Two-Way Radius Sanding Block
Beta This jig is in beta: the files are generated and checked, but the design may still change. Check the drawing against your instrument before you cut.
3D-printed block for radiusing fingerboards and leveling frets. Two working faces, each with its own radius chosen in the form. Hollow, light, no engraving on the working faces.
Two radii, one print
The block has two concave working faces — top and bottom — each with a different radius. One print covers two radii, e.g. 12" and 16", or the fingerboard radius and the target radius on a compound-radius board. You set both radii in the form before generating the files; the usual values are offered (7.25", 9.5", 10", 12", 14", 15", 16", 20") and any custom value can be entered.
The cross-section is hollow with longitudinal channels — the block is lighter and uses less filament, while thick flanges under the working faces and webs between the channels keep it stiff over its full length. The sides have a concave grip profile so the block sits well in the hand.
There is no engraving on the working or grip faces — engraved text can leave marks on adjacent layers and spoil flatness. Which face has which radius is shown in the 3D preview (each face has its own colour). Optionally the radii can be engraved on the top end face of the block.
Printing
The block prints standing on end (length = print height), so each radius arc is drawn exactly in every layer with no stair-stepping, and the channels are vertical and need no supports. Recommended material: PLA (stiff, cheap, fast); PETG also works.
After printing, check the working face for straightness with a straightedge along the full length. If it is off, true the face on sandpaper laid on a sheet of glass. PLA softens above ~55 °C — do not leave the block in a hot car or near a heat source.
Preparation and use
- Print the block standing on end, with a brim, in PLA or PETG. Do not scale the model — the radii depend on the dimensions.
- Remove the brim and any strings. Check both working faces for straightness with a straightedge; true on sandpaper on glass if needed.
- Cut PSA sandpaper to the block length and apply it to the chosen working face, pressing from the centre outwards so it follows the radius without creases. The paper must not overhang the edges.
- Radiusing a fingerboard: move the block along the neck, parallel to its axis, always using the full length of the face. Do not tilt sideways. Check the radius with a gauge.
- Fret leveling: mark the fret crowns with a marker first, then work the block lightly until the marker disappears evenly from every fret. Then crown and polish.
- Changing radius: flip the block — the other face has the second radius. Peel off the used paper and apply fresh.
- Store flat, away from heat.