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  #1  
Old 08-18-2013, 09:51 PM
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JENSWAY JENSWAY is offline
 
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Default Landing light lens cracked!

Hi all. Advice would be appreciated.

I have trashed my second Duckworks lens! The instructions call for the lens to be countersunk to accept the dimples in the wing skin for the #6 screws. Although I have worked slowly with the drill speed and only applied light pressure on the lens while doing this, I have cracked the lens...

- Garage temperature is 70F...
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Old 08-18-2013, 10:23 PM
Sig600 Sig600 is offline
 
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Quote:
Originally Posted by JENSWAY View Post
Hi all. Advice would be appreciated.

I have trashed my second Duckworks lens! The instructions call for the lens to be countersunk to accept the dimples in the wing skin for the #6 screws. Although I have worked slowly with the drill speed and only applied light pressure on the lens while doing this, I have cracked the lens...

- Garage temperature is 70F...
Use a piece of wood as a backing, I used my speed deburing hand tool to counter sink all acrylic. Light pressure.
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  #3  
Old 08-19-2013, 05:29 AM
Captain Avgas Captain Avgas is offline
 
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I recommend you read the following VansAirforce thread from 2007 and in particular my post #24 and those following.

http://www.vansairforce.com/communit...tersink&page=3

The thread is about preventing cracks in canopies but the rules apply to drilling and countersinking in acrylic generally.

Specifically, the three flute countersinks you get with a typical RV tool kit are very dangerous for countersinking acrylic. Builders use them because that's all they have and all they know about. Some have success with them but some do not.

Maybe it is good that this has happened before you moved on to your canopy....easier, and cheaper, to replace a light lens.
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  #4  
Old 08-19-2013, 06:30 AM
BillL BillL is offline
 
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Location: Central IL
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Default Zero Flute

What you want is called a zero flute countersink.

http://www.aircraft-tool.com/shop/se...ord=zero+flute

Here is an example. I used the one with no pilot and properly monitored, it will make a nice countersink in acrylic, plexiglas without any problems. Smooth surface remains when done.

You may be able to source it closer to home now that you know what to look for. I used one on my canopy and would not do it any other way now. I used a battery drill and would count the revolutions to get the final fit.

Use common sense and don't press too hard, just enough to get it to cut, you have some practice pieces now. Backup (with a board etc) as needed in practice.
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  #5  
Old 08-19-2013, 07:18 AM
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Listen to Bob...A pointed rock is your friend.
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  #6  
Old 08-19-2013, 07:57 AM
rgmwa rgmwa is offline
 
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From Bob Barrow's post: "Incidentally the Sikaflex 295UV that many builders use does not in fact adhere well to plexi at all. That is why they need an adhesion promoter such as Sika 209 Primer. The adhesion promoter "bites" into the plexi. It does so because it contains approx 25% MEK and 20% Ethyl Acetate. Both of these chemicals fall into the worst category for causing microscopic crazing of plexi. Down the track it may well be that those builders who used Sikaflex to avoid cracks may develop them anyway."

I need to bond a fibreglass fairing to the front edge of the canopy, and was going to use 295UV. Any suggestions for alternatives if what Bob says is correct?
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  #7  
Old 08-19-2013, 08:04 AM
Grumpy-Hodge Grumpy-Hodge is offline
 
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Default Another option to consider...

I installed without dimpling the six holes on the leading edge using button head screws. Reduces the risk of skin and lens cracks now and future and the button heads aren't that offensive to me in this particular install. Good luck!
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  #8  
Old 08-19-2013, 02:56 PM
ian greenland ian greenland is offline
 
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Default Plexiglass drilling

my experience drilling plexiglass led me to this method. Take an automatic map gas torch with a Mikita drill with a dulled bit set on lowest torque. Heat the tip to near red hot and virtually melt your way through, go slow heat three times, gently push through. I did this on windscreen and is perfect. Plexiglass is rubbish, I,m building an oven and will cast my first Lexan screen in a few weeks.
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  #9  
Old 08-19-2013, 06:09 PM
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BSwayze BSwayze is offline
 
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Quote:
Originally Posted by rgmwa View Post
From Bob Barrow's post: "Incidentally the Sikaflex 295UV that many builders use does not in fact adhere well to plexi at all. That is why they need an adhesion promoter such as Sika 209 Primer. The adhesion promoter "bites" into the plexi. It does so because it contains approx 25% MEK and 20% Ethyl Acetate. Both of these chemicals fall into the worst category for causing microscopic crazing of plexi. Down the track it may well be that those builders who used Sikaflex to avoid cracks may develop them anyway."

I need to bond a fibreglass fairing to the front edge of the canopy, and was going to use 295UV. Any suggestions for alternatives if what Bob says is correct?
The only thing about this is, SikaFlex was developed specifically to be used with plexiglass. I would suggest that you read through some of their technical documentation before writing it off. It's hard to imagine that the makers of this product didn't know about this, and plan for it accordingly.
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  #10  
Old 08-19-2013, 06:46 PM
Captain Avgas Captain Avgas is offline
 
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Quote:
Originally Posted by BSwayze View Post
The only thing about this is, SikaFlex was developed specifically to be used with plexiglass. I would suggest that you read through some of their technical documentation before writing it off. It's hard to imagine that the makers of this product didn't know about this, and plan for it accordingly.
The manufacturers of Sikaflex 295UV know exactly what the limitations of their product is. That is why they have stated consistently and repeatedly over the years that it is not a suitable product for formed acrylic aircraft canopies. RV builders who use Sikaflex generally know this but use it anyway, preferring to believe that the company advice is based on legal liability issues rather than on technical considerations (whereas in reality it will be based on both).
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