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Windshield
and window trimming:
Mark
the windshield for trimming with masking tape or a china
marking pencil. Trimming is best done with a band saw
using a 1/4 inch blade with 14 or more teeth to the
inch. The windshield and edges should be well supported
during the sawing and for any other work on the windshield.
Clamp a wood block up against the saw blade so that
in sawing it is not necessary to reset the windshield
or have it come in contact with the band saw work table,
or if desired mask off the work table so that the plastic
will not be scratched during sawing. After sawing it
is important that the edge be smoothed as this lessens
the tendency towards edge cracking and effects the service
life of the windshield, also it lessens the risk of
cracking during installation. Smoothing or edge finishing
is best done with a small 11/2 inch diameter drum type
sander that can be used with 1/4 inch drill motor. For
most effective edge finishing the grinding should be
done in the same plane as the plastic surface which
is also the easiest way to sand when using a drum type
sander. Edge finishing may also be done well with the
round face of a medium cut bastard file and some double
0 sandpaper.
We
do not recommend that trimming be done with a jig
saw or hand saw. If trimming is only minor and it
must be done by hand then use a coping saw with what
is ordinarily called a bone cutting blade, (about
30 teeth per inch), it may work best by putting the
blade in backwards that is with the teeth pointing
towards the handle. Extreme care must be taken in
that the teeth are not allowed to catch or hang up
in the material and cause the starting of a crack.
Keep the plastic well supported during sawing, do
not allow the sawed material to pinch the saw blade.
Sawing
flat sheet:
In
sawing flat sheet as for windows etc., this sawing
can be done very easily on an ordinary table or circular
saw, only cross cut or finishing blades should be
used. Saws should turn at least the speed as recommended
for lumber. Smoothest cuts; will be obtained by keeping
the blade adjusted to a low level and by not rushing
the work through the Saw.
Drilling
holes in Plexiglass:
The
drilling of holes in Plexiglass seems to be the big
bugaboo among many aircraft mechanics. The principal
difficulty lies in that drills as sharpened for steel
are used, it is near impossible to drill a hole in Plexiglass
without cracking using a drill that has been sharpened
for drilling steel and especially so if the hole is
much larger than 3/16 diameter. Drill sharpening for
Plexiglass is relatively simple, in general take the
drill as sharpened for steel and just grind off or dull
the cutting edge. The object is to sharpen the drill
so that it scrapes its way through the plastic. Sharpen
your drill as shown below, try it out on some scrape
plastic first.
Dubbed-off
to zero rake
Slow
Spiral-Polished Flutes
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Cross-section
of drill used in drilling steel. This will not
work for drilling Plexiglass, due to the fact
that it digs in, causing cracks.
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Cross-section
of drill sharpened for Plexiglass. The Drill does
not dig, but scrapes it's way through the Plexiglass.
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Drilling
windshields:
Whenever
screws or bolts are used through holes drilled in
the Plexiglass as for the securing of an aircraft
windshield or window, consideration should be given
to the expansion or contraction of the Plexiglass
as effected by changes in temperature. Plexiglass
expands or contracts with temperature at the rate
of 1/16 inch per foot per 100 degrees F. temperature
change. For example a line of holes at the top of
the windshield 36 inches across using a 1/8 diameter.
screw would require that the holes be 5/16 inch in
diameter to take care of a temperature spread of 100
degree F.. When holes are drilled with insufficient
clearance excessive strain is placed on the windshield
and may cause the holes to crack out at high or low
temperatures. Excessive strain on Plexiglass windshields
shortens their service life and invite cracking and
crazing.
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