Lathe chatter/pattern

[J Tiers]

Lol. It's brought up every time there is a thread about chatter. Yet somehow folks think that a 20x80 lathe with more drivetrain mass than their entire lathe, has chatter induced by minute motor speed fluctuations. Cracks me up every time.

............

You are easily amused.......

It has NOTHING TO DO WITH "MINUTE SPEED VARIATIONS". In the cases where it occurs, it has to do rather with the variation of applied TORQUE, ensuing vibrations, and resonances in the "system" of machine and workpiece.

There may be more drivetrain mass, but there is also a proportionately larger motor with more torque to drive that drivetrain. So the effect of the mass would need to be compared to the driving force in order to have any meaning at all.

What you are suggesting is essentially similar to saying that a small bell can of course be caused to "ring" (resonate) by hitting it, but that somehow, a LARGE bell will not do that. That is of course not true.

The difference with a large and heavy "bell" (lathe) is that the resonance may not occur in the frequency range that is present. And, that is a lot of the issue with "weight of the lathe". It tends to move the resonance out of the range.

You could equally well make fun of the whole thing by saying "What? You think that a tiny half lb RUBBER belt is going to make the slightest difference to this 4000 lb machine? LOL, you are cracking me up!".

But then we have the OP, who is finding a significant difference by removing one of the belts.

It is a mistake to assume that some factor is "insignificant and can be ignored".

When I've experienced chatter ( I'm tempted to say every single time, but maybe I'm forgetting something ) I've managed to eliminate it without changing from a single phase to a three phase motor.

As for the implication that there is no effect of single phase vs 3 phase, that suggestion has been made, but does not stand up. In my own case, changing the motor and drive (a smaller motor and also a smaller machine) made a large difference.

The NATURE of the difference is significant..... with single phase, the chatter would not go away, it might only be changed a bit, but all the textbook "solutions" made no overall change. With 3 phase, there might still be chatter sometimes, but changing the speed, etc would make it go away.

That strongly suggests that with single phase there was a constant "driving frequency". Obviously then, changing the speed etc , which is intended to change that frequency, would make no difference, because the problem exists elsewhere.

The type of motor also makes a difference. There are "split phase", "capacitor start", "capacitor start with run capacitor", and PSC motors. The first two are pure single phase. The last two are essentially "two phase" motors, which behave similarly to three phase.

The run capacitor adds a second "phase" that changes the pulsing torque of single phase which goes to zero twice per cycle. With even an imperfect 2 phase, the torque minimum is considerably larger than "zero", and the "pulsing' is reduced. That makes the result much more similar to three phase.

So any single phase motor that has a "run capacitor", no matter what type of start it has, will act more like a 3 phase motor. If your machine has one of those motors, then even though it is single phase, it will NOT SHOW the same characteristics.

A machine with a particularly massive drive train for its motor size may also have a similar effect, but the "fix" is less if the motor is also larger to match.

[mattthemuppet]

no useful comments on chatter, but is there any reason why you aren't working with that piece the other way round in the chuck? Looks like you should be able to hold it with the outside jaws and still clear that little stub on the other side. Would be considerably more secure turning operation - you won't have a big offset weight hanging off a small shaft and you won't need a tool hanging out in the breeze.

[Captain K]

Yes there is reason I didn’t hold it the other way. The shaft I’m trying to repair is offset from center. This is a crank mechanism. The offset is too large to take up with 4 jaw. Even if I could offset it enough it would be an absolute nightmare to center as it would probably rotate a little bit every time I moved a jaw.

edit: The piece that is available for gripping with the outside jaws is not machined, would be difficult to get it to run true. Having said that, I would probably try it that way if this job comes up again

[mattthemuppet]

I can see that it would be a pain, but it doesn't look impossible - grip the large disk with the outermost step of the outside jaws, indicate off the shaft you're trying to machine (that's what you indicated off before, right?), tap the disk into the jaws to get it to run true. I'd bet a dollar there's a 60deg center in the end of the shaft you're trying to machine, so you could even use the tailstock for set up and maybe even support if neccessary.

It might not eliminate the chatter (though it'll probably help) but it will rule out the pendulum effect of that large offset weight and the flex of your extended toolholder as possible causes.

[Captain K]

There’s a bolt hole without 60 degree cone in the end. There is no way eliminate the pendulum effect since the shaft I’m working on is offset from the center of the weight. You owe me a dollar 😃

[mattthemuppet]

doh, I'll have to check my wallet Pendulum effect = large offset weight at the end of the small shaft being supported. If you hold the crank by the large diameter the only unsupported part of the work will be the shaft that you're cutting, not the shaft+large offset weight. You'll still have the off-center mass either way, but at least it won't be trying to flex the bit of the work that you're cutting.

[Captain K]

I misunderstood the pendulum effect. If this job comes up again I will try it your way.
Thanks

[The Artful Bodger]

I do believe the work holding requires some careful thought and I suspect the answer may include a face plate!

On the question of single vs multi phase power I would say to use 3 phase power and motors if you have such things and even if you dont spending the money may or may not make a difference but at least you would feel better which is not a bad thing. In my case we have never been bothered with unicorns trampling the vegetable garden but just to be sure we always hang a bunch of onions at the back door.

[mattthemuppet]

I misunderstood the pendulum effect. If this job comes up again I will try it your way.
Thanks

you're welcome! No guarantee it'll work any better, but it's worth a shot.

[The Metal Butcher]

You are easily amused.......

It has NOTHING TO DO WITH "MINUTE SPEED VARIATIONS". In the cases where it occurs, it has to do rather with the variation of applied TORQUE, ensuing vibrations, and resonances in the "system" of machine and workpiece.

There may be more drivetrain mass, but there is also a proportionately larger motor with more torque to drive that drivetrain. So the effect of the mass would need to be compared to the driving force in order to have any meaning at all.

What you are suggesting is essentially similar to saying that a small bell can of course be caused to "ring" (resonate) by hitting it, but that somehow, a LARGE bell will not do that. That is of course not true.

The difference with a large and heavy "bell" (lathe) is that the resonance may not occur in the frequency range that is present. And, that is a lot of the issue with "weight of the lathe". It tends to move the resonance out of the range.

You could equally well make fun of the whole thing by saying "What? You think that a tiny half lb RUBBER belt is going to make the slightest difference to this 4000 lb machine? LOL, you are cracking me up!".

But then we have the OP, who is finding a significant difference by removing one of the belts.

It is a mistake to assume that some factor is "insignificant and can be ignored".

Okaaaaay.

I'm sticking to my guns. (And yes, I suppose I am easily amused. Good for me, eh?)

Were this a direct drive spindle like your lathe, you might be onto something. But suggesting that the minute vibration caused by torque variance and propagated through at least two more shafts--is causing that horribly finish--is nonesense. To me.

So why does it chatter then?

Well I'd say that this is pretty telling.



That lathe should be able to take off a half inch if he wanted to. A couple of mismatched belts isn't going to cut that down to 0.010, nor leave a mangly finish like that on their own. I've ran a lathe with crap belts before, heavily set, flapping about, mismatched, and it did nothing of the sort. However, a pendulum on the end of a small shaft running at a relatively fast speed (150-220 RPM) seems like a pretty good culprit to me. Couple that with 8" of stickout, and I think you've got your answer. I'll wager if he turns it around as Matt suggests, he can take off as much as he'd like, one belt, two belts, red belts, blue belts.

[J Tiers]

Direct drive?

Nope.... it has TWO belts in between the motor and spindle, and one of them is a FLAT BELT from countershaft to the spindle. And I STILL noticed a VERY obvious difference, in fact it was like night and day.

You may have been misled because I mentioned that it is direct belt drive to the spindle, no gears involved unless I use back gear. I think that came up in a turret lathe thread. Many lathes have belt to an input shaft, and then a gearbox. I'd like that, but no such luck with either the Logan or the Rivett.

Change around the disk?

You forgot that the OP here CANNOT put that large disk in the chuck, because the offset is too much to get the crank pin on-center. He could maybe do it with a faceplate if he has one. (Actually, I think he could do it if he turns the jaws around, but we have to take his word for it, he can see all the parts right there, and we cannot.)

In any case, use your mechanical engineering vision and see what the input frequency of that off-center is. Right, it's once per rev..... Now, what is the frequency of the chatter pattern? Well we do not know for sure, but it looks like somewhere in the area of 50 to 70 per rev, maybe more. It does not look like the pattern is from the off-center mass.

It COULD be from the stickout of the tool, which is why I mentioned that. Clearly something is vibrating at that rate, and there is some energy-inputting feature that is in synch with it. That is a pretty aggressive looking chatter pattern, although the camera may be misleading.

He says he took off one belt, and it improved the chatter.

So, what did he actually do by removing the belt?

Well, obviously, he may have removed a defective belt that is causing a problem. I am not sure just what it could do, how it could be that sort of "defective", but there it is. The belt was removed, and the problem got a lot less observable.

Also, he reduced the coupling between motor and drive. One belt obviously has half the capability of two belts, it stretches more, has half the friction area, etc. If the MOTOR were actually the source of the problem, we'd clearly expect reducing the coupling to tend to damp out vibrations, or not transmit them as much. So according to your own theory of "the belts absorb all of that", ONE belt is less well-coupled than two, and probably transmits less of the torque pulsing than two belts would.

So the motor torque pulsing theory is not dead yet......

[Captain K]

I you go back to my original post you will see that this was on ongoing problem unrelated to extreme stick out. Have got same pattern on small and large workpieces, turning and boring. I have a new matched set of belts coming, will report back when I get them. In the meantime here is a picture of the finished project.