# Determining pressure on workpiece between centers

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

Nick

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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

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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.

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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
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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.

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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

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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...

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Röhm (or Roehm) makes them.

Nick

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