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

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.

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

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

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

.

[… 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.

A photo would help. Best lit with some reflection to give some idea of the degree.

The fact that they match the leadscrew pitch is rather unusual – in my view. It suggests movement on each revolution of the leadscrew. Larger work may provide more load and prevent it from happening. It really and truly isn't unusual for machines to produce different results due to cutting loads.

I think I would be inclined to look at the saddle fit as well. Also check somehow that the leadscrew isn't angled to the bed. It could be running true but if the bearings are miss positioned it wont be true to the bed. A tighter saddle fit might mask that problem. If it's a severe problem with the leadscrew you might be able to detect it by using the handle with the drive to the leadscrew disengaged.

From my memory the spindle pulley set up on a Myford is cast iron. Vibrations there could cause problems but not ones that match the leadscrew. I can't see how they could anyway.

If the work is bending running the same cut again usually shows that is happening but make sure that there is sufficient feed on before it takes the 2nd cut. Saddle fit can make the tool move away from the work when the saddle is wound back.

A travelling steady set perfectly more or less takes the rest of the lathe out of the equation. A principle much used on production capstan and auto lathes. Everything on them can move all over the place on these when the work gets into the cutter box and they still produce perfect results – usually with a rather heavy cut. I was gob smacked when I saw just how bad the rest of the machine could be.

John

–

Three thoughts:

1) What tooling? If T C, could it be that the feed is too low for it? If this is the case, the tool does not seem to cut, and then does, before ceasing for a short time, while the pressure at the edge of the tool builds up again.

2) Presumably the motor is single phase. Not unknown for a single phase motor ro produce banding of this nature.

3) Are the belts in good condition? A belt with a "lump" in it will cause the output speed to vary, and this can cause banding or chatter. Other posters on here have reported solving problems by changing the belts. (Some swear by link belts as a solution).

Howard