instrument sizes
Just a followup to this thread (timing is everything...), I had some
success using 2-1/4" and 3-1/8" holesaws and match-drilling the
screwholes. This is pretty long...
I started the screwholes with a long 1/8" bit, and with these you want
to be very very very particular about getting it centered, then slightly
enlarged them (9/64 or 5/32 as necessary... depending on how far
off-center my match-drilling was), and finally countersunk the cockpit
side of the hole.
Minor "gotchas" and notes:
Westach 2-1/4" EGT/CHT gauges come with their screw holes pre-tapped,
and a 1/8" bit is a bit too large (it will chew up 6-32 threads).
I just about wore out my fly cutter making one very ugly 2-5/8" hole
(probably too much rpm), hence I decided to try holesaws. The hardware
store holesaws made holes slightly undersize. I had to enlarge some of
my 2-1/4" and 3-1/8" holes using a Dremel sanding drum and about ten
minutes of patience per hole. The 2" automotive instruments required
more minutes of patience.
You can trim/debur/countersink big holes in 0.063" aluminum using a
utility knife and file.
Here are the important points of how I made the big holes:
1) clamp the workpiece to plywood (the plywood will stabilize the 1/4"
bit in the middle of the holesaw, otherwise the holesaw will try to
wander ALL OVER THE PLACE!)
2) drill a 1/8" pilot hole where the center of the instrument will be.
3) use the hole saw at low speed (I have a large two-handed variable
speed/max 850rpm drill but used about half that speed... only had a few
very minor kickbacks)
(I don't have a drill press)
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