Last episode of how it’s made I showed a bit about hammering out the bulging belly of a big cookie tin. Now I’m going to add the dimples for the bridge location and adjust the bulge so that it holds up to the string pressure.Here’s that big texas fruit cake tin again, now with a round belly and a concave curve all around the edge. You can still see my guide circles marked out and the little circle marking where I want the dimples for the bridge.
To make the dimples I set up the big vise with my plastic dimple anvil. This ABS rod works well and it was very easy to machine the dimple shape. The reverse end has a smaller pit for smaller dimples. I rest the tin over the rod, feel where the pit lines up with my guide marks, and go to work with the ball end of a small ball-peen hammer.
Here you can see the dimples formed nicely. Several times during this process I will rest the tin flat on the bench and look carefully at the dimples while holding a straight edge across the tin. The dimples should match in depth and be parallel to the tin edge. Creating the dimples will always change the strength of the belly arch creating weak spots that need adjusting.
pop! from The Tinkers Damn on Vimeo.
This video demonstrates what I am looking to correct in this finicky part of the process. By carefully feeling around the belly surface (fingers are more sensitive to this than eyes) I can find slightly flattish spots that are weak and will pop (as heard in the video) functioning as a two position or bistable spring. These spots are trouble. I find they will have their own distinct natural frequency adding too much can noise (bark, rattle, and buzz) to the completed instrument. I do my best to eliminate these spots. In addition, I need to reinforce the area around the dimples to resist the string pressure on the bridge. Ideally the whole center of the tin belly will be rigid enough to vibrate with the string frequency while most of the flexing will occur in the concave curve around the edge of the tin.
To make these fine adjustments I switch to a steel rod anvil with a rounded end. This allows me to carefully stretch small areas of the steel belly with the flat face of a medium sized ball-peen hammer. It is difficult to describe this process beyond saying it is a few hours of trial and error. I will stretch one bit then go looking for more weak spots and stretch some more. Little by little I will get to a well adjusted belly curve that rings nicely when struck with a small drum stick, has no flat spots that pop, and can take the string pressure without deforming.
To test that last bit I press about as hard as I can with two fingers on the dimples and watch carefully for any crippling in the belly. The pressure should only deform the tin in the concave curve around the edge.
That completes the tin until it’s time to fit the completed neck. Next episode I will show a bit about shaping a neck.
Update! Want to see all the hammering happen really fast?
5 hours of work in 1 minute 15 seconds from The Tinkers Damn on Vimeo.