On 2010-09-15, Jon Elson wrote:
On 09/15/2010 01:32 AM, George wrote:
Hello again,
I have a 60 year old Herbert preoptive Lathe. It is quite huge and I
have a chuck with a very bad case of worn out scroll. My question is:
is it possible to replace the scroll and the jaw carriers, or what
ever they are called. or would it be easier to get a whole chuck, -
Easier? Sure, replacing the whole chuck is easier....
But, machining a new scroll is not very difficult. The main thing is
you need to be able to replicate whatever the spiral thread pitch is on
the chuck, unless you will be replacing the lower jaws, too. Your lathe
may or may not be able to match that thread pitch with whatever
settings are available. Depending on how the scroll is turned by the
pinions, that can be the more difficult part. Often they have bevel
gear teeth cut in the edge of the scroll.
You know -- while you mentioned the lathe brand, and posted a
photo of the chuck mounting plate, you did not say how big a chuck you
needed. I know that *I* have never heard of a Herbert Preoptive lathe
before. (Of course, I was only 9 years old when it was made. :-) This
might be useful information.
Actually -- often the problem in the scroll-back lathe chucks is
that the center of the scroll gets worn -- and it usually uses the
center on a pilot in the chuck body to maintain centering.
To fix that -- at least somewhat -- take the back off the chuck,
take out the jaws, and using some feeler gauges, determine how much
smaller than the bore in the scroll the center spigot is. Divide that
size by two, get shim stock of that thickness, and cut a strip to wrap
almost totally around the spigot. Slide the plate over it, and this
will minimize the walking off center.
Note that such chucks are sensitive to which of the (usually)
three key holes is used to tighten it, because this determines which way
the scroll plate is pushed off center. So -- it is best to mark one of
the key holes as the "master" if one is not already marked. (Stamp a
'0' in the body near the key hole.) Experiment to find out which one
gives the best centering for a common size of workpiece (the error
varies with workpiece diameter).
Once you have found the best key hole and marked it, consider
re-grinding (or turning if soft jaws) the top jaws.
Of course -- if you have the jaws in the wrong slots, or in the
wrong order, you will get major errors in centering.
And a 3-jaw scroll-back chuck will always have some error.
You can get plain-back chucks, and make your own backplates to
mount them to your spindle. (It sounds as though you might have
difficulty finding backplate to fit it from the vendors.) A quick
Google search comes up with Herbert No. 3, Herbert No. 4, and Herbert
No. 7 and No. 8 lathes. And what is huge to one poster is tiny to
another. My largest lathe is 12" swing, and I do not consider it large,
while those with 3" swing lathes (yes, they exist) might consider mine
huge. Personally, *I* consider the ones with 60" swing or larger to be
huge. :-)
O.K. here are the dimensions for a No. 4 -- finally found:
7-1/2' (that's foot, not inches) center height, 17' swing over bed, 8'
swing over cross slide, spindle bore 2-1/4", and 32 feet from chuck to
turret face.
This one is huge enough by my standards -- but is it the one
which you have? Tell us the size so we have some clue.
Making new lower jaws may be more difficult, as they need to be a very
fine fit into the chuck body, and have a number of bearing surfaces that
need to be ground.
Don't make master jaws -- and if possible, don't make a new
scroll plate -- just re-center the scroll plate and then true the jaws.
(Turning for soft top jaws, toolpost grinding for hardened top jaws.
Or -- consider making your own back plate for a chuck which you
*can* find.
there is no runout on the body of the chuck? I get runout that varies
over about 20 thou, depending on which bit of the scroll is in use.
What about the chances of finding a 4 jaw that would fit on the 3 stud
fixing.
I assume this is a D1-x camlock mount, when you talk about these pins.
These are still in current use, and many chucks, backplates and etc. can
be found in tool catalogs. Just measure the pin spacing and you
should be able to match the correct D1 size.
From the photo, I think not.
Good Luck,
DoN.
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