Plexiglass Countersink?

[Tbone]

After reading several post on the correct, or let me say, different ways to countersink plexi is there a problem in not using any countersunk screws at all on the canopy? It seems to me that there are many different screws that could be polished "stainless" that would not give a tall profile and still look good. Are the countersinks in the glass to keep the canopy "centered" on the frame? I also thought I saw a RV at OSH and it had tinnerman washers under the screw but were cut back to the screw edge. Not sure what for. What are some of the ingenious ideas of the RV community on this issue?

[Tbone]

Any takers?

[roee]

Originally Posted by Tbone

After reading several post on the correct, or let me say, different ways to countersink plexi is there a problem in not using any countersunk screws at all on the canopy? It seems to me that there are many different screws that could be polished "stainless" that would not give a tall profile and still look good. Are the countersinks in the glass to keep the canopy "centered" on the frame?

You could probably use non-countersunk screws on the canopy if you really want to. But the obvious penalty would be added drag from the protruding screw heads. Seems like a waste.

My advice: Don't become disproportionately frightened of this issue just because some builders have experienced problems. Remember that most builders have gotten through this with no problems, and that clear lessons can be learned from the problems that have occurred. As for the "right" technique, or "different" techniques, I wouldn't take anyone's word for it as gospel. Take some scrap plexi and experiment with it yourself until you've got a technique that you're satisfied with.

Originally Posted by Tbone

I also thought I saw a RV at OSH and it had tinnerman washers under the screw but were cut back to the screw edge. Not sure what for. What are some of the ingenious ideas of the RV community on this issue?

Tinnerman washers are a good idea to use under the screws where there is no aluminum skin to spread the load on the flat surface of the plexi, i.e. along the roll bar. I'm not sure what you mean by "cut back to the screw edge". The wide flange of the tinnerman washer is what makes it useful, so I don't know why someone would cut that off, if that is indeed what you're describing.

[schristo@mac.com]

make sure that you are using the correct sized pilot hole for the countersink and it should make a very clean and smooth cut without any chatter. when i used a thin uni-bit to open up pilot holes to 5/32 i ran a dulled #19 bit from the back side to match my piloted countersink bit and the countersinking was a non issue.

after completing the cutting, drilling, and countersinking on my tip-up i had the worst success with the so called 'plexiglass bits'... much better results using a dulled bit... i dulled several designated bits by running them against a deburring wheel per the recommendations on the EAA site (one of the online instructional videos).

in any event i would not want to work on the canopy in the winter...

[mikehoover]

I used a regular dull bit with no problem. On countersunk holes I used the microstop countersink and found that you need to use low rpm to prevent having too much heat. And it cut quicker and smoother at low rpm. I was working in very hot conditions - 90-100 degrees in the hangar.

[Tbone]

I have no problem with countersinking plexi, I was just curious if others have entertained the idea of not countersinking and using different screws. I wouldn't think the drag penalty would be an issue.