Cracked Windscreen; help requested!

[gerrychuck]

Posted on the "How cold is too cold" thread, but thought I'd post it separately to solicit advice:

I think I jinxed myself; after bragging about my New Years Day cold weather flight, I went back to the airport yesterday to fly (in much milder temps) and discovered a 2" crack in the windscreen, probably from the cold temps last time out. Bummer. The crack is at the bottom of the screen on the passenger side at the "corner" where the screen curves from the frontal plane to longitudinal. It can't be seen from inside the cockpit at all, and there is zero separation; can barely feel it with a fingernail.

I know I should stop drill it; but would appreciate advice re: techniques, bit sizes, etc for doing so. With no separation, I don't know if it needs to be glued as well or if this isn't needed. Again, any advice appreciated.

I apologize for the lack of pictures; for some reason I left my phone behind at the house when I left for the airport yesterday.

From what I have been able to learn from searching this forum and a more general google search, it appears that stop drilling is a given, but the questions I have are mainly related to drill bit type and size: Some people are adamant that you need to use a plastic bit, which I don't have, and others say that for stop drilling it isn't important since the plastic is already cracked. Also, many suggest a very small bit size, such as 1/16, and the plastic bit sets I've looked at online seem to start at 1/8.

After drilling, it appears that gluing/sealing with Weld-On #3 is in order, according to what I've read. Any hints on how best to do that would be appreciated.

Fortunately, the repair does not have to be perfectly transparent or even look particularly good cosmetically; as noted above it is not visible from the cockpit at all, so my concerns are purely to prevent further propagation of the crack.

Thanks in advance!

[Dbro172]

When I built my canopy I drilled all holes initially with a standard #40 drill bit. (Avery tools). Then for enlarging any larger than that I used acrylic bits from here http://www.aircraftspruce.com/catalo...lickkey=106441

[cfiidon]

Regular drill bits have a flute which will tend to grab the plastic and may very well cause a crack. You can dress a regular bit to remove the flutes so it won't do this, or you can play it safe and order a plastic bit.

When I put holes in my wingtip lenses I used a soldering iron to melt
The hole. You might consider using a 1/16 or 3/32 bit in vice grips to heat and do the same. It would be great if you had a piece of scrap to try it on first (whether hot method or modified bit).

Good luck.
Don

[gerrychuck]

If using a very small bit, like a 1/16 or 3/32, would the grabbing be much of an issue? That seems to be Derek's experience. Again; going to Spruce through Derek's link, they don't sell acrylic bits smaller than 1/8 at all. I can certainly appreciate how the grabbing would be a factor with larger bits. I do wish I had some scrap lying around to experiment with, but I don't think I do.

[LettersFromFlyoverCountry]

Just curious. Does the crack come from the most-forward screw hole?

[gerrychuck]

I don't know, Bob; if you were looking down on the windscreen from above, with the screen covering the clock from 9:00 to 3:00, the crack would be at about 2:00.

[flyboy1963]

Gerry, sorry to hear that.
I found a small crack starting in my canopy ( slider) and just held a block of wood tightly on the inside, and drilled with a new, sharp 1/16" bit just at the end of the visible crack. I was surprised that I almost missed getting it centred on the tip of the crack, I'd advise making a small dot or crosshairs with a waterbased marker to make it more accurate!...or use a 1/8" bit.
I did touch a drop of methylene chloride (weld-on) to help cement the crack together, but hard to say if that really helps, ( without pressure on the joint.)
been ok for 2 years, even with the flex of the canopy.
I'd venture that the windshield is more rigidly supported, but also gets prop and aerodynamic loads, so my experience is that cracks will continue.

[gerrychuck]

Thanks Perry; I went out today and did pretty much exactly what you did. I spent lot of time making sure I was lined up with the end of the crack, and then drilled through with a 1/16 bit. With the location over the glare shield I ended up not using any backing, but drilled very carefully: very light pressure, fairly low speed, frequent stops to check progress and make sure there wasn't any significant heat buildup. I tried to avoid any chipping from punching through the back, and was pretty successful; I could feel a bit of an edge with my finger after drilling, so I smoothed it off with some 1000 grit paper to hopefully avoid any stress risers. I'll put some pics up after I have a chance to load them into the computer.

I don't have any Weld-On lying around, so that will have to wait, but hopefully I have stopped the crack from propagating until I can get some. Quick question; is the 1/16 hole okay, or should it be enlarged a bit to get a bit more radius? Would a slightly larger hole be more effective, or am I okay as is?

Film at 11...

[gerrychuck]

A couple of pics: First, before drilling:


IMG-20140119-00203 by gerrychuck, on Flickr


IMG-20140119-00204 by gerrychuck, on Flickr

And finally, after drilling. Pretty hard to see a 1/16" hole in a phone pic!


IMG-20140119-00205 by gerrychuck, on Flickr

[flyboy1963]

looks good to me Gerry, but I'm not sure it's an exact science.
Just be sure to resist the urge to stick some sealer in there.....unless it's just putty, or paintable latex caulk or something, sealers tend to have a lot of things in them that can attack the raw core of the acrylic that you've now exposed, and 'presto', a bunch of cracks radiating from the new hole you drilled.
The sanding was a good idea, if you can get a cone-shaped stone, or a tool in there with some fine sandpaper and deburr the edges, just like an aluminum fitting, that would be a good idea. ( obviously tough on the inside)