"DoN. Nichols" wrote in message
...
In article ,
B.B. u wrote:

[ ... ]

Harold, When you wrote "Can I assume the mandrel is ½?" that last
character came out as Pi on my screen, and I have no idea what you meant
by it.

It comes out as "1/2" on some screens, but since it is a part of
the extended ASCII character set, there are no promises that it will
show up the same on all. This is a primary reason for sticking to only
the characters which show on the tops of your keyboard keys. Anything
which requires fancy keyboard tricks to generate (e.g. Compose-1-2 on my
Sun keyboard) will not mean the same to everybody in this newsgroup with
mixed systems in mixed locations around the world. (And it even chances
on some of them when you change the characterset used by the program.

That's one thing I need to try to keep in mind. I'm fairly comfortable with
a half dozen characters so generated and use them routinely. Probably not a
good idea when posting! Thanks, DoN.

But, bar stock, pressed the mandrel in with an arbor press.

Hmm ... bar stock? Not turned to have a very slight taper?

His answer is likely no, and it shouldn't have. Arbors have a gentle taper
and will hold an item squarely, assuming the mandrel is pressed properly and
the item held isn't very narrow.

That could be part of the problem. I think that the typical taper on an
arbor for turning between centers is 0.001 inch/foot, but I'm sure that
Harold will correct me if I am wrong.

Close, but no cigar. Standard taper is .0005"/inch. A tiny taper like
you propose would necessitate a huge array of mandrels to be useful for
run-of-the-mill parts, unless a very tight tolerance was imposed. They
would also have the ability to swage holes effortlessly, which could be a
bad idea depending on circumstances at hand. Mandrels are often used on
grinding machines, where one is grinding the likes of bushings that must be
dead concentric. Hone first for size and finish, then grind. I've ground
literally thousands of bushings in that fashion.

We're using general-purpose indexable carbide tools on all of the
lathes, but I have a fresh bit of tool steel and I'll see about making
my own tool to play with.

That could help, as typically a carbide insert is not as sharp
as a frechly ground and honed HSS bit.

Yep! Rarely do you find inserts that rival honed HSS tools, but they do
exist. I use RB inserts that are diamond ground. Very nice inserts, and
sharp, so long as you don't buy them coated. Even the coated ones are
fairly sharp, however.


If your arbors *do* have a taper, it should get it that much
tighter. (Also note that the heat generated by turning could expand the
aluminum and loosen the grip on the steel arbor, which would make the
typical carbide insert the poorer choice here.)

Yeah, all a part of that delicate balance one must achieve. In practice,
one might never use a mandrel. They are slow, cumbersome devices that
limit the ability to machine anything. Soft jaws are often used instead,
or any of a myriad of setup variations, anything to improve driving
capability and cutting ability. Still, they're a very important part of
learning to run machines, exposing the newbie to precise ways to achieve
certain functions. Learn any and all of these procedures, and learn them
well. Later on, when you have better processes at your disposal, you'll
incorporate them when appropriate. Right now you're learning the basics,
and that's the best way to progress.

Harold