Banding on turned work

I am getting banding on turned work that equals the leadscrew pitch on my Myford .Could someone suggest a cause please.

Frank

Gears not meshing correctly causing fluctuations in power to the headstock ?
Drive belts a bit too slack ?
BobH

Check the halfnuts fro cleanliness especially a single bit of swarf embedded in one groove. This will tend to be the dominant point governing the movement making it a single point following the imperfections of the leadscrew thread.
Another possibility is the gear driving it having an odd tooth or out of round. Try a careful note of the position of the pattern relative to a mark on the gear, Try a different gear. This is simplest check.
Final suggestion is influence of bearings and bend in leadscrew.

Edited By Bazyle on 04/01/2017 09:06:48

Frank ... Could you just confirm, please:

are these individual rings, spaced at the leadscrew pitch?

... or is it a continuous helix?

MichaelG.

Banding is usually something related to speed changes while feeding.

Do you power feed or hand feed?

If power feed then check if there is a stiffness at part of the turn of the leadscrew, some teeth are worn on the drive or if the leadscrew is running eccentrically.

If hand feed, then try power feed or identify the angle at which you are changing speed & try to correct.

HTH

Richard.

Adding to Michael's question - what depth of cut when this happens ?

First thing I would wonder about is if it disappears on deeper cuts or moderately faster feed rates.

One obvious thing to check is a warped leadscrew but I'd expect that to only cause problems on shallow cuts with a rather fine feed.

John

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The banding shows on diameters below 1" with cuts of .5mm under power.I bought the large bore at the Myford sale it was made in 2001,I had it serviced by Darron & Pete the ex Myford fitters.They checked the bearings and they were fine.The v pullys on the headstock spindle although tight on the mandril do not run true and could this be the possible cause.The gears are meshing correctly although I have not checked the gearbox gears.

Frank

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Frank,

You have answered John's follow-up question, but not mine.

[sorry to be a bore, but I think it might be relevant]

MichaelG.

Hi Frank

So a newish lathe, probably not abused by your good self. Which begs the question, when was it serviced and why? Did the Myford men notice during the service that the v-pulleys weren't running true, or is that new?

Anyway, you're looking for a source of vibration, and I think you're right to worry about the pulleys particularly if the motor is also misaligned, or has become misaligned over time due to the mismatch. This combination, perhaps coupled with a prematurely worn belt, might account for the symptoms.

As others have suggested there are many other possibilities. Not having access to a local expert, I have to tackle this kind of problem by asking on the forum, and by experimenting. In my case, I eventually found my Chinese lathe stand was wobbling ever so slightly on a not quite flat concrete floor. But I wonder if you have any Super 7 experts nearby who could have a look?

Dave

As Michael points out that could be important.

John

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The banding is not helical and I did mention this problem to the fitters , the belt was replaced but the pulleys were like this when bought .I have a 1957 S7 which I had reground 10 years ago and this lathe is still a joy to use but this one gets on my nerves.

Frank

Oh dear! I not sure you should have said that

Nobody has questioned whether the gibs are loose? This is something liable to changing on any lathe, over a period of use so it probably wouldn't have been spotted by the myford fitters at the time (as they would've been ok at the time). It is also a problem that can directly affect the quality of the work produced.

The force of the hand wheel moving round with the gibs loose can create a slight rocking motion, so that the tool touches back and forth, in and out of cut. On a continuous cut, the deflection created by the cutting can lead to tapered work and poor finish, as the loose gib is pushed back by the cut.

Michael W

Edited By Michael-w on 04/01/2017 12:05:10

I have an example of what vibration can do. This is what happens when my boxford bearings have warmed up.

It's silver steel and no doubt some will think what a crap finish but it isn't. It feels like glass and the shot is larger than life size.

There is a vague repeat pattern. I reckon it's caused by a wobble on my countershaft pulley. It's a pretty big one on an ME10 and it has about 1/16" + wobble. The same cut was run twice to get this but all the 2nd cut did was take a tiny amount of the end and make the pattern a little clearer. The cut was also pretty light. The effect on the end is probably down to the feed rate changing a touch as play is taken up.

What's being seen in the shot is features that probably aren't even 1 um deep. The same sort of thing can happen due to changes in the hardness of the material that is being turned. That usually produces rings that are obvious and spaced further apart and usually down to headstock bearings being past their ultimate best.Heavier cuts or a faster feed usually get rid of those.

The problem really apart from wear is that the tiniest deflection of the work or the tool can show. This is why on some lathes heavier cuts can help. The cutting forces produce enough deflection to make any other causes too small to show so well.

John

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Might just be a trick of the light, but that actually looks a lot better than you might think ​especially for steel. I'm convinced what hes on about is a lot worse than that, which you might call, if not immaculate, a nicely finished product.

But I get the point, vibration might create a very slightly linear patina on what would otherwise be a totally smooth product.

Michael W

Edited By Michael-w on 04/01/2017 12:10:03

Super 7's have been know to produce immaculate finishes Michael. Truly amazing ones. The better it gets the more apparent very minor defects can be.

I've also got a shot of the same piece of silver steel turned without giving the bearings chance to warm up. It just about shows some rings down to the bearings and material / feed variations but most likely material.

The pattern isn't so marked and the light reflections show that the finish is worse. Increasing the feed rate may have given a better finish.

I did this following buying some of those finishing tips I have mentioned. Oil would probably help with the marginal tearing.

Suppose I should mention something about me. I have used machines that can produce perfect results with a sensible tool on them on any material within reason. Trouble is when new they probably costed more than many people houses. Having used them I want my machine at home to do the same. Next escapade for me is hss grinding as a careful look tells me that for some reason they are not as sharp as the ones I ground at work.

And so it goes on. Sort of challenge really.

John

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Edited By Ajohnw on 04/01/2017 12:31:36

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[... and it appears [noticeably] on work smaller than 1" diameter.]

So ... I would look first at the cutting tool [sharpness, angles, height; everything], and then consider using a travelling steady.

The source of the 'periodicity' may be periodic errors in the leadscrew drive, but I suspect that the actual banding is caused by flexing of of the tool or the workpiece.

MichaelG.

I had exactly the same problem on my Super 7.

For all the world it looked as if the leadscrew must have a problem, but the lathe has been in my family from new and never had a crash or anything that could cause this problem.

After much head scratching and adjusting of leadscrew bearings and half nuts etc it was still doing it.

I then grabbed hold of the saddle and found I could rock it slightly (I am talking a few thou at most)

So I cleaned everything and re-adjusted the saddle gibs and the banding problem disappeared instantly.

Phil