MAKING AND USING A JIG FOR CENTRALISING PLATES

It is often a challenge to locate the centre of a rib structure accurately, in order to position an endpin or centre a neck mortise or button. Placing a flexible ruler across the bouts and projecting marks down to the edge of the plate is adequate, but a ruler can distort easily when bent over the archings, resulting in off-centre marks.

I use this jig for new instrument making.

To locate the centre of a plate with worn edges the reference is usually taken from the position of the purfling, and this jig cannot help with that. But it can locate the centre of the rib structure before the plates are attached, or with just the top attached, or with both plates attached. I can mark the button position by referencing the centre of the belly or the back, or both. I also use it to find the centre of the belly edge for marking the neck mortise and the saddle.

Equipment needed for the jig

ALL PHOTOS STEPHEN QUINNEY

1 Here is the list of parts and equipment I need to make this jig:

• 20mm-thick plywood (60 x 120cm sheet)

• 20/40 aluminium extrusion (120cm long cut into 30cm lengths)

• 2x T-slot track with tape measure slot (Veritas part#13K1202)

• 2x centre-finding tape measures

• 4x T-track elbow brackets (Veritas part#13K1207)

• Screws

Cutting the plywood

2 I cut the plywood pieces to size. It is important to keep them square, and a table saw is ideal for this. The base board is 660mm x 485mm and is big enough for violas with a back length of 16 to 16.5 inches.

The top board is slightly shorter, at 500mm x 485mm, and is centred on the base board. This leaves 80mm at both the top and bottom for the T-slot tape track, plus some extra to support the fence brackets.

The top and bottom fill-in edge pieces are 485mm x 43mm, and I also need two more fill-in pieces to sit under the T-slot track, to raise it up to the same level as the top board. In this case they are 9mm thick and 485mm long. Then I screw all the pieces together.

3a Masking tape is used to mark the centre

3b Adding the centre line with a marker

3 After sealing the plywood with sanding sealer, I draw a centre line on the top surface. This is done by sticking a line of masking tape to the surface and then clamping a straightedge to the board where I want to mark the line (3a). Then I make a knife cut along the straightedge. I run a permanent marker along the knife cut and then remove the masking tape (3b). Now the centre line is effectively engraved in the surface and less likely to rub off.

The ‘0’ mark is positioned at the centre

4 I stick the centre-finding tape measures t o the T-slot tracks at the top and bottom of the board. I make sure that on T-slots. By this point, I have also added a strip of both tape measures, the zero position is plywood along the top and bottom edges for the accurately positioned at the centre line.

Adding the extrusion rails

5 Next, I install the aluminium extrusion rails into t he elbow brackets, and slide them into the brackets to run against and keep them square.

Making the fences evenly spaced

6 I w idths of the instrument and set the fences to take a preliminary measurement across the half that distance as a starting position. Then I make adjustments to space the fences evenly, and have the instrument fit snugly inside them. I use an engineer’s square to double check that the fences stay parallel to the centre line when they are tightened.

7a A rib structure rests flat in the jig

7b A support to stabilise an instrument

7 To find the centre of a rib structure, it can rest flat on the surface of the jig (7a). If the back and/ or belly are attached, then I need a cradle or support to keep the instrument stable and flat to the surface, and to give the instrument edges some overhang on both sides (7b).

8a Using the jig

8b A square helps mark the plates

8c Marking with a pencil

8 T he jig allows me to decide whether to reference the centre p oint from the back or belly or both. When the instrument is on the cradle the side arms only touch one plate at a time. In picture 8a, I took centre lines from both plates individually and my rib mitre is centred between them. When the belly edges are the reference points, I can mark the end-button, the centre of the saddle and the centre of the neck mortise and know that they are all aligned. I have made a square with a notch that sits over the edges of the plates for marking the ribs with the plates attached (8b&c).

The centres are correctly aligned

9 Here, the centre of the neck mortise a nd the button are both aligned and referenced from the edges of the belly. I can also take a centre point from the back edges, if I want to check the alignment and then decide where to centre the button.

If I were to make another jig, I would try to find zero-centre tape measures with half-millimetre markings, as this would help to position the fences more accurately.