What's causing these bands on a turned rod?

I've been experimenting with improving the finish I get from my WM280 lathe and have noticed a banding effect. The effect appears on fine auto-feed which is 0.07mm per revolution.

The picture is a 1" diameter Aluminium rod turned with carbide taken on the machine from the rear to get the light right. If you look carefully you can see a series of bands. They cannot be detected as ridges with a micrometer or felt with a fingernail. May be cosmetic rather than a full-blown fault I think.

Giving the same rod a very quick polish with very fine emery paper (ST5000) highlights the bands more clearly.

The bands are about 2.6mm broad.

I tried the same experiment on 3/4" diameter leaded mild steel. Although it's much harder to see the bands (and I found them impossible to photograph) they can just be seen if the rod is angled carefully against the light. They are a shade over 2mm wide, which isn't the same period as the 2.6mm bands in Aluminium.

I don't think it's chatter because in both metals the swarf came off cleanly with no untoward noises. Can anyone suggest what might be causing this effect? And do the bands matter, for example if they occurred on a piston?

Thanks,

Dave

Does the pitch match your leadscrew or rack used to move the saddle? A small error in concentricity in a drive gear will cause cyclic variations in feed per rev that can show up like this. Tightening the gibs has been successful for some people who have had this type of banding.

Martin C

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

I've encountered this effect too some time back on my Chipmaster which had two matched driving belts and after removing one belt the problem went away. They were shaking when the machine runs. I assume that there must some uneveness in multi belts that cause a "tuned vibrato" or harmonics that carries this over to the cutting tool. I noticed at the time that the banding varies in broadness and intervals at different speeds and od of the work.

Brilliant! Tightening the gibs fixed it.

Many thanks,

Dave

My theory is that the CAUSE is slight asymmetry in the forces created by the leadscrew due to it be slight eccentric or even slightly bowed, that doesn't stop the CURE being proper adjustment of the gibs so reducing the impact of any variation in the drive.

Neil

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

Yes, I've been thinking about root causes too. Those repetitive bands are a hint that something is vibrating regularly.

A lathe is a rich source of vibrations: motor, fans, belts, gear-train, bearings, lead-screw, moving weights, and perhaps tool chatter. And it will get worse if anything's worn, bent or off balance. (How round are new gear wheels? I've never checked.)

I'm glad tweaking the gibs fixed the problem because it might be very difficult to pin the exact cause down. Interesting to try though. Nice to get a stroboscope on Father's Day: I'm expecting socks...

Dave

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.

Ah, got you. I'll try it tomorrow.

Cheers,

Dave

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

I agree. I've stalled on this one for the moment. I have an oscilloscope that, in theory, can separate out the individual frequencies from a mixture of vibrations. I thought it would be dead easy to stick a microphone on the lathe, press a few buttons on the oscilloscope, and identify the root cause from the period of the strongest vibration.

Silly me. It turns out I don't understand the oscilloscope display even after re-reading the manual.

When it comes to setting up lathes I've not found much advice on minimising vibration. I'm starting to think vibration is the elephant in the room, perhaps causing more trouble than generally realised.

Dave

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.

Your probably right Clive, I can only say what worked for me (but I cant say exactly why it worked). It might also depend on the type/size of lathe on how they react to unwanted vibrations, I do know that hundreds of Boxfords were installed in schools and colleges and all of them were bolted down into the concrete but never bolted to an upright wall.

Perhaps its just a combination of things setting up a certain frequency like a mini "perfect storm", my mate on another forum said he cured this by using rubber feet, as you said above.

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

Ho hum. I slackened off the gibs to test Chris Evan's hand-wheel suggestion (which didn't help in this case) and after retightening, I've got bands back again. Looks like getting gib adjustment spot on might be quite critical.

I took some microphotographs.

To get a sense of scale, the graduations are 0.5mm each. A little hard to count but there are about 14 scratches per millimetre, which agrees with a feed rate of 0.07mm per revolution.

Next photo is a 0.2mm deep cut straight off the lathe, no polishing. The banding isn't obvious.

After a quick polish the cause is more obvious. In the dark bands not all the scratches are equally deep and some are missing entirely. In the light bands, most of the scratches are present and about the same depth.

Next might be a clue. After a light cut I got a saw-tooth pattern. I think this due to run out: me not replacing the work in the chuck exactly as it came out.

Nothing proved yet. I need to take more measurements and compare them with the periods of the gears and lead-screw etc.

I like the rubber pads suggestion. My lathe sits on a pad of roofing felt with concrete underneath. I doubt roofing felt is much cop at absorbing vibration compared with proper feet.

Dave

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

I changed a sheer-pin recently after a screw-cutting 'incident' . I hadn't noticed banding before the crash but then I wasn't looking for it. I wonder...

On the vibration front, I sussed out the oscilloscope problem but now I've lost the microphone. Both issues down to operator error. A piezo-electric sounder isn't a good substitute for a microphone so zero progress.

What was interesting was bouncing torchlight off a saucer of water on the cross-slide. Where the reflection hits the ceiling you see amplified vibrations. I got:

180rpm slight vibration
228rpm no vibration
450rpm slight vibration
630rpm strong vibration!
800rpm moderate vibration
1200rpm slight vibration
1512rpm sllghtly worse vibration
2400rpm slight vibration

There's a resonance problem at 630rpm. I don't think it explains the banding though as that occurred at 1200rpm.

I'd have done more but the electric lights make my warm workshop too hot in this weather.

Dave

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.

I share your pain John!

I'm collecting data in the hope that it will suddenly make sense but I'm not much nearer to the root cause(s). There's been useful progress: the advice about tightening the gibs was good and I think rubber feet will help.

At present I think my 600rpm vibration and the banding are different issues. When I get a chance I'm going to try a cut at maximum vibration and see if I get bands.

Although vibration has caught my interest I'm not too worried about it because the lathe works well enough in practice. It would be nice to see if it can do better though, assuming I can sort it that is.

Dave