PVC. What a nightmare. While it works just fine for most of its applications like plumbing and impromptu scaffolding, it's horrendous when it comes to engineering outside of those niche areas. It machines OK, and comes off in medium-size ribbons on a lathe (I looked like I was covered in silly string when I was done turning my workpiece), but the fact is, it's just not round. It makes trying to measure the damn stuff almost impossible!
What's the context here, you ask? Well, I'm working on a few projects at the moment. One of the more interesting ones is the construction of a 6 degree of freedom robotic arm. I've chosen DuPont Delrin as my main working material, and I have to say I absolutely love it. It machines easily and holds tolerance, has low density, is quite rigid, and best of all it's cheap. Unfortunately, it's not cheap enough that I can build the base of my arm from it. I've decided to attempt to make my base out of a section of 6" ID PVC pipe instead. It was to be covered on both ends by a pair of .1" aluminum plates, turned on a lathe, which were to be secured to the pipe by means of a set of 6 #8 screws each. Of course, this means that I have to place these screws almost exactly in the middle of the walls of the PVC, and... Well, therein lies the problem!
In any case, I'm not a fan of machining PVC for tight tolerance projects such as this one, and you certainly can't rely on it being round to any more than .25" tolerances.