Author Topic: When a Rotating Centre isn't a Rotating Centre  (Read 3186 times)

Online awemawson

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When a Rotating Centre isn't a Rotating Centre
« on: October 05, 2013, 05:57:57 AM »
Yesterday the last of the bunch of special tooling for the Traub CNC lathe  arrived. The one I was really looking forward to receiving was swivley rotatey twisty one.
Andrew Mawson
East Sussex

Online awemawson

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Re: When a Rotating Centre isn't a Rotating Centre
« Reply #1 on: October 05, 2013, 06:00:25 AM »
It was in excellent condition and obviously unused, but one of it's clamping pins was missing that locks the rotation angle
Andrew Mawson
East Sussex

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Re: When a Rotating Centre isn't a Rotating Centre
« Reply #2 on: October 05, 2013, 06:07:37 AM »
Never mind, it's a trivial turning milling and tapping exercise. 9 mm pin, two flats to mill and an axial 6 mm hole to tap. Needs to be a bit more than mild steel so I decided to turn down a 16 mm HT bolt as I had a box of them.  I also decided to turn it in the hard and tempered state rather than soften, turn and re-harden and temper, as carbide tooling easily deals with HT.

As it was going to be rather long sticking out of the chuck I used a 'rotating' centre. First time I'd used this particular one, and I chose it for its relatively thin nose section to allow better access at that end of the turn.

Fitted, adjusted the tail stock, checked it was freely rotating and started reducing the bolt from 16 mm to 9 mm . Initially a nice finish but then, hang on, why's the finish so awful?

Turns out that the bearing in the rotating centre has seized and melted the tip off the centre!
Andrew Mawson
East Sussex

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Re: When a Rotating Centre isn't a Rotating Centre
« Reply #3 on: October 05, 2013, 06:14:32 AM »
A few good old Anglo-Saxon words were then to issue forth :lol:

Fortunately the resulting mess in my centre hole was soft enough from the heat to re-drill with a centre drill. And as I had only taken the bolt down to 14 mm when it happened there was plenty of meat left to finish the job using a different rotating centre. I reverted to a 'Matosa' centre that I've had probably for 25 years, but as it's nose isn't as long it meant that the tailstock barrel had to project further making things slightly less rigid.

Still finished the job and it's back together, but I'm rather miffed as that "rotating" centre wasn't cheap. Odd thing is that now the bearing turns freely  :scratch:
Andrew Mawson
East Sussex

Offline greenie

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Re: When a Rotating Centre isn't a Rotating Centre
« Reply #4 on: October 05, 2013, 07:33:03 AM »


Still finished the job and it's back together, but I'm rather miffed as that "rotating" centre wasn't cheap. Odd thing is that now the bearing turns freely  :scratch:

Better knock it apart and replace whatever crook bearings are in there, before your tempted to use it again.

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Re: When a Rotating Centre isn't a Rotating Centre
« Reply #5 on: October 05, 2013, 07:39:49 AM »
My thoughts at the moment are to dismantle it, identify the bearings and buy replacements, then re-assemble with the old bearings and lock them solid with Loctite. Put it on the cylindrical grinder and re-grind the point. Then again dismantle it and put the new bearings in.

Can't think of another way of keeping it reasonably accurate.

Andrew
Andrew Mawson
East Sussex