ron soyland
One of the most difficult parts of working with glass is cutting off glass parts exactly where you want, evenly, and without having the cut go out of control. Various methods of using scribes, hot wires, and various hand tools are proposed in literature but none of these is even close to the amazing convenience of the diamond rotary saw.
This consists of a very thin (.006 in, .15mm) steel disk that has diamond powder sintered to the outer edge. The blade is rotated at about 500 rpm and when it is pressed lightly against a glass part it cuts easily through it. Since the force against the glass can be controlled to be anything you want, you can cut thin fragile parts or heavy wall tubing with ease. An important cut that can be made with the diamond saw and not with other methods is cutting off oddly shaped pieces. The part doesn't have to be round.
If you are planning to work with glass you should make up one of these saws. It is not expensive, and you will easily save what you spend making the saw over the glass you will ruin if you try other methods.
The blade itself is made in China however don't be fooled. They are excellent quality and each blade will make hundreds of cuts. The blades are under $15 from ebay seller "gemshow7". The 4 inch diameter blades are available in .006 or .012 inch thicknesses. Either one is fine, but the .006 is best for very delicate cuts. It will easily cut 2 inch dia. thick wall pyrex in a few seconds. The blades come with either a half inch hole or a 5/8 inch hole. When they ship with a 5/8 inch hole, an adapter ring is supplied to fit the 1/2 inch shaft.
The saw above is mounted permanently to the glass lathe. If you don't have a glass lathe, you can mount the saw to a heavy block as a base. To cut off a piece of tubing, you place the tubing in the roller stand and position the saw blade at the point of the cut. With one hand you rotate the tube and the other hand you lightly press the saw blade to the glass. lubrication is WD-40 light oil. Any light oil or even kerosene will work fine. Simply stop rotating the glass tube for a second and spray some oil on the glass. If you have 3 hands it is easy!


The motor used on the saw should be a small DC motor. The type from the old 5 1/4 floppy drive is ideal. Suitable motors are available on ebay for under $10. A 12 volt motor is ideal but higher voltage motors will work just as well. The motor speed is controlled by a simple rheostat. The motor doesn't pull a lot of current so a 10 watt 100 ohm unit will do fine. Any method can be used to control the motor, the rheostat with a simple wall mounted 12 volt supply is easy to do and cheap.
The blade must be well centered on the motor shaft. If the blade is off center, only one part of the blade will do the cutting and it will wear out long before the rest of the blade. As the diamond dust wears off the blade, bare steel touches the glass and generates heat. This can cause cracking of the glass. Thus, centering the blade gives the longest service life since the entire blade wears evenly.
The chuck is made of aluminum bar stock. You could use brass or even delrin plastic. The two critical diminsions are the saw shaft extension, which should be very close to .500 inch. This is what keeps the blade centered. The machining of the shaft extension and the drilling of the hole for the motor shaft should be done in the same setup to insure exact concentricity. The other diminsions are not critical and can be modified to fit. The diameter of the chuck should be about 1.5 inches or so to provide stiff support for the thin blade.
A block of steel or aluminum can be used as a base to mount the saw on. Make an arm out of 1/8 aluminum plate and screw it to the motor face. There are usually holes in the motor for mounting like that. The arm is mounted with a fairly strong bolt to the base and slip washers are provided to allow the motor to be easily moved in an arc to make the cut. The fit should be tight enough to keep the assembly from wobbling and loose enough to allow easy movement of the arm. A means of parking the arm vertical when the saw is not being used is prudent to keep the fragile blade from getting bent.
Do a good job on your saw since this will be one of the most useful tools you have!