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Rosette Inlaying
The rosette is inlayed by cutting a groove in the guitar top that
exactly fits the assembled rosette, and then gluing the rosette into
the groove with epoxy. The outer and inner edges of the groove are cut
with a flycutter on the drill press. The principal trick is in getting
the size of the groove so that the rossete fits perfectly. This is done
by setting the flycutter directly from the rosette. The rosette, still
glued to its building board, is place on the drill press. Since the
flycutter was used to cut the inner and outer edges of the rosette
(before the rosette banding was glued on), the 1/4" guide hole for the
flycutter is used to center the building board on the drill press. The
flycutter diameter can then be set from the rosette, as illustrated.
Note that a flycutter is an inherently dangerous tool - it's a fairly
heavy, unbalanced mass of rotating steel. Be sure to protect yourself
when using it. I wear a complete facemask protector; I've even heard of
someone putting a phone book into his shirt as a chest protector.
Always use the flycutter at the slowest speed available on the drill
press, to minimize the centrifugal forces. And because it's so
unbalanced, don't even think about using one in a hand drill!
Note that in the photograph, the rosette building board is placed
directly on the guitar top into which the groove will be cut, for
convenience. The cutter diameter is set, and the rosette building board
is removed. The depth of cut of the flycutter is set to be a little
less than the thickness of the rosette, so that the rosette will sit a
little proud when glued. The excess can then be scraped off, leaving
the rosette flush with the top. The next photo shows the grooves cut in
the guitar top. Note that the soundhole is not cut until after the
rosette has been glued and scraped.
After the inner and outer radii have been cut with the flycutter, the
remainder of the material in the groove can be removed with a router,
with the depth set appropriately. The groove can then be cleaned up
with a chisel.
The rosette should now be removed from the building
board. Since it was glued onto the board with a sheet of paper in
between, the rosette can be relatively easily removed by sliding a thin
blade between the rosette and board - the paper will split fairly
easily.
The paper that remains on the rosette can be removed with a
scraper.
The inner and outer edge of the rosette banding should have
their edges chamferred (beveled) slightly so the rosette can be pressed
into the groove. The rosette ring should now be cut to permit easier gluing into the groove in the top.
The groove in the guitar top and rosette are shown in the next photo.
In preparation for gluing, the groove is next sprayed with a couple
coats of shellac. The purpose of the shellac is to seal the end-grain
on the edges of the groove near the centerline of the top. If this
isn'e done, the epoxy used to glue the rosette will wick into the
endgrain, discoloring the spruce near the soundhole. You don't have to
be careful when spraying the shellac, as the overspray on the top will
disappear during finishing. (However, if you don't use the shellac, the
epoxy staining doesn't disappear even under the finish - discovered
this the hard way...)
Why not use Titebond instead of epoxy? Wouldn't this eliminate the
problem of the glue staining the top through the endgrain? Well, yes,
but the problem is that the rosette is itself mostly endgrain. If
yellow glue is used, the tiles in the rosette tend to expand due to the
moisture in the glue, warping the rosette and even the top.
The groove can now be coated with epoxy - either 5 minute or longer
cure time will work fine - and the rosette pressed into the groove. The
top is placed on a flat workboard, a piece of plastic wrap is placed
over the rosette, and a piece of 3/8" or 1/2" thick plexiglass is
placed on this as a clamping caul. The rosette is then clamped and left
for the glue to cure, as in the next photo. The plastic wrap keeps the
epoxy from sticking to the plexiglass caul.
When the glue has cured, the clamps and caul can be removed. You can
see in the photo below the shellac overspray and the epoxy squeeze-out.
The rosette rises a little above the surface of the top; this is next scraped flush (along with the epoxy squeeze-out).
After the rosette has been scraped flush, the soundhole can be cut with the flycutter on the drill press.
