My Myford S7 was doing just the same and I was blaming the leadscrew or nut or both for a while.

In the end it turned out (no pun intended) to be the saddle gibs that needed adjusting, once this was done the bands disappeared completely.

Phil

No cure for correctly adjusted Gibs but, "dragging your hand" on the crank handle wheel sometimes eliminates the effect.

Think about it, it may just be the weight of the carriage hand wheel rising and falling. Hence the comment to drag your hand on the wheel.

A few years back the topslide on my Boxford started doing this, as OP said, nothing could be detected on the work apart from the "shading" that went along like a thread. It directly corresponded to revolutions of the screw.
I went through everything but couldn't find the cause. The only thing that had changed was I had moved the lathe a couple of inches so I could add a bench to one side. Its not bolted into the floor its standing on adjustable feet, so I checked and reset them and re-leveled the bed, the problem went away.
I agree with Mikelkie, I think its a resonance/vibration thing, I also think it can have a number of causes which can be transferred to the job through a loose gib etc. but I think there will be a root cause like a  vibrating belt or some numpty forgetting to tighten down the feet

Edited By John Horne 2 on 19/06/2017 09:39:49

Edited By John Horne 2 on 19/06/2017 09:42:47

The inherent vibrations in the lathe from motor and bearings and belts and any shafting will combine in some way, also if the lathe is on a concrete floor it will pick up any sympathetic vibrations, perhaps from other machines nearby.

The answer is to isolate the machine on the rubber pad type mounts which are quite cheap and easy to fit. This also means to eliminate any obvious play in the movements and belt alignments etc.One thing to listen for is noise as it is then obvious that it is originating from some component or set up.

Levelling the machine very accurately is not a requirement at this stage, A 20" bubble is more than enough and you need not go to great lengths to try to overtighten nuts and bolts to get extreme accuracy. Castings in a relaxed state are OK.

Have been spared these problems, (SO FAR) but have heard of a poor quality belt with a prominent join causing something similar. (Instantaneous variations in speed) In that instance, the cure was to fit a new link type belt.

Equally, as already said, it could be regular change of feed rate caused by one or more eccentric gears.

Single phase motors have been blamed for similar problems, although in this instance the motor is either three phase, or brushless, and so should be not guilty.

Howard

I was getting that banding at all diameters, materials and speeds on my WM250V, so although I considered resonance, I didn't regard it as a very probable cause.

It was worse on tapers cut on the compound slide – I was doing quite a lot of those at the time.

So far as I can tell, it went away when I spent a bit of time getting gib adjustment on cross- and compound slide as perfect as I could.

I'd also had an incident early in the machine's life where I engaged longitudinal instead of cross feed when milling, and this had culminated in a bent saddle pinion, giving tight spots all up and down the bed about every three turns of the saddle handwheel. A dropped-out crosslide shearpin forced me to remove the apron, which gave me the chance carefully to bend the pinion straight again – with a crowbar(!!) which the previous owners of our house had thoughtfully left. Note the unsplit infinitive :D.

Since then it's produced a very nice silky finish – when I get everything else right.

But if anyone's got any advice on how to adjust the saddle gibs on this machine, I'd appreciate it for the future – it gets a medium-hard life for a hobby lathe.

Edited By Mick Burmeister 1 on 19/06/2017 19:26:29

Curiosity got the better of me so I rigged up my saucerescope and went through the speeds, the first trials went well, no vibration whatsoever, which meant either my lathe is perfect, or, I'm a numpty and did it wrong.

I decided to go with numpty

My second trial kinda worked, nothing much at high or low speeds, worst at around 800rpm.

Next I watched the belt at different speeds…….. most vibration and noise from the belt at 800rpm

I use an inverter so I can watch the frequency, set it up on a low speed using the pulleys…..worst at around 35Hertz

changed pulleys to higher speed….. same result…..worst at around 35Hertz

I honestly dont know how to interpret this, it could be a belt or motor problem, or it might be perfectly normal.

Interesting that the vibration appears to be at 35Hz, which the O P says coincides with his feed of 0.17mm/rev.

So could this be a symptom of the feed rate varying, instantaneously?

Too little , (or may be too much) backlash in the gears?. I set my backlash by running a piece of paper about 0.003 inch (say 0.1mm) through the mesh, before locking all the clamps on the quadrant and gears.

Just a random thought or two.

Howard

Dave, what type of 'scope do you have? Is it a 'modern' digital type? If so, it will most probably have some Maths functions, one being the ability to display the Fast Fourier Transforms of observed signal inputs. That will show clearly the amplitude and frequency of all signals seen by the microphone. You can then place the microphone on the toolpost and do some cutting and see exactly what the vibration frequencies and amplitudes are.

Signals displayed thus , in the frequency domain, show as a series of frequency 'bins' , starting from left, lowest frequency, going to the right, the highest frequency. The display would then draw a line showing the relative amplitude at each of the frequency bins. You would then easily see at which frequency the highest level occurs.

Looking at the signals in the time domain only, ie, such as a pure sine wave on the scope display will only show a mishmash of frequencies all mingled together at various amplitudes – the brain cannot FFT that! Impossible to single out a single culprit frequency, unless it is huge and dominant.

If the scope is not a digital type…there is an almost easier way!

Use the sound card input to your laptop, and feed the microphone signal in there. Download one of the many free Audio 'Scope/Spectrum analyser tools from the SphincterNet and you can easily see the FFT and anything else you want. I use that a lot for vibration analysis and it works very well. Google 'soundcard Spectrum Analyzer' or such like…

Regards

Joe