completing the belly of the tin

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.tin shape 1Here’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.

tin shape 3To 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.

tin shape 5Here 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.

tin shape 6
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.

tin shape 7
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.

starting to beat a tin into shape

As promised, someday is here, and I am finally getting around to posting a little about the work that goes into the instruments I build.
measured drawingAfter selecting and cleaning a tin I think will make a good instrument (not too many dents or corrosion, a tight fitting lid, and good artwork) I always make a measured drawing to plan the instrument. Then I mark out the center, the axis of the neck, the bridge location, and several concentric circles on the tin bottom to guide shaping the steel.
hammer tin 4I do all the hammering to shape the tin before starting work on the neck or any other parts. Shaping the tin is the most iffy part of the process, and if the work fails I don’t get stuck with a neck that was custom made to fit a bad tin. In the picture above you can see concentric circles marked in black ink. I have already hammered down the outermost ring using the ball end of my hammer to start the downward part of the curve, and the center bulge is starting to rise as I work out from the center.
hammer tin 3This picture shows the curved top of the anvil I use to shape instrument bellies. Starting in the center of a tin, I stretch the steel by striking it between the flat side of the ball-peen hammer and the anvil head. As the steel stretches in the center it bulges up and I work out toward the edges little by little. It takes a countless number or hammer strikes. A heavier hammer or harder blows would make the work go faster, but I find that the result is less even and more likely to collapse when the string tension is applied to the belly of the instrument.
hammer tin 1The resulting steel instrument body must have a very even and rigid dome shape to stand the pressure of the steel strings and be flexible enough at the outer edge to resonate with the plucked strings. The steel work-hardens and becomes more brittle as I hammer so there is a limit to how far I can stretch things. The bigger the tin the more hammer work is required and the more sensitive the dome is to small flaws.

Next time I’ll get into the nit-picky work of adjusting the belly of the tin for the off-center bridge foot print.