How Much Clearance Should I Leave for Magnets in a 3D Printed Hole or Pocket?
For FDM prints, start with a pocket 0.1 to 0.15 mm larger than the magnet’s diameter for a press fit, or 0.2 to 0.3 mm larger for a glued fit, and 0.1 to 0.2 mm deeper than the magnet so it sits flush. For magnets sealed in mid-print, make the pocket about 0.2 mm larger in both width and depth. Resin printers need less, about 0.05 to 0.15 mm. Every printer and filament is a little different, so print a quick test coupon before committing to a design.
- FDM press fit
- Magnet diameter + 0.1 to 0.15 mm
- FDM glued fit
- Magnet diameter + 0.2 to 0.3 mm
- Pocket depth
- Magnet thickness + 0.1 to 0.2 mm
- Resin press fit
- Magnet diameter + 0.05 to 0.1 mm
Why Do 3D Printed Holes Come Out Small?
Holes and pockets in FDM prints almost always print slightly undersized. Curves are made of short straight segments, the extruded plastic squishes outward as each line is laid down, and plastic shrinks a little as it cools. The first layer is squished most of all, so pockets that open on the bottom face are tighter at the opening. That’s why a pocket modeled at exactly the magnet’s size usually won’t accept the magnet.
Recommended Clearance by Fit
| Fit | Diameter (or width) | Depth | Notes |
|---|---|---|---|
| FDM press fit | + 0.1 to 0.15 mm | + 0.1 to 0.2 mm | Holds without glue in snug prints |
| FDM glued fit | + 0.2 to 0.3 mm | + 0.1 to 0.2 mm | Room for super glue or epoxy |
| FDM captive (print-in) | + 0.2 mm | + 0.2 mm, then 2 to 4 layers on top | Pause at the first covering layer |
| Resin press fit | + 0.05 to 0.1 mm | + 0.05 to 0.1 mm | Resin prints are more accurate |
| Resin glued fit | + 0.1 to 0.15 mm | + 0.1 mm |
These are starting points, consistent with our guide to embedding magnets in 3D prints. Tune them with a test coupon for your printer and filament.
Diameter vs. Per-Side Clearance
CAD tools and slicers describe clearance two ways. Adding 0.2 mm to a pocket’s diameter leaves 0.1 mm per side. If a design guide gives clearance per side, double it for the diameter. For blocks, add the clearance to both the length and the width.
How Deep Should the Pocket Be?
- Flush magnets: make the pocket 0.1 to 0.2 mm deeper than the magnet, so it doesn’t sit proud and scrape or rock.
- Glued magnets: add a little more depth if you use epoxy, so the glue has room under the magnet.
- Captive magnets: make the pocket about 0.2 mm taller than the magnet and round up to a whole number of layers, so the first covering layer prints just above the magnet instead of hitting it. For a 2 mm magnet at 0.2 mm layers, an 11-layer (2.2 mm) pocket works well.
Keep the covering layer thin, 2 to 4 layers. Every layer is an air gap that reduces holding power.
How to Print a Test Coupon
- Measure your magnets with calipers. Magnets have a small manufacturing tolerance, typically about ±0.1 mm or tighter, and plating adds a few microns.
- Model a small strip with five pockets: the magnet’s size plus 0.0, 0.1, 0.2, 0.3 and 0.4 mm.
- Print it in the same filament, orientation and settings you’ll use for the real part.
- Test each pocket. Note the tightest one the magnet presses into by hand, and the one where it drops in with a little play for glue.
- Save those values as parameters in your CAD model or as notes for that printer and filament.
Tips for a Better Fit
- Orientation matters. Pockets whose axis points straight up print round. Horizontal holes print slightly oval and droop at the top, so add about 0.1 mm more or use a teardrop shape.
- Add a lead-in chamfer of 0.3 to 0.5 mm at the opening to guide magnets into press fits and offset first-layer squish.
- Use your slicer’s hole compensation setting, called hole horizontal expansion or X-Y hole compensation in some slicers, instead of editing every model.
- Leave enough wall around the pocket, at least two or three perimeters, so a press fit doesn’t crack the part.
- Don’t force or hammer magnets in. Neodymium is brittle; press with a flat tool or a vise, or open the pocket slightly with a hand-held drill bit.
- Mark polarity first so every magnet goes in the right way round; see identifying magnet poles.
Takeaway
Model the pocket 0.1 to 0.15 mm over the magnet’s diameter for a press fit or 0.2 to 0.3 mm for glue, add 0.1 to 0.2 mm of depth, and size captive pockets about 0.2 mm over in both directions. Then print a five-pocket test coupon and keep the values that work on your printer.
- Metric Disc MagnetsSizes in Millimeters
- Disc MagnetsEvery Diameter and Thickness
- Starbond Super Glue for MagnetsFast Bond on Nickel Plating
More Questions About Magnets in 3D Prints
How do I embed magnets in 3D prints?
Press or glue them into pockets, or pause the print and seal them in. See embedding magnets in 3D prints.
What are the best magnets for 3D printing projects?
Small N52 discs in metric sizes, such as 6 × 2 mm and 6 × 3 mm. See best magnets for 3D printing.
Can the heat from a 3D printer damage neodymium magnets?
Usually not with PLA or PETG; hot beds, enclosures and annealing are the risks. See 3D printer heat and magnets.
How do I convert magnet sizes between inches and millimeters?
Use our size chart; for example, 1/4 in is 6.35 mm, not 6 mm. See the magnet size chart.