Glass pipe cutting 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.
 A: 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.

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.
A: 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.    
A: 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

A: Take a piece of string (I just use cheap yard twine) and soak it im rubbing alcohol. Then wrap the soaked string around the glass tube along/in the score mark you made. Then light the alcohol soaked string on fire.  let it burn for a few seconds then dip the gloss tube in cold water and it will break along you score line.. 
  I personally use this method in making crafts and odd containers 1 out glass bottles and has over a 80‰ success rate. 
A: For cutting a glas pipe in dimensions about what is used in typical laboratories, you can use a simple glass cutter that would normally be used for window panels and similar. 
Use a diamond glass cutter - not one with one or more small hard metal wheels.
You use the cutter to make a scratch in the glass, but not all around the pipe. Make a small scratch of roughly about one tenth of the diameter or so. Do not make it deep or strong, and do not repeat cutting. One fine, short line is what you need - it may be hard to see. Now, use your thumb or so to support the opposite part of the pipe. Apply force as when you would try to bend the pipe. The pipe breaks with nice clean edges.
(Optionally, use a gas burner to remove the sharp edges - they melt first when you hold the end of the pipe into the flame.)
This method does not require a long scratch on the glass. A short scratch is much easier to keep straight, to apply the same force on the whole length, and does not require rotating the pipe. All this means the initial conditions of the breaking are much more uniform along the scratch.
One way of creating the scratch is free hand - because it is short, it can be kept straight. I usually use a vice to fix the cutting diamond so that the tip is vertical one radius above some surface, press the pipe against the diamond and turn the pipe a little.
