JENSWAY

JENSWAY

Member
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...
 
JENSWAY said:
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...
Click to expand...

Use a piece of wood as a backing, I used my speed deburing hand tool to counter sink all acrylic. Light pressure.
 
I recommend you read the following VansAirforce thread from 2007 and in particular my post #24 and those following.

http://www.vansairforce.com/community/showthread.php?t=23438&highlight=ceramic+countersink&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.
 
Zero Flute

What you want is called a zero flute countersink.

http://www.aircraft-tool.com/shop/search_result.aspx?keyword=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.
 
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?
 
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!
 
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
 
rgmwa said:
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?
Click to expand...

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.
 
BSwayze said:
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.
Click to expand...

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).
 
rgmwa said:
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?
Click to expand...

When we built my friend's RV-8, we simply used West Systems 105 epoxy with the slow hardener to bond the fiberglass fairing all around the front windshield to the rollbar and fuselage. We scuffed up all the plexiglass contact surfaces and metal surfaces thoroughly with 80 grit first. Three years later, it's still bonded perfectly well, and no sign of any delamination of the bond or any cracks or crazing in the plexiglass.
 
Thanks Neal. Makes sense I guess, since Van's method of making the fairing is to lay up the fibreglass strips directly on the windshield after masking and scuffing so it gets bonded from the start. I made mine using a different layup technique that requires bonding after finishing. I have plenty of West 105.
Did you (or should I) add any flox to the mix to help fill any small gaps on the underside of the fairing?
 
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