In article ,
jim rozen wrote:
In article , says...
[ ... ]
1) Stop/center drill
Whoa stop right there Don!
You have just jogged my memory from when I was
setting up my turrret lathe. I've only done
a few parts on it so far, some were brass thumbscrews
for an antique radio, and some were some aluminum
thumbscrews for an electric timer.
But.... The real question is, *how* do you get
the stock stop (in this case, the point of your
centerdrill) to hold an accurate length when you
snap the collet closed??!
First off -- the centerdrill point retracts to below the face of
the stop, so it doesn't ding the end of the work while setting length.
There was one of these shown in Moultrecht's two-volume _Machine Shop
Practice_, so I just made one of my own. Not as nice for stock stop as
the ones with a bearing-mounted rotating pad on the end, but it saves me
a station. :-)
Now -- what I do is to pull the rod stock too far by about a
half inch, then apply the lever to the point where there is some drag,
and use the turret with the stock stop to push it back (with most of the
slack taken out by the lever. If the stock is dragging on the collet,
there isn't much travel left in the collet before it locks up. Balance
by feel the drag as you approach the end of travel on the turret, and
then lock up. There will be some stock movement as you lock up, but it
is pretty minimal, and pretty consistent.
The most I can figure out is, one either
a) tries to get the variation in length of stock
protuding from the collet once it is closed, to
be about the same each time, ie. try to keep the
error small and constant, but accept it as an
overall finished part tolerance, or
This is essentially what I do -- with the drag technique
minimizing the variation.
b) get it close with the stop, and then use a crosslide
station, or a facing tool in the turret, to set an
absolute length that does not depend on the stock's
protrusion from the collet at the beginning of the
cycle.
I tried this, and decided that it wasted too much time when
doing a run of parts. As well as wasting a little bit more stock.
I think I was seeing variation on the order of between
five and ten thou in length, when I was using the stock
stop in the turret station alone. Does this sound
about right or is there a secret skill that turret
lathe operators know to keep this smaller?
Well ... I don't know about *secret*. I developed this out of
need, and didn't learn it from anyone else, but I suspect that it has
been similarly discovered by many others.
Of course, there are styles of collets which have zero stock
motion when being closed, but I understand that they are a bit less
consistent in concentricity, and the 5C with lever is what I have, so I
just learned to work with it. :-)
Thanks - Jim
You're welcome.
Now to see what others answered to your question.
Enjoy,
DoN.
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