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Posted by Joseph Gwinn on June 5, 2008, 12:56 pm
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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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