Clausing 5914 - Chatter and Self-Feeding Summary

The 5914 is acting like a lathe at last, so I thought it useful to summarize what I have learned so far.

The key is stiffness, both static and dynamic, and the 5914 had problems in both areas, leading to the frustrating series of fixes that improved things but did not definitively fix anything. One had to simultaneously solve all problems to see a real improvement.

Static stiffness is the ratio of total deflection to total force applied, and controls self-feeding and how small a cut one can make.

Dynamic stiffness is the ratio of rapid incremental deflection to rapid incremental change in force applied, and (together with damping and mass) controls chatter.

One can have high dynamic stiffness and low static stiffness. For instance, a floppy machine with a well-damped tool will cut smoothly if things are arranged to avoid self-feeding, but it will be damn hard to cut to a specified diameter.

One can also have low dynamic stiffness and high static stiffness. The classic is a rigid machine with a spring tool, used to make metal slivers as the tool chatters merrily.

The other example is the chatter experienced with the upsidedown SGIH

19-2 blade and reverse rotation - despite the lack of self-feeding due to tight slideways and a cutting force that tends to unload the tool, the lathe chattered badly when the stock was held in a 3-jaw chuck (versus a collet).

In the 5914, the static stiffness resides mostly in how tight the various slideways are, while the dynamic stiffness problems seem to have come largely from insufficient preload on the spindle bearings. Loose slideways will certainly cause low stiffness of both kinds, but once the slideways were tightened the static stiffness was greatly increased, and yet the lathe still chattered.

One real symptom of progress is that yesterday I was able to make a

0.250" cut (0.5" diameter reduction) at 600 rpm in steel with a sharp carbide tool bit, yielding very thin curly chips, and the lathe didn't complain when I later fed at 0.0026" per turn under power yielding hot and thick chips.

Joe Gwinn

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Joseph Gwinn
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