This particular question relates to some 1" bright stainless bar I have
been machining. It was a problem holding it accurately in a Burnerd
multisize collet chuck as it wasn't easy to get it to run true. When I
turned a taper on the bar the intersection of the taper to the bar
surface indicated the bar was tri-lobal. I was wondering what production
process was used to produce this. I understand this is a common fault
with centreless grinding..
If your bar has the characteristic grinding, or sanding marks on the
surface, it is likely centerless ground (usually centerless belt sanded), as
you suggest. The size of the material is often an indicator of how it was
produced. Here in the States, when I bought domestic material, it was
typically centerless sanded and slightly oversized (and not necessarily
round) so you could take a minimum cut and end up with material that was on
size and at nominal diameter. I ran a job from 1-7/8" diameter 303 S for
many years and that was predictable. The one time I bought import material,
it was slightly undersized and had a cold finish with no sanding. Never
bought it again because I had tooling made to accommodate nominal size.
You can probably tell how yours was produced by the features I've mentioned.
Cold rolled material is only as good as the dies used in the rolling
mill. Most large bar is roll finished but some is sized in a die just
like pulling wire. The format of the rolling dies depends on the mill
and lots of other operations. If you look up "turks head" in a web
search, you'll see one form of rolling/forming die. Turks heads are
generally used on smaller sizes but larger stuff is often similar.
Stainless bar is notorious for being slightly oversized. When buying
stainless bar, always assume that tight OD tolerances will require some
machining. I have customers buying this as shafting all the time and it
throws them off that it doesn't slide right into the bearings.
If you really need to start out tight on the OD, ground material is the
way to go. It's spendy but may save in overall machining costs due to
secondary work you don't have to do.
If you are buying several lengths, really watch the "random" length they
come in. Stainless is so expensive now that a lot of dollars can be
wasted if you purchase "random 20's" expecting to get 4 foot bars and
find that they are running long or short. Most good suppliers will work
with you and pick to your needs if you treat them well. Some will just
say tough beans.
Harold & Susan Vordos wrote: