hydroguy2

hydroguy2

Well Known Member
RV-7build149.jpg


the pic tells the story. not sure why it cracked during countersinking, but there it is.

Now where is the best place to get a small amount of the proper glue? I did a search for Weldon glue, but don't want to buy a pint.

On a side note: Is there any merit to putting something between the plexi and the frame? like uhmw tape as a dampener. How about sealing between the glass and AL with bead of silicone or something to keep water out?

Thanks.
 
Builders continue to get acrylic cracks while countersinking because they use their standard metal countersink tools. Many of the countersink tools supplied these days by Avery etc have triple flutes. These are exceptionally dangerous for acrylic (plexiglass). :eek:

As a rule of thumb for acrylic the less flutes the better (ie. two flutes are better than three and one flute is better than two). The problem is that it is very difficult these days to source 100 degree single flute metal countersinks.

However the best countersink of all (and the ONLY one that I would use on acrylic) is a tungsten carbide grit type....they have NO flutes. Permagrit manufactures them and you can find them at:
http://www.aerocraftparts.com/Categories.aspx?Category=b78f1293-e241-422b-83e3-82b352ba6fe5

You will not crack your plexi using a tungsten carbide countersink. :)

In fact I used the tungsten carbide countersink tool to lightly "deburr" all my canopy holes (both sides) to remove any microscopic cracking (particularly on the drill "break-out side) that could grow later due to work-of-fracture theory.

What type of countersinking tool were you using when you caused the crack....and how many flutes did it have.
 
I used the aforementioned Avery 3 flute....and will do it again. With the correct pressure and speed it cuts nice.

I'm fairly certain I leaned the tool and side loaded the pilot which caused the cracking. Failure to pay attention at the correct moment...guilty. I did practice pieces at different speeds, pressures, etc and had good results with my cordless Makita. When drilling I am using highspeed air drill with plexi-bits, also from Avery.
 
I got two of these that look almost identical. Since the crack is going from the screw hole to the edge of the plexiglass is there any reason to repair them? Just curious what you were planning on using the glue for?
 
Captain Avgas said:
Builders continue to get acrylic cracks while countersinking because they use their standard metal countersink tools...As a rule of thumb for acrylic the less flutes the better (ie. two flutes are better than three and one flute is better than two)....
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Very well stated Captain. Using a standard 100? sheet metal countersink on acrylic is just asking.....begging for trouble. Why take unnecessary risks on that expensive canopy by using improper tools on it? All it takes is a momentary lapse of concentration and you can generate a side load on the bit's pilot pin that can and will stress the plexigas to crack. Ouch. I recommend you get a NO flute countersink bit that happens to be well suited for plexiglas work. Not very expensive and certainly a cheap insurance policy. Skygeek's got em and the bit serves as an excellent deburring tool too:

http://www.skygeek.com/ats-zfc01.html
 
Looking at your photo and noticed that the edge is not rounded very much. How much should the edge be rounded? Just smooth or a complete with no edge? Seems to me that the edge should be more so. Just curious because I am at the same exact spot that you are and haven't countered the holes yet. My canopy has been rounded a little bit more than the pic.
 
I guess I'll apologize up front to all the experts out there.

I'm a country boy building by myself reading Vans instructions, using the tools recommended and used by thousands of other builders. I do my best with the knowledge and skill I have, but sometimes things happen. If we want to get technical a countersink in plexiglass is just wrong and not recommended by the plastic industry.

I went to the shop last night and tried to duplicate the problem on scrap. Sure enough, if I didn't hold the tool square and forced the pilot into the work as I started it will crack. Hold the tool square, no problem. So I'm saying the crack was not caused by the cutting action on the plastic.

Tbone-the edges have been dressed.

Let's see a show of hands: Who used an 3 fluted countersink? IF I'm all alone on this I'll eat my words.
 
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I did the same thing, but mine was a brain fart and not a consequence of countersinking. I torqued a screw down on it and the plexi was not sitting level on the aluminum...it was sitting a little proud because the plexi had not been trimmed enough to clear a joggle.

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I did nothing with it. It's completely hidden and it's already "stop drilled". I just dealt with it. Almost 300 hours later I have had no problems with it.
 
hydroguy2 said:
Let's see a show of hands: Who used an 3 fluted countersink? IF I'm all alone on this I'll eat my words.
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You're not alone on this, and don't let anyone give you any flak. I used a 3-fluted countersink bit with excellent results. In fact, before doing so, I experimented (on scrap) with several different techniques and types of countersink bits, including the permagrit ones that some have recommended. I found that the standard 3-fluted piloted countersink bit produced by far the cleanest results (permagrit on the other hand leaves a very rough surface), had no tendency to heat or melt the plexi, and had no apparent tendency to crack the plexi as long as it's held square using a countersink cage. I have no expert reasoning behind this, just my empirical observations. I then proceeded to do the entire canopy this way, and had no issues what so ever.
 
Some time back one of the aviation magazines ran a story on how to repair plexi, do any of you remember which magazine and issue it was in?

After flying up north to visit Mom my canopy developed a crack on the ground when the temp dropped below zero. :(
 
Forget it and move on... It already has a large stop drilled hole....

I used a 3 flute and no cracks yet :eek:
 
I'm ready to move past it. All I hoping someone knew a glue source(hopefully local) that I could pick up easily. Figured I may as well wick a little glue in the crack before I bolt things up.

Also cushion or no cushion between the glass and AL?
 
Any glass outlet will have that glue for the plexi... It is thin as water and comes in a small bottle with a needle applicator. It just wicks in the crack.
 
No issue with 3 flutes

I also used a regular 3 flute countersink in a microstop cage which forces the countersink to be square to the surface and had zero problems. Nice clean cuts and would probably do it again that way.

Bill S
7a almost flying
 
Captain Avgas said:
Yes, maybe it "cuts nice" but you now have a cracked canopy. :rolleyes:

Aaaah well, as they say......you can lead the horse to the water.
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Bob, I think Brian's point was that his crack was caused by improper technique and not by poor choice of tool. You may disagree with him, but that doesn't make him a horse.
 
Brian,

The article referred to above was in Kit-Plane or EAA mag. You'd have to go at least one year back to find it. It cracked on the right side and it's going to be hidden. If that's your worst mistake, you're miles ahead.
 
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