Helical glass cut attempt
But I had the crazy idea of trying to make a sort of "spring" out of glass by making a helical cut in a bottle. I didn't have any suitable bottles handy, so I used some glass jars.
For flat pieces of glass, cutting consists of scoring a line with a glass cutter, then trying to "bend" the glass along the scored line, which causes the crack started by the score to go all the way through the glass, breaking it apart. But for a jar or a bottle, there's no way to just "bend" it. However thermal stress can be used to get the score to form a crack through the glass.
I screwed the lid from one of the jars to the face plate of my lathe, then used a glass cutter to score a line all the way around.
I then poured some boiling hot water into the jar to stress it. With the inside surface of the glass hot and the outside surface still cool, thermal expansion should cause tension on the outside surface (where the score line is), which should cause the crack to go deeper.
I did this with the jar standing in some warm water to avoid shocking the bottom too much. If you pour boiling water into a cold jar it's not unusual for the bottom to pop right off.
What did I do different this time? Maybe it was because the scored line was a bit further from the top of the jar. Or maybe I poured in the boiling water slow enough that it didn't slosh around as much. Or it could be that this was just a better glass jar. I was starting to give up on the spiral cut idea, but after this one, I figured it was still worth a try.
I needed a slow steady rotation to make the score along the whole helical path. So I recruited Rachel to do the turning while I followed the line with a glass cutter. I was pleased with how consistent I was able to follow the marked line with the glass cutter.
I think the helix was too ambitious. But cutting the bottle into a series of rings 2 cm or wider might be doable.
At this point, I already broke all the jars I cared to break, and even if I was successful, a jar with a helical cut would be very fragile and not at all useful.
But I hope this encourages you to try to cut a bottle, if, for whatever reason, you want to cut one in half. For example, for making tumblers out of beer bottles.
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