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Head stock should point up and out 0.0005/12" This allows for cutting pressure and assures it will cut concave and not convex.
Quote from: steamer on July 02, 2018, 01:57:12 PMHead stock should point up and out 0.0005/12" This allows for cutting pressure and assures it will cut concave and not convex.That confuses me. I hadn't seen a spec like that.1) Is Rollie's Dad's Method affected by that?2) At the distances I usually cut (1" to 4") does this really have that much affect?3) I don't understand concave versus straight. And is this when turning, facing, or both?Thanks.
I'd like to see your sketches Steamer, I'm interested in this stuff.I've read through RDM, both in what I think is the official procedure link below and on other forums. http://manuals.chudov.com/Rollies-Dads-Method-of-Lathe-Alignment.pdfI guess I don't quite understand the nuances of RDM. When a headstock axis deviation is measured using RDM (notwithstanding all the shaft averaging business which I still can't quite get my head around), at the end of it all the recommended 'fix' is to start 'leveling' the lathe. Which is another way of saying introducing or removing lathe bed twist to compensate. But I can envision a 100% planar lathe bed resting on a 1-degree slope without introducing taper cutting issues. Providing the lathe bed is designed to be sufficiently rigid under its own weight which it should be. How is the lathe being adversely affected? But as I mentioned in an earlier post, if your headstock is adjustable in the yaw plane viewed from top, which many machines are, I would think all levelling bets are off & this deviation trumps shimming the lathe feet. Maybe RDM was documented in a time or specifically for lathes that have the spindle axis cast in place (meaning there is no HS adjustability). This I can see as a correction of last resort. But for 2-part headstock/bed lathes, I would think this would be the focal point especially if they were ever moved or never adjusted over time. Think about it exaggerated: if my lathe was 100% level & 100% untwisted, then I loosened the HS bolts & cocked the spindle axis 10-deg in our out relative to lathe bed, how much shimming & jacking of the feet would have to be done to twist the lathe bed to compensate? Answer: a lot. Unless I'm missing something fundamental, its attempting to solve the problem with the wrong lever.Back to convex/concave I think this might be related to facing cuts on the end of stock? The way I envision it is: - if extended HS axis is pointing to front of lathe then a facing cross cut will yield a shallow inward concave cone. In non-TS supported longitudinal cutting, the carriage travel will cut a taper where the TS side diameter is smaller than the HS side, no different that TS supported offset taper cutting method.- if extended HS axis is pointing to rear of lathe then facing cross cut will yield a shallow outward protruding convex cone. Longitude cutting will yield taper in opposite of above, TS diameter larger than HS diameter.Of course cutting forces & material & cantilevered vs. supported stock can mask what are probably tiny deviations.