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I want to know how to quickly create the straightest possible breaks in glass pipes

I apologise if this is only borderline suitable for a physics forum - I just hope experts with a lot of experience in stress, strain, thermal expansion, material science and similar topics might have some insight into.

The glass being cut: in short, all kinds. Varying quality, some tempered and/or annealed. Varying thickness, varying diameter, varying types - lime soda, borosilicate. Typical diameter 20mm to 40mm, thickness 1-2mm.

So far, I've had most success simply using a glass scoring tool which I use to manually score round the outside. Then I apply a bending force at that cut. I have less than 50% success rate.

I found videos on youtube where amateur glass-workers use various combinations of scoring, heating over flames, dunking in water hot or cold, applying ice. But the results seem variable, and I have had even worse success applying these methods.

I've heard of professional glass tube cutting machines that use a flame and water. I don't know if they also score the glass. I couldn't find any such machines on the net, let alone how they work.

How would you design such a machine to work? I presume the key factors are glass tolerates higher compression than tension, and glass is not a good conductor of heat.

For example, would it make more sense to apply heat evenly around the tube in a narrow band, while next to it, applying water in a narrow band? Or applying one followed by the other? Or perhaps heating on one side of the tube and cooling on the other?

Thanks!

Addition: After seeing this post: Why does glass break at the line where you score it? Perhaps it would make sense that heat is applied at the score line, equally and simultaneously around the circumference? If so, where would the water be applied? I'm clutching at glass straws with this one.

Further addition: This PDF on glassworking, with references to M P Groover, “Fundamentals of Modern Manufacturing", explains that tubes are made in extremely long lengths, and implies they are cut by scoring and then breaking along the score line. But it doesn't say how exactly the breaking is done. I should see if I can find a copy of that book.

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What is the context of the question? That is, why do you need such neat breaks? –  dmckee Mar 6 '13 at 16:45
    
Part of my income is from cutting and supplying glassware, I want to offer glass tubing. My clients use it for anything from, brewing to scientific projects to art, plus a surprising number of bizarre applications such as replacement parts for old military vessels –  Jodes Mar 6 '13 at 18:50
    
In that case, the cost of a diamond wafer saw may not seem that much. –  Mark Rovetta Mar 7 '13 at 3:33

3 Answers 3

I would refer you to any of many manufacturers of abrasive water jet cutters. Wikipedia has a good initial discussion an image of an abrasive water jet.

abrasive water jet

Essentially these put water under high pressure which sucks grit from a feeding line before leaving the nozzle at high pressure.

There are also multiple self help guides online on how to make cuts.

As far as the physics of breaking glass, since glass is an amorphous substance, there are no dislocations or grain boundaries to stop crack propagation. So once scored, the glass will tend to break at the defect and the crack will propagate very rapidly in the direction along shear boundary that is most closely aligned to the force. A well placed notch should serve to direct the force in the desired direction.

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It really depends upon your needs and application. When I had to routinely cut (0.125-0.250 inch dia.) glass tubes (to length) I had to use a precision diamond disk-saw.

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Try to first make two scores in the pipe and then making perpendicular score in between the two line and the breaking the small one I think it would help here is a pictorial representation of my idea if I interpreted your question correctly enter image description here

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