Tom,
I send appropriate CAD drawings to the vendors. They build from those, and I accept or reject according to the outcome. This is all done in close contact with those involved, of course. The slate, for example, can be cut to an accuracy of .0006". Metal is even more accurate than that. One particular company, IDG Mfg. LLC., is owned by a machinist named Colby Lamb who has done work with me from the outset, so he is the key vendor. It is nice to have those who understand what is expected beyond that of a simple job that has been contracted, and he is that guy. If you think about it, the tolerances of most anything in a turntable are subject to the whims of temperature or humidity, or both, but in the case of slate, this is diminished over wood and some other materials. Unacceptable variance is still a big concern, however, so great care in the process is still necessary for all aspects. That is the most painstaking part of the assembly. In other words, I haven't discovered a way to snap the design together. It requires a lot of time and effort from those involved.
Win