Re: Headlight Lens Crack

[e6941tb@gnn.com]

Al Powell wrote:
> 
> About the only way to stop a crack from contuing is to stop-drill it
> at the end of the crack.  This means drilling a hole at the end of
> the crack, which distributes the strain in a circle (the hole) and
> sometimes stops it from spreading.  Of course, it doesn't remove the
> reason the crack started in the first place.
> 
> On a headlight lens, the thermal cycling from having that halogen
> lamp on and off would be likely to spell doom for an attempt to
> stop-drill....BUT you might get a diamond-tipped bit (or something
> similar - you have to drill glass, remember...) and try it.  Then you
> can use clear silicone to seal up.  Result: a faulty lens with
> silicone impairing the light pattern.  Yuck.
> 
> If you try something like this, be prepared to sacrifice the lens NOW
> instead of waiting for it to finish cracking on its own.
> 
> Igor, any comments?  You seem to be the most clued-in on ideas like
> this.  Am I leading the poster who asked this question astray???

Al, this technics work well on plastics. I wouldn't try this on the
glass, however. Drilling the glass will generate a lot of local heat,
whereas the rest of the lens will remain to be at the ambient
temperature. The temperature gradient will crack the glass immediately.

Incidently, I work for one (out of two nationwide) high precision
speciality glass houses. Although I spend the best part of the day in my
electronics lab, I get to see a lot of very high tolerance (0.0001")
glass work. It is still more art than science, and we have some awesome
glass technicians. One of them once in a while repaires stuff for me as
a favor. He once had virtually resurrected a clear Pyrex tea brewing
pot, that mom brought from the old country and which has a sentimental
value for me. (BTW, the tea, brewed in it IS much better, than it is
from a conventional porcelan pot. No paper tea bags need apply here).

The only way to repair a partially cracked lens would be to remove it
from the reflector housing, properly anneal it and seal the crack with a
torch. If done right, the crack would disappear before your very eyes.
Looks fascinating. BTSeenT many times. I even tried to learn the skill
myself (out of curiosity), but proved to be rather dumb and decided to
stick to my soldering guns, er pencils that is.

I once also tried to polish a wiper arm scratch off my windshield. I've
put a thick felt wheel on a hi-rpm drill and dressed it up with a mildly
abrasive compound. I also had a VIN number engraved in the upper left
corner of the windshield as the antitheft ID (it all happened 10 years
ago in a windshield- and, generally speaking, any-car-parts-challenged
old country of mine). I fearlessly proceeded to polishing the windshield
and within 2 minutes I had it cracked. "Oh sh*t, haven't you been
learning Physics all your life?!,-helplessly sed I, shelling out 1/2 of
my (official) monthly salary for a replacement shield. A costly lesson
has been learned indeed.

Igor Kessel
'89 200TQ