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Date: Fri, 25 Nov 2011 09:12:53 -0500
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From: "Bill Wade" <super_chipmunk@roadrunner.com>
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Subject: Re: [LML] Re: Window Bonding
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  Drills for plexiglass are ground so they don't cut as aggressively as a 
normal twist drill. Aircraft Spruce sells them and you could check out the 
description. The design won't grab the material and/or pull the drill in so 
it either tries to tear a chunk out or creates a wedge effect, resulting in 
a crack. Also if the drill speed is too high the plexi will melt. Heed the 
voice of experience.

A Forstner is very good for wood but I wouldn't use it for plexi.

  My Navion's windshield has dozens of screws securing it- no problem. With 
composites I wouldn't drill since you could get a good bond by roughing the 
surface as Matthew Collier advises. OTOH my plane isn't done and the proof 
is in the pudding, whatever that means.

This may be covered by the instructions but if not I think it's good 
practice to break (chamfer) the edges of the window and any holes. I also 
sand or file the perimeter to remove any marks that could be stress risers.

See AC 43.13-1B , Chapter 3: 
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/list/AC%2043.13-1B/$FILE/Chapter%2003.pdf 
Among other things it recommends step drills for larger holes. -Bill Wade