Lathe runnout question

Hi,
If you stuck a 1" dia bar in a 3-jaw chuck so it protruded out by say 8" and took a 30 thou cut off of the farmost 6" what would you expect the
turned diameter to vary by ? Lathe is a newish Super7 and the bar was not supported at all at the end furthest from the chuck.
Thanks,
Boo
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10-20 thou larger at unsupported end perhaps?
If you want to test the run-out on the lathe take a 2 thou cut under similar conditions.
Bob
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You should check the runout at the chuck jaws as well.
Jim.
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it will be more if using carbide cutter rather than hss one, but the correct way for checking is to fully chuck, centre drill rechuck then setup your tailstock with a centre and do your test cut, 8'' inch hangout is a big ask of a little lathe for accuracy under the conditions decribed.
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olmod
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its potential accuracy or otherwise. This procedure is very bad engineering practice as I found out many years ago when I did my Toolroom apprenticeship. The bar could quite easily roll over the top of your tool with dire results. I did actually try to cut a fairly long slender job last week on a cnc lathe with massive headstock bearings without tailstock support but the result was predictibly rubbish so I then had to do it the correct way.
Best wishes
Tony Pratt

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That depends on how well you have set the lathe up in the first place.
If you look in the Myford Super7 Owner's Manual that should have come with the lathe, you will find a section on aligning the lathe using an unsupported bar in the three jaw chuck and taking very light cuts at the same cross slide setting close to the chuck and at the end away from the chuck. The lathe feet are adjusted to remove the twist that occurs if the feet are not aligned correctly.
Boo wrote:

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Thanks for the answers, I know it's not a good test but that's the only piece of work I've seen from the lathe. From the responses it looks reasonable anyway.
--
Boo

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