When you say that sometimes the machine can cause b grades, what does that mean? does the lathe sometimes messes up with the weight distribution, or what?
It can be a machine running too fast, or the calibration can have shifted after running for too long or not long enough. Or it could be a problem with the bar stock, or any number of other weird mechanical bugaboos. Doesn't happen often though, when you have a machine shop that pays attention. Most of our b-grades are the result of anodization problems more than anything. -Steve
