Unless you're taking vectored surface points individually, the 'cosine error' is not present because the systems usually build the feature out of center-point data then apply the compensation. The disk is a cut down sphere so there is virtually no difference if the manufacturing of your stylii are the same...and I assume they are. As far as wear, unless you are using continuous contact scanning, in a program that is ran frequently, there will be almost no wear on the ruby nor disk, however, if contact scanning is used, I have found some wear and am trying out silicon nitride and zarconia stylii to see if it will help. I don't think you need to worry about that from what I read. I do think that the ball/disk size is important. Generally, the larger the ball/disk, the better for accuracy and repeatability on a practical side, however, you will minimize surface form errors that are present. If you want to show these form errors for what they are, you need to watch going too big with your stylii. tas