On 2009-07-11, Michael Koblic wrote:
Jim Wilkins wrote:
On Jul 9, 8:37 pm, "Michael Koblic" wrote:
Jim Wilkins wrote:
You can check if the tool will shave off a fraction of a thousandth
while you're still a few thousandths out and can safely remove and
hone it.

Presumably this is possible only with a QCTP without removing the
toolbit from the holder? If you took it out of a lantern or a rocker
you will never be able to re-insert it in exactly the same way. Or
am I missing something? ... Michael Koblic
Campbell River, BC

You don't have to reinsert it exactly the same way, you remove and
sharpen the bit while there is still enough extra metal left to take a
fine trial cut and measure.


I see.

One of the things I was not fully aware of is that if one does three passes
with the same (sharp) tool over the same area without changing anything, the
first pass produces sort of peaks and valleys. The second pass will not
follow the path of the first pass exactly and will flatten these peaks. The
third pass will do so even more especially if done at higher speed. Thus the
first measurment would catch the peaks which will not be there after the
second and third pass and the measurment will be less.

I assume this is true even in a perfectly rigid system with an infinitely
sharp tool.

That depends. If the feed is coming through the carriage drive
or hand fed, yes it will pretty much improve things -- if the tool nose
is too much of a point for the feed. If you have a sharp tool point, go
for a finer feed. If you need a faster feed, round the point somewhat.

But -- if your feed comes from the leadscrew and half nuts (not
present on your Taig, so that does not matter) then it will take
precisely the same path and not cut the peaks left by a previous pass,
but rather move through the existing valley. Your experience with the
Taig involves hand feeding of the carriage by necessity. There is no
other option present, so the path of the tool is going to be somewhat
unpredictable no matter how closely you attempt to feed at precisely the
same rate.

Note that some of the less expensive lathes do not have a
separate carriage feed in addition to the leadscrew and half nuts (which
*should* be reserved for threading, not turning to minimize wear on the
leadscrew and half nuts which can reduce the precision of subsequent
threading. An example of this is my ancient Atlas/Craftsman 6x18"
lathe.

Thus the trick is to find out how much the tool will remove
between the first and third pass. Was this what you were referring to in
your first response?

I believe that he is assuming a slow enough feed for the
geometry of the tool tip, and is assuming that you are adding a little
more infeed for each pass -- especially for the next to last pass (just
after honing the tool) and the last pass (also with the freshly honed
tool).

Enjoy,
DoN.

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