Re: [CAD_CAM_EDM_DRO] Bar stock

Hi Jon,

I have replied to the list admitting that, if production nowadays is as
Ballendo suggested, then the centreless ground stock is likely to be very
true. However, the problem I mentioned earlier about lobing of the stock
with the older 3-wheel method is valid and the biggest problem with it was
that it didn't show up using 'normal' measuring methods. That is, you could
check the diameter at as many places as you wanted to and with as accurate a
micrometer as you could find and the answer would always be the same - but
the stock would still have 3 lobes. The only true way to check it was to
mount the bar between centres on the lathe and use a fixed micrometer to
measure any eccentricity (not a dial indicator as these are subject to
'stiction' - stick-slip and are not particularly accurate as they are only
intended as comparators). By the time you had done all this is was quicker
to turn a bar true anyway.

Ian
--
Ian W. Wright
Sheffield UK
www.iw63.freeserve.co.uk

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: 27 December 2001 07:52
Subject: Re: [CAD_CAM_EDM_DRO] Bar stock


> Smoke wrote:
>
> > Happy holidays to you too.
> >
> > How accurate a pice of stock is as it comes from the manufacturer will
> > depend on the manufacturing specifications. I'm sure there are
centerless
> > ground items available that are not out of round as suggested. I'm also
> > sure that the higher degree of precision, the higher the cost.
> >
> > Several things should be borne in mind:
> > 1. Do you NEED centerlessground stock?
> > 2. How much precision are you willing to pay for?
> > 3. Is it worth your time to do it yourself?
>
> I have a fairly large variety of hardened and ground shafting. I use it
for
> a number of metrology-grade measurements, ie. test bars on lathes and
> indexing heads, round parallels, and many things I can't think of at the
> moment. I can vouch that these shafts are NOT out of round, but are
> extremely accurate cylinders, and straight, too. They are round to the
> limit of my measuring instruments, which goes down to about .00001"
> or thereabouts. (That is, indeed, 10 micro-inches, or 254 uM if I
> converted it right.)
>
> Jon
>