Lathe Facing Problems

I've developed a problem when facing on my lathe, I do a lot of small dia disks (20-40mm) in 6082 Aluminium and when it comes time to polish
the face I'm noticing concentric circles appearing on the finished surface.
They're not to pronounced, I'm able to polish them out easily enough, but I'm worried it might be indicating a developeing problem.
I've tried cutting at 0.5mm down to 0.1mm with little differance both manually & on powerfeed, I've adjusted to gibs to the cross slide (tried it loose, spot on & too tight), varied the RPM's, differant cutting tools/holders and I can see that the saddle isn't moving from the DRO.
I've put a dial guage against the surface with no discernable movement when rotating !
The circles are easy to see but impossibly difficult to photo, you have to catch the light just right but they appear to be 2-3mm apart. I cannot tell if they are truly concentric or spiralling.
I'm thinking 2 things . . . either play in the head bearings or the crossfeed leadscrew. Please don't let it be the head bearings but I can't see how the leadscrew can affect the finish if the gibs are correctly set????
Any ideas?
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HeffaLump wrote:

I had a similar problem on a Boxford, it eventually went away when I replaced the motor so it may be worth looking at the motor, motor bearings, V belt (if appropriate), etc
Dave
Dave
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HeffaLump wrote:

I have a similar problem on my Boxford when turning diameters in aluminium to a good finish. The problem is a visual "banding" like you describe at around the same 3mm scale. There is no measurable difference in diameters at the bands and there is no feeling of roughness to the turned surface. I once saw a reference somewhere (most likely ME or else one of the GH Thomas books) to something called "leadscrew shadow" where the author ascribed the problem to minute irregularities in the relevant feedscrew.
It would be interesting to measure the repetition "pitch" of the pattern and see if it relates to the pitch of the leadscrew you are using (maybe modified by the gearing in use) ?
Unfortunately, I don't remember the upshot but it does seem to suggest that changing the feed rate may help, or alternately using a metric/imperial cross changewheel in the rive to the leadscrew so as to avoid having the spindle rotate in a fixed simple ratio with the leadscrew (as far as possible I mean).
Hth,
--
Boo

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On or around Tue, 13 May 2008 12:33:44 +0100, HeffaLump <"Percussion Engineering"> enlightened us thusly:>I've developed a problem when facing on my lathe, I do a lot of small

if the feed is non-linear it could do. I'd pull the saddle and examine the feed, clean it all and adjust if it has any, and put it all back together, see if it affects anything.
what machine is it?
I've noticed when hand-feeding you tend to leave marks on things if you momentarily stop the feed.
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If they are equally spaced then play in the cross slide could be the problem, as the handwheel is turned you slightly lift then press but if you are getting it under power then not so likely.
Is the saddle locked? could be play there
Are you using any cutting fluid, could be metal build up on the tool tip that builds upto a certain level then comes off before starting again.
JAson
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Try facing from inside to outside if that's possible ...this will drag your tool ... and not push it .
you may have to set it at a different angle or grind it differently or use a completly different tool to achieve this
and use wd 40 as lube.
All the best..markj
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On May 13, 12:33pm, HeffaLump <"Percussion Engineering"> wrote:

Your rings could be caused by an imperfection in the lead screw bearings or drive, which causes a momentary change of feed speed. The outcome of which is a slight polishing at the point of the stiffness. To cure it you could try taking, having already locked the saddle firmly, a second or third cut at the same setting. Doing this will tend to polish the whole surface. You could then "texturize" the surface with one of the "Scotchbrite" type abrasives. Another thought occurs, you are not using a carbide tool, are you? Aluminium responds well to a VERY sharp HSS or even silver steel tool with WD40 or parafin as a lube. T.W.
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HeffaLump wrote:

I don't know how to adjust gibs perfectly - and I don't know anyone who does. They can be adjusted as best as they can be, but the cross slide will still wobble a bit if it experiences a twisting force. Always. That may well include the change in sideways force when/if you are turning the knob by hand.
The best solution is to not let the cross slide gibs experience any twisting forces - ie, if you can, mount things so the cutting point of the tool is vertically somewhere between the two horizontal cross-slide bearing surfaces.
Milling machines mostly work like that.
This may be impractical when eg boring - best advice I know of then is to mount the boring bar a bit high, so it cuts shallower if it sticks, and do an extra thin cut at the same slide position after the major cut if you are going to take a measurement afterwards - the relative measurement of the second cut will be more reliable in a twin-cut scenario than the measurement of the first cut. Tiresome, but it may be necessary.
And always lock any slides you aren't going to deliberately move.
-- Peter Fairbrother
(doing horrid things to/and with a tiny C3 lathe)
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On or around Mon, 19 May 2008 22:24:19 +0100, Peter Fairbrother

and on most machines, it'll be impossible to adjust them perfectly anyway due to uneven wear. new stuff excepted, natch.

that's a good point. when parting, especially, I try to have compound slide wound in near the middle.

IME, boring bars are by nature flexible and it takes several cuts to actually get the dimension you set it for.

now that's the one I can never be bothered with. Should lock the compound, really, for some things especially.
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This is OK(ish) if you don't need a very precise length. If you do, and don't have DRO fitted, the markings on the carriage handwheel are nowhere near precise enough though. With a leadscrew handwheel I can get a length to within a fraction of a thou.
If I get around to buying a larger lathe, as I plan to do, then fitting a leadscrew handwheel will be one of the first things I do, while cursing the makers for not bothering.
David
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