On 2008-10-01, Michael Koblic wrote:
[ ... ]
Thank you. I just knew it was going to be simple...:-)
So the bottom line is you need a set of perfect cylinders of varying
diameters to get your chuck right on.
Not exactly -- you need them to see how much the off-center
behavior *varies* with diameter. You aren't going to find a 3-jaw (or a
universal 4-jaw or 6-jaw) chuck which is truly on center at all
diameters -- though if it has the adjust-tru feature, you can make it
very close at any given diameter, while it is off a bit more at other
diameters until you re-adjust it. Think of zeroing an adjust-tru for a
given diameter as similar to zeroing a 4-jaw chuck. That can be pretty
good for more workpieces of the same diameter, but worse than useless
for other diameters until you re-adjust it.
This made perfect common sense to me
but oddly enough I could not find any reference to this in any literature I
perused.
That is because it is not a permanent fix -- just a way of
checking how bad things are to decide whether it is time to purchase a
new chuck.
Note that if you check at a number of diameters (perhaps those
which cover a range representing one full turn of the scroll plate in
the back of the chuck) and find out that you have close to the same
runout through the range of testing, and much more runout than the range
which you measure from min to max, you can perhaps improve things by
grinding the tips of the hard jaws while the chuck is closed on some
spacers between the faces of the jaws) using a toolpost grinder. (Be
sure to protect the bed and ways from the grinding grit while you do
this.) This won't be perfect, but it will deal with worn jaw tips and
make things significantly better.
Are such sets being sold for this very purpose?
Not as such -- for the reasons suggested above. But if you want
a set which could be used for such testing -- consider buying an index
of hardened and ground drill blanks. (They are also called "reamer
blanks". Here is an example at MSC's web site.
http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=2044515&PMT4NO=51073658
It is on page 64 of their current "Big Book" catalog, if you have that.
Be warned that they are expensive.
O.K. I've found a set:
http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=1680286&PMT4NO=51074150
for only $29.52. That is metric sizes, nine of them from 1mm to 6mm by
0.5mm steps. That makes the biggest just a little under 1/4" which
might be too small to cover the full range of a scroll plate rotation.
Pull a jaw, and measure from the start of one tooth to the start of the
next tooth. This is the range of sizes which you will need. You'll
probably need a fractional inch set, which is noticeably more expensive.
Try: http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=1676488&PMT4NO=51074150
$135.49 import quality MSC part number 01105295
1/16" to 1/2" by 1/64" steps.
But you probably don't really need to do this. Just learn to
live with your 3-jaw chuck, or get a 4-jaw and learn to set it properly
when (and only when) you need the accuracy.
Enjoy,
DoN.
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