Centre Finder
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.
One printed body, two rollers on ball bearings and a marking hole in the middle. Laid across a workpiece and turned until both rollers touch the edges, the marker sits on the exact centre — on a tapered neck as well as a parallel one, because the rollers roll instead of dragging.
Rollers, not edges
One printed body carries two rollers on ball bearings at a fixed span, and a marking hole exactly between them. Lay it across the workpiece and turn it until both rollers touch the edges: the marker is then on the centre line, because the two rollers are equidistant from it by construction.
The rollers roll rather than drag, which is what makes it work on a tapered neck or a neck blank. A fixed-width gauge measures one width; this one finds the middle of whatever width it is resting on.
One body, modular ends
The body knows nothing about what a module does — only the interface. Three identical sockets take a left roller, a right roller and a centre insert, each held by a single M3 screw entering from the top into a nut pocket printed in the body. Nothing protrudes below the base plane, because every measurement this jig reports is taken from that face.
Roller modules: a fixed roller on a bearing, a spring-loaded roller that keeps contact on a carved profile, and a plain pin for anyone without bearings. Centre modules: a wood pencil, a mechanical pencil, a scriber, and drill guides in three sizes. Modules from a later catalogue fit an earlier body of the same interface version.
Where it is wrong
On a carved neck, use the spring-loaded roller, or set the roller height so the bearings ride the fingerboard edge rather than the belly of the neck. A roller resting on a curved surface touches it somewhere other than the widest point, and the mark moves by half that error.
On a fretted neck the rollers must clear the frets. If they ride on fret ends rather than on the fingerboard edge, the span is not the width you meant to halve.