Just added to the dropbox:

http://www.metalworking.com/DropBox/...ET_CENTERS.jpg
http://www.metalworking.com/DropBox/...ET_CENTERS.txt

---------------------------------

BEARING BALLS FOR OFFSET CENTERS


This is an idea that sprung to my mind a while ago, although I'm
sure I'm not the first one to think of it.

When offsetting the tailstock for taper turning, or using a special
tailstock fixture for the same purpose, the 60 degree center points
don't fit well in the centerholes of the work being taper turned.

This method needs custom-made lathe centers for both headstock and
tailstock.
The sharp point is turned off for a short distance, and centerdrilled
just as is done for the work being turned.

Hardened steel balls are captured in the centerholes between the lathe
centers and the work, at each end.

The correct centerhole size is important in relation to the bearing ball
diameter.

For a standard 60 degree centerdrill, the opening of the hole at the
ends should ideally be between 88% and 90% of the diameter of the ball.

If larger, there may not be enough clearance between the lathe center
and work to allow any offset.
If the hole's opening is smaller than 87% of the ball's diameter, only
the corner of the hole's opening will contact the ball and the whole
thing may come loose under heavy cutting pressure.

In practical experience, I've had very good results with this technique
while turning morse taper shanks.

For the purpose of accurately setting the tailstock setover, the
effective length of the workpiece is measured between the centers of
each ball.

Just mike the workpiece with the balls in place, and subtract the total
of one half the diameter of each ball.

Be sure to use your favorite tailstock center lube on that end
(I use white lithium grease).

Hope this is useful,

Ken Grunke
West Lima, WI
Jan. 09, 2005


--
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In article , Ken Grunke says...

When offsetting the tailstock for taper turning, or using a special
tailstock fixture for the same purpose, the 60 degree center points
don't fit well in the centerholes of the work being taper turned.

Your approach is novel and I'm sure it works well.

However, did you ever wonder why this was not done, back
in the days when tailstock setover was a routine approach
to manufacturing tapered items?

Basically, even though the centers don't seem to fit
well, they still allow a true cylinder to be turned on
the part.

Those old-time folks really knew their stuff.

Jim


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==================================================
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==================================================

"jim rozen" wrote in message
...

However, did you ever wonder why this was not done, back
in the days when tailstock setover was a routine approach
to manufacturing tapered items?

Basically, even though the centers don't seem to fit
well, they still allow a true cylinder to be turned on
the part.

Those old-time folks really knew their stuff.


At school, we use bell-style center drills for turning tapers with an offset
tailstock. Works very well.

Regards,

Robin

Robin S. wrote:

At school, we use bell-style center drills for turning tapers with an offset
tailstock. Works very well.

Hmmm, never heard of or seen those. Judging by the name, they must cut a
curved-shaped cone?

Ken


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jim rozen wrote:
In article , Ken Grunke says...


When offsetting the tailstock for taper turning, or using a special
tailstock fixture for the same purpose, the 60 degree center points
don't fit well in the centerholes of the work being taper turned.


Your approach is novel and I'm sure it works well.

However, did you ever wonder why this was not done, back
in the days when tailstock setover was a routine approach
to manufacturing tapered items?

Yup, I have wondered, but I suppose that in a production situation, they
got tired of dropping the balls in a pile of swarf never to be seen
again :-) My solution is to glue them in with sticky grease.


Basically, even though the centers don't seem to fit
well, they still allow a true cylinder to be turned on
the part.

Sure, although there are only two points of contact--one at the outside
edge of the hole, and then at the inside edge, where the 60 deg. cone
ends and straightens out to the pilot hole. I haven't done enough taper
turning to know--do those edges wear into the cone center after a while?

thanks,

Ken Grunke


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"Ken Grunke" wrote in message
...
snip-----

Sure, although there are only two points of contact--one at the outside
edge of the hole, and then at the inside edge, where the 60 deg. cone
ends and straightens out to the pilot hole. I haven't done enough taper
turning to know--do those edges wear into the cone center after a while?

thanks,

Ken Grunke


Yes, they do, and in the process the shaft is constantly creating more and
more clearance between the centers as it cold flows to achieve the form. .
Look closely at centers that have been run offset to see how badly they are
deformed from the center drilled configuration. Further, if, when turning
between centers, if the face on either end of a part is not perfectly
square, it has the effect of creating an out-of-round (oval) turn. This
very concept has been the subject of endless debate, with almost no one in
agreement, but all it takes is a little experience in precision grinding to
put it directly into focus. Be certain to maintain right angles on the
ends of offset turned parts unless you don't mind oval turns.

Your ball turning is a very good concept, for it eliminates that problem,
but the same results can be achieved with center drills that are made with a
large radius in place of the 60° cone. DoAll is one of the makers, but
I'm sure there are others. They have a name, but it escapes me at the
moment.

Harold

In article , Harold & Susan Vordos says...

Yes, they do, and in the process the shaft is constantly creating more and
more clearance between the centers as it cold flows to achieve the form. .
Look closely at centers that have been run offset to see how badly they are
deformed from the center drilled configuration. Further, if, when turning
between centers, if the face on either end of a part is not perfectly
square, it has the effect of creating an out-of-round (oval) turn. This
very concept has been the subject of endless debate, with almost no one in
agreement, but all it takes is a little experience in precision grinding to
put it directly into focus. Be certain to maintain right angles on the
ends of offset turned parts unless you don't mind oval turns.

Your memory is incorrect in this regard. Under normal conditions,
the centers don't open up, and in the case I tested, the end of
the part *wasn't* square to the machine axis, and it *did* turn
a round, not oval piece. To remind folks of the tests that were
done:

http://www.metalworking.com/DropBox/_2001_retired_files/offcenters.txt
http://www.metalworking.com/DropBox/_2001_retired_files/offcenters1.jpg
http://www.metalworking.com/DropBox/_2001_retired_files/offcenters2.jpg
http://www.metalworking.com/DropBox/_2001_retired_files/offcenters3.jpg
http://www.metalworking.com/DropBox/_2001_retired_files/offcenters4.jpg

One of the regulars here at that time tested the roundness of the
turned part, it showed no systematic deviation from round to the
limit of the tallyrond tester.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

"jim rozen" wrote in message
...
In article , Harold & Susan Vordos says...

Yes, they do, and in the process the shaft is constantly creating more
and
more clearance between the centers as it cold flows to achieve the form.
..
Look closely at centers that have been run offset to see how badly they
are
deformed from the center drilled configuration. Further, if, when
turning
between centers, if the face on either end of a part is not perfectly
square, it has the effect of creating an out-of-round (oval) turn.
This
very concept has been the subject of endless debate, with almost no one
in
agreement, but all it takes is a little experience in precision grinding
to
put it directly into focus. Be certain to maintain right angles on the
ends of offset turned parts unless you don't mind oval turns.

Your memory is incorrect in this regard. Under normal conditions,
the centers don't open up, and in the case I tested, the end of
the part *wasn't* square to the machine axis, and it *did* turn
a round, not oval piece. To remind folks of the tests that were
done:

http://www.metalworking.com/DropBox/_2001_retired_files/offcenters.txt
http://www.metalworking.com/DropBox/_2001_retired_files/offcenters1.jpg
http://www.metalworking.com/DropBox/_2001_retired_files/offcenters2.jpg
http://www.metalworking.com/DropBox/_2001_retired_files/offcenters3.jpg
http://www.metalworking.com/DropBox/_2001_retired_files/offcenters4.jpg

One of the regulars here at that time tested the roundness of the
turned part, it showed no systematic deviation from round to the
limit of the tallyrond tester.

Jim

Chuckle! Or perhaps big belly laugh!!

Yep, I remember, and I commend you for the great pictures, but that's not
what we're talking about. My point is turning a taper with an offset
*tailstock* center, although it's possible I never made that clear in my
original argument. It's not the same thing. The degree of error in
drilling offset centers remains constant and there is no movement of the
part as it relates to the fixed, but *in line* centers in the test you
performed. The machine centers, in your specimen, would pick the high
spots and run there, likely not fully seated, but with enough area of
contact to perform without distorting. When you offset the tailstock,
everything changes. You didn't prove your point originally, I simply quit
talking about it because I had quit following RCM (sort of like not talking
to your family, I discovered).

Try that same test, this time offset the tailstock, and for purpose of
proving whether you're right, or I am, turn a much shorter piece, with a
large offset, so it's exaggerated. Be certain that the faces are not at
right angles to the center, which is a part of my argument. You'll not only
mush the centers, you'll detect an oval. Grinders (the machines, not the
operators) don't lie. By the way, you shouldn't need any special machine
to learn what I'm talking about. Simply measuring the part will disclose
the oval. It will be fairly obvious.

Harold

In article , Ken Grunke says...


Basically, even though the centers don't seem to fit
well, they still allow a true cylinder to be turned on
the part.

Sure, although there are only two points of contact--one at the outside
edge of the hole, and then at the inside edge, where the 60 deg. cone
ends and straightens out to the pilot hole. I haven't done enough taper
turning to know--do those edges wear into the cone center after a while?

The contact area is larger than you might think in this case.
There was a thread on this a while ago, and I took some photos
of this. It really winds up being a line, and fairly large
contact patch:

http://www.metalworking.com/DropBox/_2001_retired_files/offcenters2.jpg

Unless one is really cranking on the tailstock, they don't wallow
out. And if one is really cranking on the tailstock, they'll
wallow out even on straight turning.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

So what drives the part here? It looks like it's free to just stop turning
to me. Or is the dog/driver plate setup just left out of the sketch for
clarity?

GWE

Ken Grunke wrote:

Just added to the dropbox:

http://www.metalworking.com/DropBox/...ET_CENTERS.jpg
http://www.metalworking.com/DropBox/...ET_CENTERS.txt

---------------------------------

BEARING BALLS FOR OFFSET CENTERS


This is an idea that sprung to my mind a while ago, although I'm
sure I'm not the first one to think of it.

When offsetting the tailstock for taper turning, or using a special
tailstock fixture for the same purpose, the 60 degree center points
don't fit well in the centerholes of the work being taper turned.

This method needs custom-made lathe centers for both headstock and
tailstock.
The sharp point is turned off for a short distance, and centerdrilled
just as is done for the work being turned.

Hardened steel balls are captured in the centerholes between the lathe
centers and the work, at each end.

The correct centerhole size is important in relation to the bearing ball
diameter.

For a standard 60 degree centerdrill, the opening of the hole at the
ends should ideally be between 88% and 90% of the diameter of the ball.

If larger, there may not be enough clearance between the lathe center
and work to allow any offset.
If the hole's opening is smaller than 87% of the ball's diameter, only
the corner of the hole's opening will contact the ball and the whole
thing may come loose under heavy cutting pressure.

In practical experience, I've had very good results with this technique
while turning morse taper shanks.

For the purpose of accurately setting the tailstock setover, the
effective length of the workpiece is measured between the centers of
each ball.

Just mike the workpiece with the balls in place, and subtract the total
of one half the diameter of each ball.

Be sure to use your favorite tailstock center lube on that end
(I use white lithium grease).

Hope this is useful,

Ken Grunke
West Lima, WI
Jan. 09, 2005

Grant Erwin wrote:
So what drives the part here? It looks like it's free to just stop turning
to me. Or is the dog/driver plate setup just left out of the sketch for
clarity?

Yup--and to save time. Just a quick 3D CAD sketch!

Ken Grunke

--
take da "ma" offa dot com fer eemayl

On Sun, 09 Jan 2005 18:27:48 -0600, Ken Grunke
wrote:

Just added to the dropbox:

http://www.metalworking.com/DropBox/...ET_CENTERS.jpg
http://www.metalworking.com/DropBox/...ET_CENTERS.txt

---------------------------------

BEARING BALLS FOR OFFSET CENTERS


This is an idea that sprung to my mind a while ago, although I'm
sure I'm not the first one to think of it.

When offsetting the tailstock for taper turning, or using a special
tailstock fixture for the same purpose, the 60 degree center points
don't fit well in the centerholes of the work being taper turned.

This method needs custom-made lathe centers for both headstock and
tailstock.
The sharp point is turned off for a short distance, and centerdrilled
just as is done for the work being turned.

Hardened steel balls are captured in the centerholes between the lathe
centers and the work, at each end.

The correct centerhole size is important in relation to the bearing ball
diameter.

For a standard 60 degree centerdrill, the opening of the hole at the
ends should ideally be between 88% and 90% of the diameter of the ball.

If larger, there may not be enough clearance between the lathe center
and work to allow any offset.
If the hole's opening is smaller than 87% of the ball's diameter, only
the corner of the hole's opening will contact the ball and the whole
thing may come loose under heavy cutting pressure.

In practical experience, I've had very good results with this technique
while turning morse taper shanks.

For the purpose of accurately setting the tailstock setover, the
effective length of the workpiece is measured between the centers of
each ball.

Just mike the workpiece with the balls in place, and subtract the total
of one half the diameter of each ball.

Be sure to use your favorite tailstock center lube on that end
(I use white lithium grease).

Hope this is useful,

Ken Grunke
West Lima, WI
Jan. 09, 2005


This is a pretty good technique. It's perhaps
worth emphasising that it removes the length uncertainty
that's always present when turning between centre points.
With centre points the bar pivots about a point a bit inside
the pivot hole so the effective length is always an
uncertain bit less than the overall bar length.

With balls this is not a problem. The measurement
of overall length with both balls in place less 1/2d +1/2d
(d=ball dia) precisely defines the effective length.

With this checked with a decent vernier and the
offset set by gauge blocks pretty precise tapers are
possible.

Jim

wrote:

This is a pretty good technique. It's perhaps
worth emphasising that it removes the length uncertainty
that's always present when turning between centre points.
With centre points the bar pivots about a point a bit inside
the pivot hole so the effective length is always an
uncertain bit less than the overall bar length.

With balls this is not a problem. The measurement
of overall length with both balls in place less 1/2d +1/2d
(d=ball dia) precisely defines the effective length.

With this checked with a decent vernier and the
offset set by gauge blocks pretty precise tapers are
possible.

Exactly!
thanks,

Ken


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