Lathe Facing Problems

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HeffaLump - 13 May 2008 12:33 GMT

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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NoSpam - 13 May 2008 12:44 GMT

> 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
[quoted text clipped - 21 lines]
>
> Any ideas?

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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Boo - 13 May 2008 13:05 GMT

> 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
[quoted text clipped - 19 lines]
> can't see how the leadscrew can affect the finish if the gibs are
> correctly set????

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,

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Boo

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Austin Shackles - 13 May 2008 13:28 GMT

>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
[quoted text clipped - 19 lines]
>can't see how the leadscrew can affect the finish if the gibs are
>correctly set????

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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jasonballamy - 13 May 2008 13:32 GMT

If they are equally spaced then play in the cross slide could be th
problem, as the handwheel is turned you slightly lift then press but i
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 too
tip that builds upto a certain level then comes off before startin
again.

JAso

--
jasonballam

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mark - 13 May 2008 15:08 GMT

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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the wizard - 13 May 2008 18:08 GMT

> 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
[quoted text clipped - 21 lines]
>
> Any ideas?

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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Peter Fairbrother - 19 May 2008 22:24 GMT

> 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
[quoted text clipped - 7 lines]
> 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 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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Austin Shackles - 20 May 2008 20:04 GMT

>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.

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

>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.

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

>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,
[quoted text clipped - 3 lines]
>scenario than the measurement of the first cut. Tiresome, but it may be
>necessary.

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

>And always lock any slides you aren't going to deliberately move.

now that's the one I can never be bothered with. Should lock the compound,
really, for some things especially.

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David Littlewood - 25 May 2008 12:20 GMT

>>And always lock any slides you aren't going to deliberately move.
>
>now that's the one I can never be bothered with. Should lock the compound,
>really, for some things especially.

I used to be the same, but a year or two back I carried out the GHT mod
to my topslide (pinning the gib strip and adding a dinky little locking
handle) and now it takes so little time to lock the slide (a second or
two) that I now do it any time I'm not actually turning with the
topslide.

This, BTW, is almost always; I hate trying to do parallel turning with
the topslide. Why do nearly all lathes not have a leadscrew handwheel?
The time it takes to set the topslide to turn parallel must add up to
quite a large part of your workshop time, if that's all you have.

David

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David Littlewood

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Austin Shackles - 25 May 2008 16:53 GMT

>>>And always lock any slides you aren't going to deliberately move.
>>
[quoted text clipped - 11 lines]
>The time it takes to set the topslide to turn parallel must add up to
>quite a large part of your workshop time, if that's all you have.

You can hand-feed the student, but it takes a careful touch on the wheel.
For anything over a few mm I tend to engage the power-feed. For little
fiddly jobs I use the top/compound slide.

'course it helps having a decent power feed, which not all lathes have.

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David Littlewood - 25 May 2008 18:58 GMT

>>This, BTW, is almost always; I hate trying to do parallel turning with
>>the topslide. Why do nearly all lathes not have a leadscrew handwheel?
[quoted text clipped - 6 lines]
>
>'course it helps having a decent power feed, which not all lathes have.

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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David Littlewood

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Model Forum / General / Models / May 2008

Lathe Facing Problems