"Zero Glue Line". Talk about a Holy Grail.
For years, craftsmen (and craftswomen) making top quality pool
cues, and musical instruments have relied on years of
experience and not a little luck to consistently inlay mother
of pearl, abalone shell, turquoise, malachite, and precious
metals into exotic hardwoods with no visible glue line. Given
the vagaries of hardwoods, the difficulty of cutting brittle,
abrasive minerals, and the absence of tools specifically
designed to cut these materials, this amounted to nothing less
than a heroic task.
Shell and mineral inlay media have most of the aggravating
properties of very hard woods combined with an abrasiveness
that quickly dulls just about anything you try to machine them
Glue Line inlay means that the inlaid component
seems to be part of the wood in which it is
embedded, as if the tree grew that way. The
grain of the wood seems to originate and
terminate in the inlay, with no intervening filler.
This cue butt, designed and built by Keith Josey of Josey
Custom Cues, is an excellent example of what can
be accomplished when a master craftsman, with the
right tools, sets out to create a functional work of
problems encountered include edge
chipping, rough sidewalls, and
imprecise dimensions of the finished part.
For the past 9 years, Think & Tinker has been designing
custom cutters for a wide variety of soft, non-metallic
materials. In 1998, we began working with some of the
best pool cue artists / craftsmen in the United States to
develop a set of tools that would bring the precision of metal
machining to CNC based woodworking. With the
introduction of the 1/32"
CueBitę SMF end-mill in 1999, this goal became a reality.
Ongoing development has expanded the range of this family of
tools from 0.005" (0.125mm) to 0.1250" (3.175mm).
The speed of the cutting edge and the chip load
can be tuned with great precision in CNC based
woodcutters. The sharpness and geometry of the
cutting edge are another matter altogether. While
you can always start with an new, sharp tool, the
shape and attack angle of the cutter will determine
how long the tool stays sharp and how well it cuts
as it inevitably wears down. Historically, one way
to tell when a tool was getting dull was to observe
the degree of splintering and change the bit when
the splintering became too severe. It is
probably not possible to fabricate an end-mill that
never splinters any kind of wood. However, proper
design and testing coupled with fine tuning
rotational speeds and feed rates make it possible to
fabricate tools that will reduce splintering to such
a degree that the splinters can be removed with a
soft tooth brush as with this test cut in black
walnut ( 0.0625" dia., 0.125" depth, 50
in./min. feed @ 40,000 RPM).
One aspect of metal
cutting that has been missing from woodworking is
the precision and accuracy of the shape and size of
a finished part. At the heart of the problem lie the
distorting effects of the grain of the wood and the
inconstant density encountered as the cutter moves
through the material. Wood, even hardwood, is a
compliant media that tends to press away from a
cutting edge and then swell back into place after
the edge has passed. As a result, pockets tend to be
smaller than expected and sidewalls end up in the
wrong place. Efforts to overcome this tendency by
increasing the attack angle and sharpness of the
cutting edge, often result in a cutter that pulls
into the wood, creating pockets that are bigger than
desired. Making matters still worse, a poor match
between the shape of the cutting edge and hardness
of the wood can result in the grain being ripped
apart instead cleanly cut.
The SMF family of cutters are designed to cancel
the tendency to push away from the wood by using a
novel geometry that exerts a very slight pull into
the wood, even when climb milling. The result
is pockets that are exactly the size that they
should be and edges so precise that clocks with
wooden gears can be made to keep time with a
precision of +/- 30 seconds a day (like the clock
hanging in my office). All of the SMF tools are
intended for full plunge operation eliminating the
sidewall stepping commonly encountered with
multi-pass cutting. Pretty neat, huh? (
0.0313" dia., 0.125" depth of cut, 32 in./min.
feed @ 40,000 RPM).