Sunday, March 29, 2015

Positioning the Bridge

There are several handy gauges and gadgets available that help the guitar builder determine the position of the bridge, but the method I've adopted works well enough that I haven't felt the need to try them out. To give credit where it's due, I believe I first saw the method I'm about to describe on Matt Mustapick's now defunct and sorely missed workshop blog.

On this particular Port Orford Cedar soundboard, the grain is incredibly fine and the centre join is extremely difficult to detect. I've lightly pencilled a line along the join to highlight it, then taped a sheet of graph paper to the soundboard with the centre grid line aligned with the pencil line. With the neck securely attached, I measure from the nut end of the fret board and mark the notional scale length on the graph paper - it's the longer of the three marks in the picture below.

I make a second mark a little under four millimetres further along the centreline to allow adequately for compensation, acknowledging that with wider saddles gaining favour among many builders these days - myself included - there's plenty of scope for fine-tuning of intonation once the guitar is strung up.

I measure the distance from the centre of  the saddle slot to the front of the bridge at its mid-point, then measure back from the compensated mark on the paper towards the nut by the same distance, placing a third mark on the graph paper to represent the front edge of the bridge. I extend this mark outwards in either direction.

The underside of the bridge has been sanded to conform to the dome of the soundboard, initally using a domed platform to which I've stuck some 120-grit sandpaper, and then, by way of fine tuning, by taping some 240-grit sandpaper to the soundboard itself then moving the bridge to and fro across the sandpaper until the white pen marks I covered the bridge underside with have been removed.

I insert a clamp through the soundhole, positioning a piece of MDF that conforms to the bridge plate outline inside the body and over the bridge plate as I do so. With the bridge clamped lightly in place over the graph paper, I wriggle it into position, using my third mark to align the bridge's front edge at its mid-point, and the grid lines that run perpendicular to the soundboard centreline to align the bridge's front corners. The clamp is tightened, and I drill through the first and sixth bridge pin holes so that the bridge can be accurately pinned in position when the time comes to permanently attach it to the soundboard. The MDF inside the body prevents chipping when the point of the drill bit exits the bridge plate.

Removing the bridge, I replace the graph paper with a thin, low-tack adhesive film known as frisket film. I give the film a light scuff with fine sandpaper so it's easier to mark. I pin the bridge into position through the first and sixth bridge pin holes, and lightly mark the bridge outline onto the frisket. I remove the bridge and, using a craft knife, carefully cut through the frisket around 3/16" inboard of the pencil line, being careful not to cut into the top wood. I can then peel off the frisket that sits outside the scored line. I've found that by leaving a generous margin between the edge of the remaining film and the actual bridge outline, it's much easier to level-sand and buff the finish in the area of the bridge perimeter, bearing in mind that the frisket film is removed only when the finish has been buffed and I'm ready to remove the remainder of the finish within the bridge outline prior to permanently gluing the bridge.

In the interests of a neatness, I'll leave around 1/16" of finish inboard of the bridge outline, and rout a ledge fractionally over 1/16" wide around the underside of the bridge perimeter to a depth equivalent to the target finish thickness. To do so, I clamp my laminate trimmer to a MDF platform that's been domed to replicate the curvature of my soundboard. To prepare the MDF, I laminated two 3mm layers of MDF in my go-bar deck using a 25' radius dish as a base, then drilled a hole in the centre large enough to accept my chosen router bit. As previously described, the underside of the bridge has been sanded to conform to the dome of the soundboard before I rout the ledge.

As always, I'm open to suggestions and I welcome any comments.