Lexan Windscreen Installation

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Postby JPXman » Thu Mar 10, 2005 1:33 am

Hey lads/lasses

Before we tackle the installation of my new lexan windscreen on the enclosure, does anyone have any tips for drilling, sanding, or general installation of lexan windscreens? I've heard its best to use a special bit for plastics, but I've also heard to use a slightly oversize hole so the pop rivets don't put pressure on the walls of the drilled hole. I welcome all advice!
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Postby jb88ci » Thu Mar 10, 2005 3:50 pm

In general, use a dull bit at a slow speed. A sharp bit will break through and cause chips on the backside of the material. Bingellis says to drill a 1/32 flat on teh cutting edge. Others recommend a flatter angle.

Make the hole large than the fastener so that there isn't any stress. Use a large flat washer under the screw head. Rivets really shouldn't be used...you can't control the torque.

I read about a zodiac builder who used sikkaflex (SP?) to fasten his canopy to the frame. It's a flexible autobody adhesive....he claims it worked great.

there are some good articles here

http://members.eaa.org/home/homebuilder ... icles.html
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Postby Billm » Thu Mar 10, 2005 7:30 pm

I have never tried this for drilling plastic but those really cheap carbide tipped drillls used for drilling concrete look like they have the exact point (no "relief") that you are supposed to use for plastic. A little experimentation on a scrap piece of plexiglass (acrylic) would be recommended first. Be careful!!
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Postby flyalaz » Thu Mar 10, 2005 9:37 pm

Long process but clean:
End mill with flat or negative rake, your lexan well clamped onto your drill press table with a back up board under it to prevent break-out. You only need a few thou clearance, so choose a bit that works with your fastener. For fasteners, you might try looking in a motorcycle shop for some small size "well nuts". These are rubber grommet type do-dads with a molded in nut that expand when you tighten them preventing them from pulling out. My bike fairing lowers are attached with 5 each side, and haven't moved a fraction at over 140 mph. Only thing with these is you will need a little sealing strip between them as they don't sit flush in the part you put them on. The pic I have is of a larger version, but the attachment is the same.

Cheers,
Karl
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Postby Chappy » Thu Mar 10, 2005 11:34 pm

Darn, Karl beat me to it. I also would highly recommend small wellnuts and stainless steel Phillips pan head screws. Drill the holes slightly oversize. When I replaced the windshield on my Avenger, I did it this way rather than use pop rivets again, and it worked out very well. I used very small wellnuts, IIRC #6 screw size. I purchased a box of 100 from a commercial hardware supplier and they weren't too pricey. On some machines, like my Avenger when I owned it, it was very convenient to be able to easily remove the windscreen.

Something else that might be of value. The first Lexan windshield that I made didn't hold up too well. It had a fair curve imposed on it when mounted, and over a period of a year or two developed a lot of very small stress cracks and stars on the outer surface. Eventually, I made another new one, but after I mounted it, I used a heatgun on it passing it over the outside to stress relieve it some. I used enough heat to get it somewhat warm, but not near warm enough to melt it. That windshield never developed the little cracks and stars that my first replacement did.

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Postby jb88ci » Fri Mar 11, 2005 9:34 am

good call on the heat for stress relief....if you put a big curve in in it, it'll start to craze for sure.
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Postby flyalaz » Fri Mar 11, 2005 12:19 pm

Another little heat trick is to flame polish the holes with a micro torch. This will remove any little nicks and stresses caused by a small break out. This does take a fair bit of practice, and you should only try on scrap lexan first.

Karl
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Postby Shannon » Fri Mar 11, 2005 9:46 pm

Tyler on the fairings I've installed I've used oversize head aluminum rivets (sorry no part number)with small washers(hardware store purchased) on the inside. The washers have the same ID hole size as the rivets. The wellnuts may work fine on the rear panels of the enclosure for ease of removal for warm flying. I use a large set of metal cutting shears to cut the lexan. They are basically nothing but a big- ass set of scissors and work pretty well for cutting curves.

To start you can make a taped together posterboard template to get the exact shapes you need. Trace that on your sheet of Lex and go to town.

As mentioned definitely do some heat treating as the Lexan will craze on you in a year or so.

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Postby rayjb60 » Fri Mar 25, 2005 6:49 pm

I have a thought!

Why not melt a hole using an oversized nail in a soldering iron?

Fill the oversize hole with Silicon rubber, oozing out on both sides,
trim the silicone to leave a washer sized 1/8" thick pad on both side and use
a screw with washers on both sides.

You will end up with a little rubber isolation joint that firmly holds the lexan in place, but does not stress the hole area due to the silicon rubber isolation....and hopefully you will never have a crack.

I have to install a sailplane front canopy made from a flat peice of lexan, and thats what I will use.

My 2 cents.

Ray

PS: Thank goodness my new Monerai Sailplane project arrived AFTER I finished my Lazair project.

Still waiting for the dry lake at El Mirage to dry a bit....Going for a recon visit this weekend....and to Visit Chris, new Lazair owner this Month....and in my area too....cool...formation flights and pics this season for sure.
<H5>Nothing is impossible...Even the word tells you Im-Possible!!!</ H5>
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Postby flyalaz » Fri Mar 25, 2005 7:06 pm

A little more info:

In the Aircraft Spruce catalog, which by the way. is now free for Canada, their plastics section for plexiglas says that you should allow 1/16" per foot to allow for expansion and contraction. And for attachment screws, to snug up and then back off 1/4 turn.

Karl
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