Originally Posted by
barnbwt
Greetings,
I would like to pick your brains regarding some basic aspects of lathe spindle designs. Namely, those of the bench-top variety with a servo-controlled spindle that can operate as a fourth-axis. My slant-bed design is to be a modest size, with a 5C spindle about 8" in length through the headstock, and I am seeking confirmation or advice on my approach to this most crucial portion of the machine.
I've decided to go with the tried-and-true opposed angular contact pair at the nose end, and a simple deep groove ball bearing at the drive pulley. The workpiece is generally going to be as large as ~2" x 8" long & running at a max of 4000rpm, driven by a 1.5hp servo motor and ~400oz-in steppers. My first instinct is that the added capacity of roller bearings will be unnecessary, and would mostly serve to generate waste heat turning so fast. The goal is a very compact (3ft cube) unit that is rigid enough to do entry-level-quality light CNC prototyping (400+ lbs total weight) and since I won't have lots of damping mass, reducing the vibrations generated at the spindle and toolpost takes priority. My chief concern is that the length of my system seems to be somewhat short for it's 1.25" bore diameter, and that putting both ends of the angular contact pair at the same end may be counterproductive to reducing off-axis motion (compared to the thermal expansion advantage of keeping them close together on the spindle shaft)
The other question has to do with seal systems, which it seems run from very simple (the sealed rubber skirts of my machine) to highly sophisticated (positive pressure through non-contact labyrinth). Is it typically sufficient to rely upon the seals of a bearing pack alone for a machining/coolant environment, or is a secondary contact seal preferable? Vendors suggest that 4000rpm is safe for contact seals with grease, but should I expect greatly reduced seal life and go with non-contact shields instead? If I do a small amount of fine live tool machining or finish grinding that produces dust-like particles while the spindle is stationary, will a labyrinth plate behind the chuck be sufficient, or should this change my approach to something more proactive? I would like a fairly maintenance-free solution that doesn't entail taking apart the spindle for frequent cleaning if practical.
Lastly, though I think I know the answer already, has anyone found success using the more-compact double-row angular contact bearings in lieu of an opposed pair of single-rows, with an additional angular contact or deep groove bearing at the tail end to add preload? My guess is that absent great precision, such an arrangement moots one of the double rows, resulting in only a single row of balls at each end of the shaft carrying all the load.
TCB