1920 Dyer Harp Guitar - Model 7 Serial # 910
Back
The first attempt left the center ring
.012 too narrow so I had to bump it out  
.006 on the inside and outside of the
ring.
Using a single 1/16" bit the center ring
requires 4 passes to complete.  The
other rings require just one pass each.
Here is the console of the CNC
machine just before pulling the trigger.

The router bit is positioned on the
cross hairs at 0,0 with the tip of the bit
just touching the surface.
1/2/09

I have been working on cutting rosettes
with my homemade CNC machine.  After
a few days of tinkering, I was able to
model the harp guitar rosette geometry
and give it a go.
Fortunately, the machine worked flawlessly.

A laminate trim router does a far superior job
routing rosette channels over a Dremel tool.
The moment of truth.  This is the first
time the homemade CNC machine is
turned loose on an actual top, an
Adirondack harp guitar top no less.  
The pucker factor meter is pegged to
the right.

Here the large main rosette is being
cut.
Now the smaller harp arm rosette is
carved.
Rosettes have been glued in place and
sanded flush.  We can now move on to
installing the top braces.
Top bracing is now glued in place.
The process of removing the finish from
the sides and back involves applying
alcohol to the shellac finish to soften it
and then scraping it off with a razor
blade.

Once the shellac is removed, the red
mahogany stain can be sanded off
exposing the original mahogany and
some additional small cracks.
After uncovering the back inlay strip, it
looks like it is in reasonable shape to
keep.
Finish removal is complete.
Back binding on!
Lower bout back crack repaired and back
stain applied.
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All back braces have been removed and are
being reglued.
The back is now removed and clamping the
sides vertical.
Harp side tuner clean up is complete.  The
screws and washers will be replaced.  One of
the harp side tuners escaped.

If anyone has an extra, 1920 vintage, banjo
friction tuner laying around that looks like
these, please drop me a line.  If it has been
stored in a blazing hot attic for 60 years with
water dripping on it, that would be a great
match.
The bones are now firmly attached.
The alignment of the sides back to the
original position to match the back was
difficult until creating a masonite insert that is
fined tuned to match the shape of the back
exactly.  This was time intensive to make but
provides the best result.  The insert is resting
on 1.5 " blocks that get it positioned about
half way up the sides.
The back and the new back kerfing is attached.
Now that the sides are positioned where they
need to be, I can make and install the kerfing
doubler and kerfing.

The insert will need to be destroyed to take it
out.  The installed kerfing will trap it from
being removed gracefully.
Update:  A special thanks to Alton "Bear"
Acker for generously providing a couple
vintage banjo tuners.
Back side kerfing completed and back fitting
begins.
Kerfing trial fit.
Kerfing doubler is bent and ready for
installation.
On the first round of crack repair, the open
portions of the crack filled in to appear black
and was noticeable when looking at the back
head on.

I decided to try a color match filler to the red
mahogany stain.
Ultimately I felt the state of the original back
strip may affect the integrity of the back.  The
strip was in bad shape and not resting in a
routed channel but went all the way through.  
The back strip was carefully removed and a
fitted Spanish cedar strip was glued into the
channel and the new back strip was laid on
top.
This is a better result that may make the
crack harder to locate after finishing.
The back binding is taped on for a trial fit.
The center strip is masked off to apply some
clear to protect it from a staining mishap.
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