Notes · 3D printing & CAD
Designing 3D printed holes for 1/4" hex bits
A 1/4" hex driver bit measures 6.35 mm across the flats. Draw a hole at exactly that size and the bit won't go in. Draw it too big and the bits fall out of your holder the first time you tip it over.
Getting it right comes down to two things: knowing which radius your CAD tool is asking for, and adding the right amount of clearance for your printer.
Across flats, inscribed and circumscribed
A hexagon has two "radii", and CAD tools don't agree on which one they mean.
- Across flats (the bit)
- 6.35 mm
- Inscribed radius: centre to middle of a flat
- 3.175 mm
- Circumscribed radius: centre to a corner
- 3.667 mm
Those are the nominal sizes with no clearance. A hole drawn at these sizes will be too tight once it's printed.
How much clearance to add
For a comfortable drop-in fit, aim for about 6.55–6.6 mm across the flats, which is roughly 0.2–0.25 mm bigger than the bit.
| Fit | Across flats | Inscribed radius | Circumscribed radius |
|---|---|---|---|
| Snug, friction hold | 6.45 mm | 3.23 mm | 3.72 mm |
| Comfortable, drop-in | 6.60 mm | 3.30 mm | 3.81 mm |
| Loose | 6.75 mm | 3.38 mm | 3.90 mm |
Which number does my CAD tool want?
This is where most first attempts go wrong. The polygon tool asks for a radius or diameter, but programs use the words "inscribed" and "circumscribed" differently, and some of them the opposite way round to what you'd expect.
- If the dimension goes to a corner, enter the circumscribed size: 3.81 mm radius, or 7.62 mm diameter, for a drop-in fit.
- If the dimension goes to the middle of a flat, enter the inscribed size: 3.30 mm radius, or 6.60 mm diameter.
- Watch Fusion's naming. Its "Circumscribed Polygon" draws the hexagon around the circle, so the circle touches the flats and its diameter is the across-flats size. "Inscribed Polygon" puts the corners on the circle.
After you draw the hexagon, add a dimension between two opposite flats. It should read 6.6 mm (or whichever fit you chose). If it reads about 5.7 mm or 7.6 mm, you entered the right number into the wrong kind of polygon.
In OpenSCAD, the radius of a six-sided cylinder goes to the corners:
af = 6.6; // across flats, drop-in fit
cylinder(r = af / sqrt(3), h = 12, $fn = 6);Hex hole calculator
For other bit sizes, such as 4 mm precision bits or 5/16" (7.94 mm), put in the nominal size and the clearance you want.
- Hole across flats
- –
- Inscribed radius (to a flat)
- –
- Circumscribed radius (to a corner)
- –
- Across corners
- –
Print a test block first
Printers vary, and so do filaments. Before you design a full bit holder, print a small block with holes at 6.45, 6.55, 6.65 and 6.75 mm across the flats and try a bit in each. It prints in well under an hour and tells you the right clearance for your printer.
Download the test block (STL, free)
Practical tips
- Chamfer the openings. On a vertical hole, the first layers squash outward (elephant's foot) and close the bottom of the hole up. A 0.5 mm chamfer at each opening fixes that and helps the bit find its way in. The test block has one at both ends.
- Horizontal holes: put a flat at the top. The top of the hole then becomes a short 3.8 mm bridge, which prints cleanly. With a corner at the top, the two upper faces are only 30° from horizontal and will droop.
- For a bit holder, go snug. Somewhere in the 6.45–6.5 mm range holds bits by friction, so they won't fall out and you don't need magnets.
- Write your result down. Once the test block tells you your printer's number, put it in your CAD template as a parameter. You'll never have to guess again.