Expanding Mandrel

I machine my spool ends (vs. spinning or forming) and it is something of a problem to hold them so that all surfaces can be cut.
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On seven reels to date, I have clamped the 0.75 diameter back side hub with a 3 jaw chuck. If I ream the center hole and trim the O.D. with the same chucking, I get the concentricity that I need. But the hold of the 3 jaw chuck is somewhat precarious; I once had a part come out of the chuck while turning. So I have to get along by taking light cuts, and more of them than I would like.

So I have now made an expanding mandrel to clamp on the bore.
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The steel plug has a tapered flange that expands the mandrel fingers to clamp on the bore of a spool plate. The taper has a slope of 25 degrees (i..e., 50 degrees of included cone angle) so that there will be no problem with unlocking the taper after machining.

Sherline lathes do not have a compound feed (except as an accessory), but tapers can be cut by rotating the headstock.
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I used a slitting saw to slot the mandrel and create the flexible fingers.
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Here is the mandrel mounted on the lathe by a 4 jaw chuck. Behind the slots is a turned surface that allows me to run an indicator when centering the mandrel in the chuck.
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The grip of the mandrel seems adequate for milling a spool end blank…
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… and for turning at the O.D.
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There was no problem with unlocking the taper.

But this mandrel has a problem, it grips the bore whether or not the part is square with the bore axis. On repeated trials of mounting the blank, I typically got 0.010 axial TIR near the O.D.

The mandrel has a good grip and would be a good way to hold parts of smaller diameter. But it is not going to work for spool end blanks of 3 inch diameter. It probably would have worked much better if made from 1 inch rod (vs. 0.5 inch). There would then be a better shoulder to make the blank sit square.
I am not going to immediately pursue a revised mandrel; I have another scheme for spool ends involving a faceplate.

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