how it's made

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.
After 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.

I 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.

This 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.
The 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.

And now for some time lapse!

5 hours of work in 1 minute 15 seconds
from The Tinkers Damn on Vimeo.

completing the belly of the tin

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.

from The Tinkers Damn on Vimeo.

The video above 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.

shaping the neck

Last episode, I finished shaping the tin. For part three of how it’s made I’ll give some details of cutting and shaping the maple neck and its extension through the tin. This won’t include the fret board. That will have to wait for another episode. Even without the fret board this may be a bit long, so hold on to yer hats’n glasses! After the initial work on the band saw, table saw, and drill press this is all hand work with rasps, chisels, and sand paper. Apologies for the sharp focus on the backgrounds. One day I’ll learn to force a digital camera to focus where I want it.

Following the measured drawing I made way back when I planned the instrument around a particular tin, I transfer all the measurements to a plank of maple. This picture shows a neck after a rough cut on the band saw, holes drilled for tuning machines and the strap, some lengthwise cuts to narrow the head and tail, and some initial profile smoothing with the rasps. Those holes were drilled before the lengthwise trim cuts were made. The drill press makes them nice and square and tear-out is eliminated when the waste pieces are removed. Some center lines in pencil are visible here. After squaring up the neck blank and all this early work I replace any guide marks that have been cut away.
This is looking at the butt end of the neck where it will someday meet the tin. The brighter part in the foreground is the extension of the neck that passes all the way through the center of the tin to the string anchor at the tail. This one piece neck and extension take all the string tension. You can see the pencil line marking the intended taper where the neck will join the tin. I like to get this taper set up before shaping the middle of the neck. This sets a limit on the curve of the middle section.
Above is one side tapered to meet my guide marks. I’ll flip the piece over and do the other side right away. I like to take small bites on each side while checking the symmetry at every step.
Now to get my head straight. This work will help set limits on the curve of the center part of the neck too.
I have already trimmed the waste piece off the other side of the head. The table saw cut is set a bit wider than the finished head. I’ll have to do quite a bit of work to smooth and clean up what the table saw leaves behind.
Time to clean the gum off the saw blade. It got a bit hot and left some burn marks.
after grinding with the rat tail rasp.
 after some chisel work to make one flush surface…

and several grades of sandpaper later, the head and the curve leading to the middle section of the neck are well roughed out.

The center section is still a tapered block itching for some relief, but first lets freshen up those guide marks.
If you’re not familiar with the tool pictured above, you’re missing out. It’s one of the best tools ever, a carpenter’s scribe, and I depend on it for making guide lines that are parallel to this or that edge. The tiny black rod at the Left end is a piece of graphite pencil “lead” set so it can be inched down as it wears out. That’s a thumbscrew sticking out of the block. It locks the block in any position along the roughly graduated stick.
Scribe in action!… marking a center line…
and marking a pair of “don’t cut anymore!” warning lines along the edge where the fret board will join. When the uppermost of these lines gets faint or disappears I know my curve is getting close to removing wood that I want to keep on the edge.
Next, I check that center line with a short straight edge and back-light. I want the center line to fit nice and tight on that straight edge. The light getting under the straight edge indicates low spots and the dark shows high spots. Hmm… needs work. I will rasp and sand carefully until I see very little light getting through, and finish by redrawing my center line.
OK, last part of this post, I swear.

For this bit, I like to clamp the neck to a beam that sticks out from my bench. This makes it easy to work both sides of the middle section for a uniform result.
This is the first of MANY cuts to make nice even facets roughing out the curved cross section of the center of the neck. Each added facet gets narrower and closer to a smooth curve as I work. Note that everything must cope with the taper from head to tail…. lots of standing back with chin scratching is in order here.
After making all these swell, even spaced, symmetrical facets now I must destroy them. Back in the bench vise, with raking light from my faithful swivel lamp, I look carefully for the edges of those facets, and eliminate them one by one with many grades of sand paper. Blue lines in the inset image highlight the edges of several facets visible in the photo. All these have gotta go. In addition, if you look close at the inset (you can click on the image for a closer look) there are two green arrows indicating that my “don’t cut anymore” lines are indeed still there.
This is getting close to a smooth curve. The raking light still shows some work to be done. I find that taking the neck out into sunlight and watching as I roll the piece in and out of shadow will point out the flaws. Curves, man… they’re finicky!

fitting the neck, cutting the tin

In the last episode of how it’s made, I finished shaping a maple neck. Now it’s time to fit that neck to the cookie tin.
Here you can see the maple neck, blank fret board (in lovely red padauk), and the tin with its shaped belly. Note the long tail end of the neck. This will fit all the way through the tin body and will support the string anchor at the butt end. The neck angle relative to the tin body has already been set by the angle of the maple shoulder where the neck will meet the tin.
The neck will fit across inside the tin something like what you see above. Note that the butt end has some excess length. This excess will be trimmed to fit later.
 To get the neck through the tin I need a rectangular hole in the side that will line up with the guide marks I made way back when I was shaping the tin. With a ruler and a carpenter’s square I will extend that fading horizontal sharpie line down the side of the tin.
 The tin above is resting on a handy scrap of 2×4 (indicated by the big arrow). I clamp that scrap 2×4 in a vise with enough sticking out fit inside the tin to support this part of the work. Above you can see the sharpie guide marks I have laid out for cutting a rectangular hole with tabs in the side of the tin. These marks may not yet make sense, but bear with me. The small arrow points to the outline of what will become one of the tabs that will be bent inward to form the edge of the rectangular hole.

I use a standard box cutter knife with a fixed blade to push through the sheet metal into the supporting 2×4 scrap. I work through the metal with an old school can-opener-like motion and lots of downward pressure. I do not try to draw the blade along as though cutting paper. Yes, the bade is immediately dull. Yes, the razor tip will break. Yes, I wear eye protection. Like Norm, I ALWAYS wear eye protection. If you don’t have yours on right now while just reading about forcing a box cutter through sheet metal you should put on a pair of goggles without further fuss. Ahem… In the image above, I have cut through the T shape at the top of the opening. I will continue cutting (or can opening) along the red dots.

The three tabs, two at the sides and one at the short end, disappear into shadow but you get the idea. I will finish up bending the tabs to form a nice rectangular opening using some duck bill pliers.

That’s all for now. Next time I’ll get into trimming the butt end of the neck and getting the shoulder to fit snug to the tin. Buon metallo.