Mathematical Basis of Rollie's Dad's Method

Joseph Gwinn · 2011-07-15T03:16:49+00:00

I had been working on this and posting about it in September 2010 or so, and was getting confused. I retreated to the shop and took a bunch of measurements on lathe and mill, and tried to fit data to the then math model, and was getting odd results. Then my day job got too busy, and all else drifted to a stop. Now I'm back on the task, prompted by a passing mention on RCM, and may have figured it out. At least mathematically, as I have not done any added shop work, but it may be a while, so I'll post what I have.--------------------------------------------------------------------The original purpose of ³Rollie's Dad's Method² (RDM) was to align the spindle rotation axis of a metalworking lathe parallel to its bedway, such that the lathe would machine perfect cylinders. Many people found the instructions somewhat difficult to use, even though they seemed simple enough. It was not clear what RDM's abilities and limitations might be, and it seemed likely that some unknown limitation was causing the difficulties of application. It also appeared that RDM could have applications to other machine tools. For these reasons, a mathematical analysis was undertaken, and is summarized here.The physical setup is that a quite smooth and round and reasonably straight bar is clamped very firmly in the machine spindle. Nearby there is a dial indicator mounted firmly to a machine component that is designed to move parallel to the spindle rotation axis. The tip of the dial indicator's probe rides on and is perpendicular to the side of the bar. The bar is manually rotated by rotating the spindle, and the following things are measured and recorded at various places along the bar: the maximum and minimum indicator readings, the distance from the spindle face, and the diameter of the bar.The maximum dial indicator readings are those where the probe tip is at the greatest distance from the spindle rotation axis, while the minimum dial indicator readings are those closest to...

Jul 15, 2011 24 Replies

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Jim Wilkins 15 years ago

The wear on the AA lathe bed was greater on the front way, under the tool bit, so the carriage tilted further away from the spindle axis near the chuck than towards the tailstock. It wasn't very evident until I milled the ways and saw the pattern of cuts.

jsw

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Denis G. 15 years ago

Thanks, Joe.

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pentagrid 15 years ago

Snip

Because I had made the point that ANY pair of traverse measurements could be used to correctly define the spindle axis it was necessary to add the observation that the angular accuracy degrades as the spacing decreases.

The measured spindle axis orientation does not change if the location of the pairs of measurement points is moved, so choice can be a matter of convenience. Close the headstock and near the end of the test bar is normal.

Jim

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