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.
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.
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
Originally posted on
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
Originally posted on
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.
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.
