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krems
12-14-2012, 01:57 AM
Is there anything wrong with using a live center in the tailstock to aid in support of a workpiece when chucked up in a three jaw or collet chuck or are live centers only to be used when turning between centers. I was thinking the out of center/round stock would be exerting a lot of pressure against the live center tip. Something is going to flex either the material or something on the tailstock/ live center. I ask this because when I put an indicator on the live center tip there was a lot of movement when the part was clamped down in a three jaw. The movement was in the cnc tip on one of the expensive royal live centers. (brand new). The movement was not on the tailstock ram or large diameter of the center. I'm getting close to .0008" TIR on the live center tip which is a lot considering they are claiming .00005". After calling Royal on this they suggested running the live center for an hour or so with a good axial load on the live center to get it to seat properly. No luck same result. I sent the live center back to Royal today for them to look at. But then I started thinking that maybe I was inducing a lot of stress on the live center by using the there jaw on the other end instead of another center. Any thoughts

Krems

Krems

Fasttrack
12-14-2012, 02:22 AM
But then I started thinking that maybe I was inducing a lot of stress on the live center by using the there jaw on the other end instead of another center. Any thoughts


I often use a dead center in the manner you describe but haven't tried it with a high end live center. I've got two and one is still in its box. However, my initial thought is that the radial load generated by holding one end in a chuck isn't likely to exceed the radial load generated when turning a work piece, regardless of how it's driven. Sounds like that live center might have been bad.

The only real problem I've run into with this "method" is the tendency for the part to "spring" after you remove it from the lathe. Basically, you are forcing the tailstock end to run concentrically by flexing the work piece slightly. You end up turning a bit of an hour glass figure. It's usually off set by the flex caused by the cutting tool, so the hourglass figure isn't too pronounced. In my experience, this effect has introduced an inaccuracy comparable to using a three-jaw chuck in the first place, i.e. I can live with it and, if I can't live with it, I do the work in a 4-jaw or between centers (actually haven't tried between centers on my big Pacemakers ... maybe it's time to try?)

danlb
12-14-2012, 02:36 AM
Was this still the case when you center drilled the piece after putting it in the 3 jaw chuck? Once you do that the center should be concentric with the axis of rotation. If you re-chuck it the accuracy is lost.


Dan

Toolguy
12-14-2012, 10:37 AM
You can eliminate any side load the part might be putting on the center by chucking the part at the tip of the jaws, maybe 1/4" to 1/2" into the chuck.

krems
12-14-2012, 10:38 AM
Dan,
I center drilled w/ the piece chucked 1/2" from the chuck face. Then the 10" piece was removed and chucked w/ the 3 jaw and live center. All Accuracy was lost when removed. I never have used a dead center in the tailstock. Does the part / dead center need a lubricant of some kind

Items

grumpygator
12-14-2012, 12:09 PM
Yes it does and a lite touch on the tailsock.Think run it in and then back off a rch.
****Just Saying *****************Gator*******************

michigan doug
12-14-2012, 04:52 PM
Any grease will work for a dead center. In the old days, they had a grease with lead in it which was great for this high pressure application, but you couldn't eat it. All gone and against the law now I suppose.

A dead center will give you less flex and more accuracy in many situations compared to a live center. Cheaper too. It will also "grow" as it heats up, so you have to keep an eye on proper pressure from the tailstock so it doesn't heat up too much.

doug