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  #1  
Old 09-22-2009, 09:07 PM
koda2 koda2 is offline
 
Join Date: Mar 2007
Location: West Texas
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Default Tip: Drilling plexiglass take 2

Fellow Vanziens,
Okay I've read everything I could find and experimented a bit and I am ready to drill plexiglass. Well not quite. As usual, instructions and tips are all over the spectrum.

Perhaps the most daunting words were Tony Bingelis' warning, "In my estimation countersunk screws should not be used...." (page 214, SCT). Well that's great. Only 100 or so on the tipup.

I also looked into putting a washer between the screw head and the plexiglass in the thinking that it might distribute any stresses, but it requires more countersinking than for just the screw and maybe weakens the plexiglass too much.

No consensus on how much relief the hole has to have on the body of the screw for expansion concerns; the new plans say 5/32 but older posts say 3/16 and the aluminum to #23.

I wouldn't obsess about it but the post that stated that 35%, IIRC, of respondents had cracks develop in their canopy, either during fabrication or thereafter, is worth one's attention.

For those interested, here is a close up of the hardware I rounded up.
Top row is 6's, bottom row 8s. From left, AN507 screw, respective tinneman washer, NAS finishing washer, mockup of screw seated, hi-dollar zero flute countersink and 110 degree 3-flute countersink 3/8"dia. (edit: the NAS finishing washer is second, tinneman third. DA).



As with every other non-piloted c-sink I've ever used, the zero flute consistently wandered off center when used freehand (by me), especially when trying to countersink before "drilling up".

The 110 degree c-sink leaves a much better fit for aluminum side skirts, but will not be wide enough for washers. It takes a .5 inch dia. size. Also I am assuming what you want is a "sloppy" fit, so the plexiglass can expand and contract a little everywhere, not just in the area of the screw body.

Finally, I got out a scrap of canopy, beat it on the workbench until my hand hurt and then bent it 200 degrees without it breaking. Took another piece and carefully drilled a 1/8" plexiglass hole. Halfway that is. Snapped like cheap window glass.



So much for the reassurance of plexiglass bits. The kicker here is that if you look closely, one side of the scrap of plexiglass was not polished. Also, the temp in the hangar was 68F. The crack appeared to originate at the bottom of the drill hole but could have come from the stress being transferred to the sharp edge.

So far all I am sure of is that plexiglass must be heated till it softens enough to work on it and everything gets polished. I have kept heaters on the canopy at all times during the fitting to keep its temp 80F or above before moving, cutting or polishing it.


Dave A.
RV-6A

Last edited by koda2 : 09-22-2009 at 09:16 PM. Reason: grammatical error
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  #2  
Old 09-23-2009, 08:20 AM
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DanH DanH is offline
 
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Dave, since you're experimenting with technique (bravo for you!), grab an ordinary drill bit and a fine whetstone. Hone a small flat to replace the sharp cutting edge on each flute, said flat in plane with the long axis of the bit. Drill some sample plastic using high speed and low pressure, examine the hole with a 10X magnifier.
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  #3  
Old 09-23-2009, 07:25 PM
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Tbone Tbone is offline
 
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Dave,
I am curious on the finishing washers. Are you planning on using them on the "open" countersunk screws? That being those that are not covered by a skirt or other alum piece? Where do you aquire such? I do like the look, more finished. Are they "proud" of the surface by much? I used an 1/8 plexi drill initially and then used the permagrit w/pilot until nearly finished. Then used a wilton non-flute countersink to finalize the hole. I too could not get my non-piloted c/s to stay on center. I then drilled w/plexi bit to 3/16 behind skirts and 5/32 on the open screws. I was very pleased with the result and canopy was drilled on 80 degrees or higher temps.
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  #4  
Old 09-23-2009, 07:58 PM
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LifeofReiley LifeofReiley is offline
 
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Default Use...

The dullest bit you can find and go SLOW.
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  #5  
Old 09-23-2009, 11:30 PM
koda2 koda2 is offline
 
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Location: West Texas
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Default Washers

Terry,
Look in the Aircraft spruce catalog on the washer page or try the web. I have not made a decision on using them for acrylic. They may work better in fiberglass situations. They require a bigger countersink and may be a problem but the "flush" style of the NAS washers almost get to the point of being flush. I read somewhere that someone made some kind of stainless washers work on the open plexiglas but I never found any details. Things are on hold here waiting for warmer weather.

Good site I found with lots of acrylic info.

http://www.rplastics.com/plexdesign.html

Dave A.
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  #6  
Old 09-24-2009, 04:42 AM
Tony Spicer Tony Spicer is offline
 
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Quote:
Originally Posted by koda2 View Post
Fellow Vanziens,
Okay I've read everything I could find ...
Including this?

http://www.lpaero.com/DRILTRIM.html

Was the thin strip that broke supported on the back side of the hole while drilling?

Tony
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  #7  
Old 09-24-2009, 08:57 PM
koda2 koda2 is offline
 
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Default plexiglass bits

I think I have discovered part of the difficulty in drilling with the "plexiglas" bits.



The photo shows the two bits I got from Brown Tool. The right one is 3/16" and exhibits all the characteristics described in articles I found on the net: 60 degree included angle, approximately 0-4degree rake, flattened cutting surface, and 12-15 degrees of spiral relief. The bit on the left is 1/8" and has no flattened cutting surface. This was the one I that was cracking the plexiglas.

The 3/16 bit also shows the desired two spiral chips being made while cutting:



The finished hole is pretty good, a little rough maybe. The sharp front and back edges should probably be chamfered.



The small holes in the second picture are made using a discarded #40 bit which I put on the lathe and used a homemade tool grinder to get the 60 degree angles. I filed the flat as Dan suggested using a Norton stone. The result was still too rough to use on the canopy but if I can improve my bit making, thats what I will use for the initial holes.

Dave A.
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  #8  
Old 09-25-2009, 11:50 AM
molson309 molson309 is offline
 
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I've had good luck drilling initial holes in plexi with a #40 bit, sharp or dull (dull is probably better); I then enlarge the hole to the required size with a step drill (Unibit). Trying to enlarge an already drilled hole in plexi with a standard drill bit has a high probability of starting a crack when it emerges on the other side.


Drilling the #40 hole slowly without a lot of force is probably a good idea.

I've built two canopies using this method with no problems - the #40 can drill through the plexi and then into the steel tubing underneath; plexi drills have a hard time drilling into the steel once they get through the plastic.

One other trick I learned is how to find the correct place to drill to hit the center of the steel tubing underneath (round tube, square isn't a problem). Assuming the plexi is pushed against the tubing, take a small flashlight and hold it flat (light shining into the plexi) on the plexi just above or below where you want to drill the hole. You may have to slide the light further along the tubing to get the proper angle. What you will see is an arc where the light is shining on the tubing just through the plexi; mark the spot on the plexi that corresponds to the apex of the curve. This will be very close to the ideal position to drill.

As always, practice drilling plexi on scrap pieces to build confidence that you aren't going to crack your expensive canopy.
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  #9  
Old 09-25-2009, 03:12 PM
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Black8 Black8 is offline
 
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I found that if you place masking tape(I used the green painting tape) along the tubing and then press the plexi onto the tube you will see a line where they touch. I used two drills. An electric one with the plexi bit and my pneumatic drill with the regular bit for going into the tube...most important of all is TAKE YOUR TIME !!!!
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Old 09-25-2009, 03:29 PM
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Geico266 Geico266 is offline
 
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Quote:
Originally Posted by LifeofReiley View Post
The dullest bit you can find and go SLOW.
There are instruction in the RV-12 kit to take a good bit and drill 1/8" - 1/4" into concrete before using the drill on plastic.
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