ron soyland
The glass lathe is the most useful of tools used in tube making. Virtually every operation in the glasswork is done on the glass lathe. While some very experienced professional glassworkers may be able to do the work freehand, it is completely out of reason for the amateur to try it. It takes many years of practice to get to where you can do freehand glasswork of such precision that you could successfully make tubes. Some simple tubes can be easily made freehand by the amateur and are presented on this site. However if you intend to make a lot of different types of tubes, a glass lathe will make success far more likely. A beginning amateur using a glass lathe can do tasks successfully in an hour that would take sometimes dozens of hours of practice to do freehand. Even an experienced glassblower will admit that a lathe makes some critical jobs much easier.
Unfortunately, the glass lathe is the mose expensive and hard to find piece of equipment you will attempt to buy. Commercial glass lathes are large, usually made for working laboratory glassware. These lathes are far too large to be convenient for small precision work like making vacuum tubes. Also, commercial glass lathes are very expensive, even used.
A possible solution is to use fixtures for all of your tube making. This can be done. I will show you a simple fixture that can be used to make some surprisingly complex tubes. To use the fixture, you will have to develop glassworking skills far more advanced than that needed if you have a glass lathe. This means a lot more time from when you first start out to the time you have a working tube.
The second solution is to build a small glass lathe from scratch. This is beyond the capability of those of you that do not have access to machine shop tools. There are a lot of parts that must be made on a metal lathe and milling machine. If you do have access to machine shop tools, you can make the small lathe shown here for about $500, depending on how lucky you are in finding bearings.
The picture above is one of the chucks on the glass lathe. The hole through the center is 3 inches diameter. The face of the chuck is made of 1/2 inch aluminum plate. An aluminum sleeve to fit the bearings is pressed into the face plate. The bearings are 3.5 inch small cross section bearings. The cross section of the bearings is 5/16 inch, so the size of the housing is quite small. If you use too large of a bearing, it will make the linking of the chucks more difficult as finding parts larger than 4 inches diameter is expensive.
The chuck jaws are made of brass and have a gear pinned to them. A fixed worm with a hex drive is used to adjust the position of the jaw. The jaw extensions to grip the glass are made of 1/4 diameter rod with teflon bushings on them. The teflon works fine when around the flame. Rubber hose can be used but it tends to catch fire or melt under certain conditions. It is important to have some kind of resiliant material that contacts with the glass to keep it from breaking.

A glass lathe is different from a common metal working lathe or woodworking lathe. The glass lathe has two chucks that are mounted on the bed opposite from each other. These chucks are designed differently than metal working chucks. They are much lighter in construction since in glass work there is very little force involved. The second difference is the through hole. It is common in tube making to have parts 2 to 3 inches in diameter that have to fit through the center of the chuck. Thus, the chuck is designed to have as large of a through hole as is practical for the size of bearings on hand.
The photo above is the homeade glass lathe.The base is made of lightweight 1 inch steel tubing welded to be a flat bed. The right hand chuck is fixed to the bed. The left hand chuck is mounted to a movable carriage such that it can be moved along the bed with a hand wheel. The distance between the chucks in the photo is 12 inches for size comparison.
Above is the drive mechanism for the chucks. Across the back of the chucks is a splined shaft. (visible in the chuck photo up above) The splined shaft is directly geared to the right hand chuck since that chuck doesn't move. The sliding rider on the splined shaft is geared to the left hand chuck so as it moves along the bed it stays linked to the shaft. Thus, both chucks are positively linked no matter what the position. Cog belts could be used to accomplish this cheaper than the gears.
The clutch disconnects the splined shaft from the drive motor when the lathe is off. This allows the chucks to be easily turned by hand for positioning the work and centering pieces.
Another way to design the linked chucks is to use two stepping motors. The motor windings are wired in parallel so that the shafts turn in exact step. The shafts are linked to the chucks with cog belts. A controller is necessary for the motors so the chucks can be kept in sync at all times even when stopped. A slow jog either direction is necessary since it is not possible to turn the motors by hand while the power is on. Of course if the power is turned off, the chucks would immediately be out of sync.
A third way is to use what are called slasyn motors. These are AC multiphase motors that have a set of windings that are connected between two of the motors. They were used in WW2 radar indicators to link the rotating antenna to the display. While powered up, any movement of the shaft on one motor is directly reflected in the other. These have the advantage of not requiring any controllers. They maintain their sync as long as the power is on and turn freely by hand or by a linked drive motor. The main disadvantage is finding the motors. They are only available surplus and are rare. They do come on ebay.
The left hand chuck needs to be movable along the bed to allow positioning of the tube parts lengthwise. This is accomplished by mounting the chuck bearing housing on a moveable slide that is driven by a screw. The screw is geared to a hand wheel on the front of the lathe for convenient use. Above is a carriage that came from an old copy machine and was used in my lathe. This kind of carriage is regularly available on ebay. This one has a 6 inch linear motion, which is adequate but marginal for a tube lathe. An 8 inch movement would be best.