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First, plexiglass is not always plexiglass.
Every manufacturer (and probably every batch) has a different composition,
sometimes very different. Some formulations have a lot more styrene in
them, extruded rod and cylinders, e.g., and these are much worse to work
with. They crack and craze and chip, etc. Sheet material from name
manufacturers like Rohm & Haas or DuPont is infinitely superior to the
unbranded stuff that comes from Mexico or Asia. Dimensions are always
approximate; due to variable shrinkage during polymerization of the sheet, a
nominal 1/4" sheet can be anywhere from 0.210" to 0.280" in
thickness.
Any cutting or machining of plexiglass will induce
stresses in the material, and the more unrelieved stress, the more likely the
material will craze later on, or even crack and break. The ideal treatment
is to anneal the material after all the machining is done, but I doubt many of
us have the ovens that would be needed for doing, say, a windshield. Also,
the temperature is critical, so most home-type ovens would probably end up
melting the material.
All of that said, the best procedure is the one
that will produce the least stress. Abrasion is better than any cutting
action with a percussive component. A band saw is better than a saber saw,
and so forth. Lubricate with water, with dishwashing detergent a good
additive (a few drops in a cup of water). The only solvent that doesn't
seem to affect it is ethanol, but I would only use that sparingly for removing
adhesives and other gooey stuff. As for drill bits, the ones ground for
plexi have zero rake, so they really abrade the material away rather than
cutting it. The main risk with a normally ground bit is when the bit
starts to come out the back side (or bottom) of the hole. As soon as it
pokes through, it wants to grab, usually with disastrous consequences --
chattering, shattering, and other such events.
Plexi's a funny material, and requires its own
special set of tools and techniques. DuPont and other quality
manufacturers have good tech bulletins on working techniques, and also have most
of the info posted on the web.
Jim Cameron
Lancair Super ES N143ES
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