Boring holes in fibreglass

I have been mounting acrylic (I am going to use the word lexan from here on) and have come upon something that some of you craftsman probably already know. When I made my rear window and mounted it, I found that, no matter how careful I was- drilling both the lexan and the flange in the body together and then opening the hole in the fibreglass to receive the well nuts, some of the screws didn't line up. I am using 6-32 rubber well nuts as the anchor for my screws. I figure the rubber will provide a nice non-rigid, self sealing attach points in the fiberglass. The OD of the well nuts is 5/16" and I had been starting with a very fine twist bit to drill through both the lexan and the body. I then would take the lexan to 5/32". *** TO CLAIFY: 5/32" was the final hole size in the lexan.*** Then I started using progressively larger bits *[in the fibreglass of the body]** until I got to 5/16". **I wasn't very clear with my description here at first.** When the bits were still small there was no problem, but the last 2-3 bits as I approached 5/16" wanted to grab and pull through too fast. This caused a fair amount of chipping. As I started doing my headlight covers, I noticed that one of the two cutting edges often grabbed first, which caused the twist bit to want to pull to one side. This is what I believe was causing the misalignment of my screw holes with the well nuts.

I went to my neighbor, a wood worker, and asked him if he had any other style of bit that might stop this grabbing of the larger twist bits. He told me to try one of his forsner bits, although he only had a 1/4" or 3/8" bits. I tried the 1/4" and it seemed to bore the hole straight and true. I went to a supply store as soon as I got off work and bought a 5/16" forsner bit. It worked great! So well, that I can now mark my hole locations on the flange on the body and bip the center with a 1/16" drill bit to a depth of ~1/8". I then tape the lexan on and transfer the center point onto the surface of the lexan with a titanium scratch pencil and drill the lexan with my self made 5/32" plastic bit. (I took my bit and made the cutting edge nearly perpendicular or even a slight reverse lean using one of the tiny cutoff wheels from my dremel kit. I also decreased the point angle nearer to 90 degrees.) I then used the forsner bit to bore out the 5/16" holes in the body flange to accept the well nuts, which have had their rubber shoulder ground down to near paper thinness. On the first headlight cover I completely installed three mount screws before doing the rest as above. On the two turn signal covers, I was able to follow the procedure above completely without so much as having to drill out one hole in the lexan larger to allow the screw to align properly. Boring the hole once with the forsner vs progressively larger bits has saved a lot of time also! Do note that Mark Clapp uses heli-coil threads and the high quality twist bit that comes with the heli-coil kit. He too found the use of progressively larger bits not worth the time it took and has found that drilling the hole to the desired dimension in one pass has worked well for him also.

[ March 20, 2003: Message edited by: Lynn Larsen ]

[Mark Worthington]

Nice technique, Lynn. Your post inspired a somewhat tangential question which I posted in the body/chassis forum.

[Chris Melia]

Hi all
I remember an article in our club magazine Fortyfication about heating the drill bit using a hot air gun to assist in drilling polycarbonate. I personally have not tried it but the guy that wrote the article said it was a good way of doing it. He used a hand drill to turn the bit.
regards
Chris

[ March 21, 2003: Message edited by: Chris Melia ]

[Bob Putnam]

Just for reference:

Lexan (the tough stuff) = polycarbonate

Plexiglas (the brittle stuff) = acrylic

Bob, I spent a couple of hours pouring over pictures of GT40s, primarily originals, to figure out how many screws to use and their locations. I finally gave up as there just is no consistent way that it is done. So, I used your guide and put 13 in the headlight covers and 8 in the turn signal covers. Thanks!

[Canuk40]

Re: Boring holes in Plexi-glass. Try this on a piece of scrap first. I know this works when cutting sheet stock. If you run the blade backwords and feed into it, it won't grab the material and you get a nice cut, soooo running the drill bit in reverse might just do it... make sure you have some kind if dimple or pilot mark to start from.