The grooved barrel parts in the magnetic levitation molecular (vacuum) pump have extra-large lead multi-start threads. This type of thread is not easy to process with a CNC lathe. I have processed two types of threads like this. One type is a cylindrical internal thread with constant pitch height, that is, the crest and groove bottom are both on the cylindrical surface. The photo below is one of them.
picture
It is not difficult to program this kind of thread, but the craftsmanship and processing must use a certain skill, otherwise the precision of the machine will never meet the requirements.
Another type of internal thread has the crest on the cylindrical surface and the groove bottom on the conical surface. The picture below is one of these types of threads.
picture
Programming this type of thread is also more difficult. The maximum lead I have ever driven is 280mm, and the maximum number of heads is 14.
