It always annoys me when I'm installing strap buttons on my instruments because I always seem to get them slightly off-center between the faces of the instrument. So I came up with a little system and thought I would share. This is hard to describe, but basically you want to make a jig that will slide onto the edge of the guitar body and "point" to the centerline between the two faces.
Use a vernier caliper to measure the distance from the back face to the front. Then, with the caliper still "holding" the measurement, lay the caliper onto a piece of cardboard or card stock so that you can draw the lines of the inner part of the caliper jaws onto the cardboard, near the edge. Cut out along the lines so that the cardboard makes a squared-off "C" shape that will slide tightly onto the edge of the guitar and stay in place. With a pencil or pen, mark the center point of the guitar body onto the inner part of the cardboard "C". Now, when you slide the cardboard onto the body edge, the mark you made on the cardboard "C" will show you exactly where the center point is on the body edge. I then use a nail set to make a little detent at that point so I can drill my holes without the drill bit travelling.
JBD
Use a vernier caliper to measure the distance from the back face to the front. Then, with the caliper still "holding" the measurement, lay the caliper onto a piece of cardboard or card stock so that you can draw the lines of the inner part of the caliper jaws onto the cardboard, near the edge. Cut out along the lines so that the cardboard makes a squared-off "C" shape that will slide tightly onto the edge of the guitar and stay in place. With a pencil or pen, mark the center point of the guitar body onto the inner part of the cardboard "C". Now, when you slide the cardboard onto the body edge, the mark you made on the cardboard "C" will show you exactly where the center point is on the body edge. I then use a nail set to make a little detent at that point so I can drill my holes without the drill bit travelling.
JBD
