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Re: Glass Teeth

Posted By: SVP Neon Equipment
Date: Friday, 23 June 2006, at 9:22 a.m.

In Response To: Glass Teeth (Dana)

> When cutting in a close quarter on an
> obviously strained piece, you can get this
> 'tooth' or serration when cutting tubing.

> Is there any prep you can do to maximize
> success with this type of face-beside not
> doing it in the first place?

> I can file it flat(er), but the weld will be
> problematic without question, especially
> when you factor in an attempt not to re-melt
> adjoining areas.

Dana, I know that for the most part you are teaching yourself, so you do not have the benefit of a seasoned bender looking over your shoulder giving you pointers. Even if they were, it has been my observation and experience that in situations such as this, most would not give the right advice anyway. (The advice to stretch the glass where the cut will be made is just one small example).

It has always dumbfounded me that most veteran benders have never heard of a polariscope (much less used one) and why "neon schools" do not teach their students how to use one. It can be a very valuable educational tool for instructing someone. It will show you where stress occurs and the severity of it. The astute instructor can show what can be done to reduce or eliminate stress and why, and the results can be visually seen.

If a polariscope were an expensive piece of equipment I could understand why some shops do not have one, but not schools. Admittedly, some commercial grade 'scopes' are rather expensive, but there are ones under $200.00 as well. For our purposes, viewing clear glass tubing, a home-made one is more than adequate and inexpensive to make if you don't want to spend a couple hundred.

With that said, if you looked at your work with a polariscope you would notice that for simple bends, such as a 90* bend, double back, etc., the majority of the stress is adjacent to the bend - next to where the glass has been heated to the softening point, not in the actual bend itself. This is assuming you have not laid the bend on a cold table surface before the glass temperature was below the strain point.

Example, 15mm: You may have noticed that when attempting to cut the glass 3/8" to 1/2" away from the bend the cut is usually not too good. (Beyond that point is obviously not a problem). Whereas if you cut it as close to the bend as possible it is usually more successful. This is because the majority of the stress is adjacent to the bend. In this case 3/8" to 1/2" away from it. The stress closer to the bend is more mild - sometimes almost non-existent. Again, this is assuming the bend was done in the air, not laid on a cold table and put in a rack to slowly cool.

If a cut HAS to be made where you know the stress is going to be, there are a couple of "quick 'n dirty" things you can do:

1) After the bend is made and it solidifies, heat the glass in the area where the cut will be made, barely to the softening point. Do not stretch the glass as this will make for a thin seal. Let the piece slowly cool. Remember, the stress will be most severe adjacent to where the glass has been heated. What you are doing is "moving" the stress away from the heated area.

2) Immediately after the bend is made and it solidifies, thoroughly flame anneal the area. This takes more time and some practice to get it right, and you really don't know what you are accomplishing unless you look at the results with a polariscope. But if done correctly it is the better of the two methods because it reduces the stress on both sides of the heated area, thereby making the bend stronger.

Heating the glass to a higher temperature to make a bend will actually increase the amount of stress adjacent to the heated area because it creates a greater temperature differential between the heated and unheated areas. This is assuming a decent bend was made to begin with. You obviously have to have the glass hot enough to *easily* make the bend and not force it.


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