In article 51701a28.0408011333.7e68d461
@posting.google.com, says...
Ahh, the infamous test bar.
I've searched, but can find nowhere to get one.
I'm getting the impression you have to make your own.
But since I can't make straight cuts yet, I wouldn't
know how to do it.

You can buy a piece of 1" X 12" Thomson shaft from McMaster
for about $16, for example. It's guaranteed to be straight
within .001"/ft, but I think it's likely to be better in
this large a diameter.

Ned Simmons