Here is an excellent example of what we consider poor design regarding glass tubing...

We were referred by a colleage as a possible source to retrofit the glass components of some existing test equipment. The glass itself consists of two lenghts of concentric tubing that are held and sealed with metal flanges as shown on the right and below. This is not a difficult problem, but the original design did not allow enough room to acommodate the standard tolerances allowed for borosilicate tubing. 

The seat for the inner tube is machined into a metal flange which is smaller than the manufacturer's specs. The glass will be secured by an oring that is compressed aganst the inside of the glass tube, as shown in the detail images below. In crossection, you can see that the end of this tubing has been tooled for the correct thickness and annularity. Scroll down to see a couple of images of this tubing being resized (tooled) in a lathe.

This expense should be unnecessary. A better solution would have been to accommodate the standard glass tubing sizes during the design and machining, making the glass components inexpensive and easier to source.