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.
Centre Finder, assembled
Every physical part, bought ones included. Hover a row to find it in the model, click a part to read what it is, and colour by function or by part.
Click a part in the model to read what it is.
- Printed
- Hardware
Assembling the centre finder
The names in bold are the names the model uses. Hover a part in the list and it lights up in the model; click it in the model and it tells you what it is. The two bought parts are in there too.
You print one part, the beam. You buy two 623ZZ bearings and two Ø3 steel dowels — the bearing's bore fixes the dowel's diameter, and the model shows the dowel's length.
1. The dowels go in from above
Each end of the beam has one plain bore, straight through. Stand the beam on its underside — the flat face that lies on the board — and press a dowel into each bore from the top face.
It is a press fit: use a vice or a small arbor press with a block of wood on the dowel, and press rather than hammer, so the beam is never struck. Press until the dowel's lower end stands out of the underside by the width of a 623ZZ — the assembly model shows exactly how far. On a tool made for boards thinner than a bearing is wide, the dowel's upper end then stands a little proud of the top face; that is right, and it is what the model shows.
The fit alone holds the dowel. No screw, no nut, no glue. If a dowel ever turns freely in its bore, put one drop of cyanoacrylate on it before pressing it back in, and wipe the squeeze-out before it reaches where the bearing sits.
2. The bearings go on from below
Push each 623ZZ up onto the end of its dowel from below, until it stops against the beam's underside — or, on a tool made for thin boards, until it seats in the shallow recess cut there for it.
Nothing clamps the bearing. It runs on the dowel and the dowel's end is flush with the bearing's underside, so nothing below the bearing can touch the board. It must turn freely with a fingertip.
3. The pencil goes through the middle
One plain hole through the middle of the beam, top to bottom. Put the pencil in from above and let the point show just below the underside — only as far as it needs to mark. How far the graphite protrudes is yours to keep small.
4. Check before you use it
- Both bearings turn freely. A bearing that drags has been pushed onto a dowel that was not pressed square, or into a recess that grips its outer race; back it off and look.
- Both stand equally proud. Stand the beam on its bearings on a flat surface: it stands on both and does not rock.
- It does not rock on the thinnest board. Lay the beam across a board as thin as the thinnest you gave the form: the underside sits flat on the board, both bearings hang beside the board's edges, and neither reaches below the board's underside.
5. Using it
Lay the beam across the board, at an angle, and turn it until both bearings touch the board's edges. The pencil is then over the centre line. Slide the tool along the board and it draws it.