Chipped canopy holes

I may have really done it this time.

I upsized the screw holes in my canopy today, drilling through the fiberglass skirt from #40 hole to #30 using a regular drill bit. Some of the holes chipped. Here is a picture of the worst two. It happened on the sides, where the plexi had the most flexibility.

Since this picture was taken, I cauterized the insides of all the holes and chips with a soldering iron, then deburred them with a piece of flexible 150 grit sandpaper. The canopy is edged along the inside with a strip of .032 aluminum bonded with proseal. The holes will be filled with #4 screws countersunk into the glass skirt and sealed with 3M 5200 flexible marine caulk. My question is, is there anything else I can do to prevent cracks, or am I screwed? Should/can I find a small ball-shaped sanding bit and smooth out the chips even if it creates a dish around the holes?



I should have used Sikaflex! Gaahh!

Update...

For some reason I had it in my head that using a metal deburring tool/countersink bit would just make things worse by either making the holes too big or making the chips more severe. I got a text from Tony Spicer right away telling me to just countersink the bad parts out, so I found the sharpest bit we have and it worked great. Some of the holes are really big on the surface, but the canopy is thick enough that it will be OK. At least now all the jagged parts are gone. I gave all the holes a decent chamfer to take care of any small unseen chips and I think it will be ok.

Tony suggested I fill the holes with epoxy to support the fiberglass skirt, and then redrill. I thought I had this all figured out, but I'm going to do some more research on the Sikaflex option.

He also said to never melt acrylic. High heat can cause stresses in the surrounding material, leading to cracks. So:

1. Don' t cut the bubble.
2. Don't melt the bubble.
3. Don't file the bubble.
4. Don't touch the bubble.
5. Don't look at the bubble.
And you'll be golden!! Open cockpit, anyone!?
(Can you tell I'm getting tired of this!?)

You can still use the rubber grommet trick!

Get some surgical rubber tubing that is 1/4" OD, and approximately 1/8" ID (try a hobby shop). Cut the tubing into "washers" as thick as the plexiglas. Now enlarge all holes in the plex to 1/4" - yes, make them bigger! (That will also get rid of your chips). Insert a soft rubber washer in every hole, and insert the pop rivest throught the skirt, then through the washer, then pull it on the frame. Now the canopy has some "float" for thermal expansion. This has worked well for a number of folks, including myself.

Katie, don't make this more difficult than it has to be. The fasteners that hold Plexi in position, are doing just that. Holding the plexi in position rather than tightly fitting the plexi to the structure. It's not like when you are riveting or bolting pieces together.

Having holes the are oversized are much preferred to holes that are a tight fit. Giving the plexi a little room to move is a good thing.

When I was doing the canopy on my -7A slider, I made most of the holes at least one size larger than the minimum required. When I say one size, I mean 1/32 larger using a Unibit. As long as you are getting decent contact between the fastener and the plexi, going bigger won't hurt.

In my opinion, a unibit is the best way to enlarge holes in plexi.

I was nervous the first winter my canopy was exposed to cold weather. It survived just fine and after 5 winters its still fine.

And while I'm bloviating, try not to pre-load any stresses into the plexi. Using the least amount of force required to hold thing in position is a good thing. Sitkaflex is not a miracle cure - it has a whole different set of issues.

I like the epoxy fill and redrill idea - just be sure to use G-flex not West Systems 105. It sticks much much better. Nearly equal to the aluminum peal strength.

I got some fractures like that on my 7 - it was the clecos dragging through there. I sanded the areas smooth, as they were already countersunk. I might try the G-flex as I just got some.

I like Paul's surgical tubing filler idea.

Grommets

Another vote for the tubular grommet method Paul describes.. I open all my side skirt holes as such to prevent preloading and for thermal expansion.

Worked great.

Frank
RV7a.. sold 2 years ago.. fantastic airplane.

Do not upsize! Drill the correct diam. right from the beginning!

Hello Katie

Sad to hear, remember me about my landing-light-lens, made it twice.

Also done alot of other plexy parts. It happens nearly always, when upsizing a hole.

It happen even if you are using special plastic drills! Drillls with 60deg. tip angle, diamant-dust coating or one hole countersink cutter and other special plastic tool.

Do not do this:
- Try to drill directly to the final diameter. So the tip of the drill is blocking (get pushed back) the dril of moving to fast into the material. If you upsize, the center of the drill is FREE and get pulled in.
- ALWAYS put something behind the material you are drilling. Hard wood or a scrap piece of plastic, without any gap, clamped tight against the material to drill. So when the drill nearly comes out of the first layer, the tip of the drill get still the blocking force from the next layer. If there is no other layer it gets pulled throu.
- For more experianced tool-specialists, take a grind stone and manually reduce the "cutting angle, helix angle" so much, that it is at least parallel to the drill axis. It can also a bit over this, then the drill is more "scrapping" the hole, but also does not get pulled thrue.

I hope this helps!

Update:

Taking Paul's advice, I upsized the holes to accept some 1/8" ID silicone tubing "washers" (RC fuel line). I used a good quality sharp Unibit and the holes came out pristine. I'll continue with the plan to use 3M 5200 sealant along with the screws.

Dominik, I'll try to heed your advice next time before I drill any holes! (When necessary to upsize, I will not use a regular drill bit. Should've known that.)

Thanks for all the help!

Anybody ever used epoxy and microballoons to fill a large hole in acrylic (1/4" dia.) ? I'm wondering about thermal expansion over time. (Don't ask why... please) I Google this stuff and get things like:

Preparation and thermal properties of epoxy composites filled with negative thermal expansion nanoparticles modified by a plasma treatment...


Yikes. I don't want an engineering degree, I just wanna know if it will cause a crack after a while!

that's not a large hole....>..THIS is a hole!!!!

My experience is mostly in gluing, bending, welding acrylic in the sign business, but identical chemically.
You can just put a piece of tape on the back, and put a blob of Weld-On 16 in the hole....chase it around with a toothpick so it's slightly proud of the surface. it will shrink as the solvent flashes off. You now have an acrylic resin filler in your acrylic windshield/canopy.
or
you can make your own thicker stuff. The solvent is methylene chloride. we used acrylic chips left from routing acrylic, dissolve them in an old coke bottle, just like mixing micro slurry, balance solids to liquid until you have the consistency for the job.
Tightly capped, you can keep the stuff for some weeks.
Caution; fumes are toxic, wear gloves, use in a well ventilated area etc.

That sounds intriguing. Can either of that stuff be bought at a Lowes or Home Depot? I want to stick a fork in this thing this weekend!

KatieB said:

I want to stick a fork in this thing this weekend!

Click to expand...

Don't do that...it'll cause more holes!