Friday, April 20, 2012

Archtop Guitar Build - Carving the Inside of the Plates

When I read the Benedetto book a decade ago the one thing that struck me as really cool was using a drill press to drill index holes to insure a uniform thickness on a curved plate.  I finally got the chance to try this out by carving the inside profile of the back plate.  I started with the back plate because if I screw up while trying this for the first time, I'd rather not have the flaw in the top sound plate.

I made a stand off for the drill press table that bolts to the table and has a 1/4 inch dowel that sits directly under the 1/4 inch drill bit.  The plate is held parallel to the table and the thickness of the back is a uniform 3/16 inch as measured perpendicular to the drill press table.  I chose a drill bit that matched the diameter of the dowel to make sure the bit's tip cut to the exact depth at the bearing point of the dowel.  As I figure it, a larger bit runs the risk of making a cut that is too deep because the profile of the plate might drop off at a pitch that is greater than the pitch of the drill bit's cutting edge.  This would make the hole too deep at the perimeter and make the plate too thin at this point.  I spent around 45 minutes peppering the inside of the plate with depth holes.  I set the distance between the top of the dowel pin and the tip of the bit at a little over 3/16 inch since it is much easier to remove wood than it is to add wood.  This is what the plate looked like at this stage.  The amber color in the middle was a test of dyes for finishing.  I figured since the wood was going to be removed, this was the best chance to play around with dyes on the actual plate to see how the grain behaves.

 I then mounted the plate in the carving jig and used a 3/4 inch chisel and mallet to remove most of the excess material.  I wondered if I should have titled this post "How to turn expensive quartersawn flamed maple into saw dust and wood chips."  One nice aspect about using a drill bit is that the profile of the point works as a really good indicator of how much material remains to be removed.  You start out with a hole that then turns into an inverted cone and the diameter of the cone continues to decrease until you are left with a tiny dot just prior to reaching the final thickness.  I used my home-made plane and it did the job it was intended for but the hand chisel worked better for me because it is much more aggressive for hogging out wood.  I then switched to random orbital sander with 60 grit paper to get to the final depth.  Here is half of the job done and the other half most of the way to completion.  On the unfinished side you can see what I mean about the dimple diameter decreasing to a dot.
 Here is the completed plate. 
Earlier, I had built a depth gauge for measuring plate thickness.  The three markings are for 1/4, 3/16 and 1/8 inches.  I chose to use brass rod for the trigger rather than the bar stock described in the book.  This gave me the option of rotating the trigger should I need more clearance for the plate near it's edge.  The D string is run around a fixed brass rod bearing point, threaded through a hole in the trigger rod and then seized to the rod with thread.  I placed a self adhesive address label on to brass shim stock for the depth gauge.  A steel wire is used for the depth indicator and I built a corresponding pyramid shaped opposing stop.  When you use the gauge, be sure to use only enough pressure to touch the surfaces of the wood.  The trigger mechanism has enough power that if you horse on it, you can cause the long unsupported upper wooden pieces to want to come together and give an inaccurately shallow reading.
One problem that I noted as I drilled the first hole is that the end grain of the dowel was harder than the top of the plate and the plate top had little shallow dimples in it that corresponded to each hole drilled on the other side.  This struck me as funny because it looked a lot like my first metal working project.  I was about 9 years old when my dad set me down at the bench with a 1/16 inch thick copper plate and a ball peen hammer and showed me how to hammer out a bowl.   It is a good thing that I set the depth at a hair over 3/16 inch because when I sanded the top of the plate to remove these dimples, I ended up at 3/16 inch.  My previous attendance at the "school of hard knocks" paid off...


  1. Hi Scott.

    It's really fun watching you go through the same process I did so recently. Your plates look beautiful! Also, that's a really excellent depth gauge.