mm under. Ball bearings go the other way, 0.05 mm over the outer ring, because the race must not be squeezed. A 608 bearing (22 mm outside) sits in a Ø22.05 pocket.
- Slip fit
- Slides or turns with almost no play: 0.20 mm over the pin or shaft.
- Free fit
- Drops in and lifts out: 0.30 mm over a pin, and a slot 0.30 mm wider than the sheet it holds.
- Glued
- Room for a film of glue: 0.1 to 0.3 mm over the rod, more for bigger rods.
- Bolt clearance
- A bolt that passes through: Ø3.4 for M3, Ø4.5 for M4, and up to a millimetre over for the largest sizes we stock.
For holes that take heat-set inserts and magnets, see insert and magnet hole sizes.
When the fit really matters
- Know the tolerance
- Our printed dimensions hold to ±0.5 mm or ±1%, whichever is greater. A press or slip fit asks for less than that, so the first print is a test.
- Order one first
- Upload it on our online 3D printing service, order a single copy, and try the fit before ordering the rest. There is no quantity minimum.
- Adjust and reorder
- Too tight: open the hole by 0.1 mm. Too loose: close it by 0.1 mm. Then order the run.
Before uploading, check the STL for units and size, and see what one test part costs.
FAQ
From our catalogue: 0.2 mm over the pin for a slip fit, 0.3 mm for a free fit, and 0.1 mm under for a press fit. A bolt needs more: Ø3.4 mm for M3 and Ø4.5 mm for M4.
A 608 bearing is 22 mm across, and our catalogue gives a Ø22.05 mm press-fit pocket for it.
Ø2.9 mm to press it in, Ø3.2 mm for it to slide or turn.
Measure the sheet, then add 0.3 mm: our catalogue gives a 3.3 mm slot for its 3 mm acrylic. Sheet sold as 3 mm is often not exactly 3.00, so measure yours.
Within ±0.5 mm or ±1% of the drawing, whichever is greater. When a fit matters, order one, test it and adjust the drawing.
Measured, drawn, printed.
Upload the part on the parts page for its price in seconds. Order one to test the fit, then as many as you need.
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