Before I do something irrational, is there a way to measure the
pressure exerted on a shaft being held between centers? This will allow
me to calculate deflection.

The object to be used is a solid 1" diameter Thomson shaft that is
18" long.

Louis Ohland wrote:

Before I do something irrational, is there a way to measure the
pressure exerted on a shaft being held between centers? This will allow
me to calculate deflection.

There do exist life centers for the tailstock that do exactly this.
Investment at payday!


Nick
--
The lowcost-DRO:
http://www.yadro.de

I remember a center with some sort of a dial on it. Damned if I can
remember the trade name. I think it was in an MSC catalog... Let me surf
a bit...

Nick Mueller wrote:
Louis Ohland wrote:

Before I do something irrational, is there a way to measure the
pressure exerted on a shaft being held between centers? This will allow
me to calculate deflection.

There do exist life centers for the tailstock that do exactly this.
Investment at payday!


Nick

Nothing on MSC yet, but I bet it's a magic term.

Louis Ohland wrote:
I remember a center with some sort of a dial on it. Damned if I can
remember the trade name. I think it was in an MSC catalog... Let me surf
a bit...

Nick Mueller wrote:
Louis Ohland wrote:

Before I do something irrational, is there a way to measure the
pressure exerted on a shaft being held between centers? This will allow
me to calculate deflection.

There do exist life centers for the tailstock that do exactly this.
Investment at payday!


Nick

Louis Ohland wrote:

Before I do something irrational, is there a way to measure the
pressure exerted on a shaft being held between centers? This will allow
me to calculate deflection.

The object to be used is a solid 1" diameter Thomson shaft that is
18" long.

I think you are worrying about the wrong thing. Tailstock pressure assuming
a ball bearing live center should be somewhere in your finely calibrated
this feels reasonable range.

The tool pressure doing some thing to an Thompson rod is going to be your
source of deflection. I hope you have a follow rest.

Wes

Wes wrote:
Louis Ohland wrote:

Before I do something irrational, is there a way to measure the
pressure exerted on a shaft being held between centers? This will allow
me to calculate deflection.

The object to be used is a solid 1" diameter Thomson shaft that is
18" long.

I think you are worrying about the wrong thing. Tailstock pressure assuming
a ball bearing live center should be somewhere in your finely calibrated
this feels reasonable range.

The tool pressure doing some thing to an Thompson rod is going to be your
source of deflection. I hope you have a follow rest.

Wes

I'm not going to machine the Thompson shaft at all. Concept- drop the
shaft between a dead center in the head and a live center on the
tailstock and use it as a test bar. I was wondering about how much
deflection would be caused by the tailstock being tightened. I err on
the side of cross section thickness.

The normal straightness on Thompson shaft is .001 per foot, with .0005
being available. Well within the tolerance of the lathe that I'm using.

On Sep 7, 4:53 pm, Louis Ohland wrote:
...
I'm not going to machine the Thompson shaft at all. Concept- drop the
shaft between a dead center in the head and a live center on the
tailstock and use it as a test bar. I was wondering about how much
deflection would be caused by the tailstock being tightened. I err on
the side of cross section thickness.

The normal straightness on Thompson shaft is .001 per foot, with .0005
being available. Well within the tolerance of the lathe that I'm using.

I suspect the deflection will be far less than you can measure with a
dial indicator unless the tailstock or its spindle shifts under
pressure (my lathe, for example).

Lets see: 0.785in^2 * 29E^6 = 22.7 million Lbs to stretch it 100%,
22,700 Lbs for 0.1%, 227 Lbs should shorten it by 0.001%. [Insert
Poisson's Ratio calculations here] If it expands anything close to
uniformly you shouldn't see the difference.
Bending with centered pressure isn't much of an issue until length/
diameter approaches 50.

It's easy to test, though. Leave the carriage stationary while you
tighten up the handwheel and see if the needle moves.

The bigger problem is drilling centered holes in the rod. That's why
the suggestion is to make the alignment bar, finishing both ends to
final diameter at the tailstock end without touching the crossfeed.

I align the tailstock with a cylinder square that I got cheap at a
machine-shop auction since no one else realized it was a precision
measuring tool and not just some shiny cutoff.

jw

Jim Wilkins wrote:
On Sep 7, 4:53 pm, Louis Ohland wrote:
...
I'm not going to machine the Thompson shaft at all. Concept- drop the
shaft between a dead center in the head and a live center on the
tailstock and use it as a test bar. I was wondering about how much
deflection would be caused by the tailstock being tightened. I err on
the side of cross section thickness.

The normal straightness on Thompson shaft is .001 per foot, with .0005
being available. Well within the tolerance of the lathe that I'm using.

I suspect the deflection will be far less than you can measure with a
dial indicator unless the tailstock or its spindle shifts under
pressure (my lathe, for example).

Lets see: 0.785in^2 * 29E^6 = 22.7 million Lbs to stretch it 100%,
22,700 Lbs for 0.1%, 227 Lbs should shorten it by 0.001%. [Insert
Poisson's Ratio calculations here] If it expands anything close to
uniformly you shouldn't see the difference.
Bending with centered pressure isn't much of an issue until length/
diameter approaches 50.

It's easy to test, though. Leave the carriage stationary while you
tighten up the handwheel and see if the needle moves.

The bigger problem is drilling centered holes in the rod. That's why
the suggestion is to make the alignment bar, finishing both ends to
final diameter at the tailstock end without touching the crossfeed.

I align the tailstock with a cylinder square that I got cheap at a
machine-shop auction since no one else realized it was a precision
measuring tool and not just some shiny cutoff.

jw


I was going to ask the folks at Danaher Motion about doing the centers
at each end. Way more precise than I could do. If they can hold .001 I/m
a happy camper.

http://www.danahermotion.com/product...?parent_id=353

On Sep 7, 7:22 pm, Louis Ohland wrote:
...
I was going to ask the folks at Danaher Motion about doing the centers
at each end. Way more precise than I could do. If they can hold .001 I/m
a happy camper.

If they have good-quality collets 0.001" should be easy. You could
probably center them that well yourself by running the end in a steady
rest and cleaning up a drilled center with a lathe tool. I think the
core of Thompson rod is soft enough.

jw

Jim Wilkins wrote:
On Sep 7, 7:22 pm, Louis Ohland wrote:
...
I was going to ask the folks at Danaher Motion about doing the centers
at each end. Way more precise than I could do. If they can hold .001 I/m
a happy camper.

If they have good-quality collets 0.001" should be easy. You could
probably center them that well yourself by running the end in a steady
rest and cleaning up a drilled center with a lathe tool. I think the
core of Thompson rod is soft enough.

Apologies, my News ISP is bothering me about too many connections again...

Uh, the purpose of this exercise is to get a decent test bar that does
not depend on me or the lathe being in proper alignment...

The Thompson rod I'm looking at has the hard surface, but "soft" center.
Crunchy on the outside, chewy in the middle. I'll ask the engineers what
they'd recommend for what I'm doing (besides more lithium, that is). I
like their concentricity...

Louis Ohland wrote:

I remember a center with some sort of a dial on it. Damned if I can
remember the trade name.

Röhm (or Roehm) makes them. http://www.roehm.biz/

Nick
--
The lowcost-DRO:
http://www.yadro.de