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Screws & Plexiglass

Skybolt205WD

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A potential Canopy "Gotcha".
I have been fabricating the clamshell canopy per drawings from Steen. I bought my canopy from Steen as well. The build is straight forward. However, I ran into a potential problem that some of you would avoid anyway but that the less experienced might encounter.
The plans call for cs holes to be drilled in the canopyat unspecified locations along the frame. The tubing is tapped for 6/32 screws. Then once assembled, a 3M structual adhesive applied. I have worked with plexiglass and polycarbonates in the past, but never c/s holes in that material. My procedure herewas to drill the hole oversize to about .040 clearance to give room for expansion, etc. I of course used a drill with the rake of the twist ground away, which is typically used for plexiglas work. I then used a microstop counter sink with a 100 degree cutter to match the screw head I had chosen. On the back side of the canopy, I used a conically shaped dremel style grinder just enough to break the sharp corner. I then used some 400 emery around a piece of dowel rod to polish the holeso as not to leave any sharp edges or areas where stress risers might develop.
Before installing screws, I assembled with clecos. In the process of installing the screws, I was very careful not to tighten the srews at all....barely snug....because my plan was to back of the srews 1/2 turn before applying the adhesive per AC 4313. Once installed and snugged, I quit for the evening. The next morning, to my horror, I found that about 10 of the 56 holes I had drilled had subsurface cracks propagating some as long as 1/8. I immediately loosened all screws while cursing prophetically. I then stop drilled all of the cracks. So, There is my confession.


What I gather is that even a snugging of the screws is too much and risks cracking. I suppose that even a torque of 1 inch pound with the screw driver translates in to a high actual PSI on such a small area represented by the underside of the screw head. Once I calmed down,I pulled out Tony Bingellis's book and found that he believes c/s plexiglass is risky for cracking and he notes that backing up the screw is absolutely necessary to reduce the risk.The adhesive really stabilizes the frame to the canopy so that there is no twisting.
Once I was satisfied that I had corrected the problem, I applied the adhesive, and I seriously doubt that there will be no problem with propagation. If I were totake a mulligan on this effort, I think that I would c/s more deeply and make the holeeven more oversize. And, even at the mock up stage, notput any pressure on the screw to the canopy.
I hope that this helps someone avoid what I encountered. Of course, if anyone has any comments (other than I have revealed my incompetence once again) I welcome them.
 

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