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08-18-2013, 09:51 PM
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Join Date: Apr 2011
Location: Pretoria, South Africa
Posts: 17
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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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It doesn't matter what AVGAS costs, it's worth it...
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08-18-2013, 10:23 PM
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Join Date: Nov 2005
Location: KRTS
Posts: 1,798
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Quote:
Originally Posted by JENSWAY
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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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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Next?, TBD
IAR-823, SOLD
RV-8, SOLD
RV-7, SOLD
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08-19-2013, 05:29 AM
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Join Date: Oct 2005
Location: Melbourne, Australia
Posts: 1,868
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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.
__________________
You’re only as good as your last landing 
Bob Barrow
RV7A
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08-19-2013, 06:30 AM
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Join Date: Sep 2007
Location: Central IL
Posts: 5,516
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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.
__________________
Bill
RV-7
Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
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08-19-2013, 07:18 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Listen to Bob...A pointed rock is your friend.
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Dan Horton
RV-8 SS
Barrett IO-390
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08-19-2013, 07:57 AM
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Join Date: Feb 2010
Location: Perth, Western Australia
Posts: 1,647
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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?
__________________
rgmwa
RV-12LR 912ULS
120346
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08-19-2013, 08:04 AM
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Join Date: Apr 2007
Location: Leonard, Texas
Posts: 33
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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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Grumpy
RV-9A
Finish and Firewall
JLMTIK
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08-19-2013, 02:56 PM
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Join Date: Jul 2013
Location: goondiwindi qld australia
Posts: 3
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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.
Ian
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08-19-2013, 06:09 PM
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Join Date: Jul 2007
Location: Molalla, Oregon
Posts: 955
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Quote:
Originally Posted by rgmwa
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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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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08-19-2013, 06:46 PM
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Join Date: Oct 2005
Location: Melbourne, Australia
Posts: 1,868
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Quote:
Originally Posted by BSwayze
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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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).
__________________
You’re only as good as your last landing 
Bob Barrow
RV7A
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