Turning Question: Fine Chatter / Wave Pattern

[DiogenesParticipant @diogenes]

Banding can be caused by oscillation of the work as well as the ‘tracking’ of the tool point, maybe a consideration in this case where you were using a somewhat elderly (did you say?) rotating centre for offset turning – the point might turn once a rev, but the balls will be cycling the track at a much slower rate..

..this is an ‘ask how I know..’ comment..

 

[Nigel Graham 2Participant @nigelgraham2]

Thankyou Diogenes.

A good point, given the unknown age and provenance of all this equipment. I will follow that by examining the device. I don’t know how or if I can dismantle it, though

 

I spent a peaceful couple of Christmas morning hours giving the lathe’s lead-screw, feed-shaft and rack-&-pinion a good cleaning and oiling. Whether it would have made any difference to work quality I cannot say, but it would have done more good than harm.

To clean them I drenched them in WD-40 (whose MSDS tells it is really only thin oil in white spirit) then used a worn-out paintbrush and tissue-paper (not cloth!) wipes to chase the muck out.

Wipe down, then squirt oil over the pinion, along the screw and down all the oil-holes I could find.

Finally, I verified the headstock still holds enough oil for the largest internal gear to pick up and fling all over the works.

[Nigel McBurney 1Participant @nigelmcburney1]

I think your problem is the use of a 6 mm dia cutter which is in effect a form tool,and its being used on a small diameter workpiece and using the least rigid on any set up on a lathe,ie set over tailstock with work between centres, dsecribed in many engineering text books but I do wonder if the writers of these books have actually carried out this method,and it stems back to victorian times when the faceplate and between centres working were the principle workholding methods and using carbon steel cutting tools, Now on your lathe would you consider using a parting tool 6mm wide,no you would you would use a 3mm tool.you are expecting too much from the circular insert.use a hss tool bit with a rad of around 1 mm . I would put the job in a three jaw supported by tail stock centre,then turn the taper with the top slide ok the travel is limited so do it in two bites and carefully blend in the two cuts.  I had a job come in to make a Crossley oil engine con rod from solid about 30 inches overall,using 51/2 inch bar ,the customer insisted on it being from solid rather than two ends welded on, so i had to do it between centres with tail stock set over,to start each end was machined first ,ncluding the bullk of the large blend radii the centre section turned down parallel to nearly the largest dia the the tapered part the holding on the parallel centre sectionof the centre machined off the ends to give the flat areas,then i mounted the rod between centres ,tail stock set over, and roughed out the taper with carbide tooling. Finished off with  HSS tool with minimum rad,even with a large job in a 71/2 inch Colchester it is very easy to get tool chatter so feeds and speeds are kept low.In over 60 odd years of engineering I have found the majority of vibration and chatter marks and simlar finish problems are due to too high speed,too high feed, and expecting too much from tools with large rads or other forms, its better to get forms on large poorly supported rads by twidling the handles on cross and sliding feeds. By the way the clicking that was mentioned in a geared headstock can be due to the tooth on a gear or pair of gears (Hardened & ground)being slightly distorted due to a crash caused by an accident with a workpiece coming into violent contact with the bed or saddle of the lathe, and its nigh impossible to locate the distorted tooth as the damage does not show,the distortion can be minimal but it will click .

[Nigel Graham 2Participant @nigelgraham2]

Bit of an IT hiatus but managed to sort that with advice from the Forum management.

Sometimes I think using a computer is analogous to needing know how the lathe was designed when I only want to face the end of a bar.

Anyway, I’ve not actually measured the marking and rack but they do look suspiciously similar. I can’t detect any unwanted play anywhere, but neither have I discovered any adjustment. It looks as if the saddle is guided simply by a single inverted-Vee, own weight and a restraining bar under the rear bed overhang.

I’ll try consulting the manual again to see if I have missed something.

[Simon CollierParticipant @simoncollier74340]

I’ve started getting banding on short bar and sharp HSS tangential cutter when using power feed. It’s a 6” geared head lathe with separate feed shaft. It’s does not happen if I manually feed. I’m hoping the lathe experts can suggest a cause. It has only recently started happening.

[Fowlers FuryParticipant @fowlersfury]

No doubt a worthless response to Simon C ~ since you have a “geared head” lathe and furthermore perhaps it’s unlikely to be powered from a 240v 1ph motor.
Nevertheless, for what it’s worth, I had the banding effect irrespective of tightening slides etc etc….on my Super 7 with 240v 1ph motor whenever non-manual feed was employed.
The infuriating banding effect appeared reduced but definitely not eliminated by replacing the V belt with a poly-link belt. (Maybe because, as others claimed, the joint in the V belt, as it ran over the pulleys, caused a specific pattern depending on mandrel speed c/f lead screw feed rate?).
The banding problem disappeared completely when funds allowed the purchase of 3ph motor and Jaguar CUB inverter.

 

 

 

[parovozParticipant @parovoz]

Hi !

 

My £0.02….

Have had a read through here and a lot of knowledge and good ideas….

Banding and chatter obviously two different phenomena. Chatter is a resonance effect and in general increasing rigidity / support or slowing the surface speed or reducing the width of the cut, lubrication etc. can all improve things.

Banding is a different thing.

It’s most annoying and is present from totally invisible to “looks like a rat tail file” in every cut we make. The key cause is instability in the cut at the interface between the tool and the work. That instability ( it’s a LONG time since I studied the mechanics / physics of this ) is a factor of the cutting tool angles, tool material, work material, and lubrication. All of these have to be correct for a good cut. In addition as mentioned above, wear on a machine can also add to this by varying the ‘pressure’ of the tool onto the work causing a further avenue for instability of the cut. depending on the way in which the tool is cutting the degree of built up edge will vary and the degree of lubrication in how that BUE breaks off the tool is also slightly random in nature.

Ways to combat it.

Sharp tool at the correct angles for the material being cut.

Correct surface speed and depth of cut for the material.

Correct lubricant.

Taking up slack in the machine that may be allowing tool deflection.

Even choice of material. Some grades of steel are more challenging to machine if the set up is sub optimal, whereas some are easy in all conditions. Easiest being a free machining grade with a slight lead content i.e. 12L13 or 12L14.

We can also resort to increasing the radius of the tool, which puts the pressure over a wider area ( take care that it is not so much as to cause chatter ) and reduces the tool’s tendency to ‘dig’ more easily.

Carbide tools with chip breakers change that dynamic again.

And finally if power and rigidity are good, there are negative rake tools where the cut is not actually a cutting action but by plastic flow. This is often used in high volume manufacture and produces a very polished surface with an appearance of fine even rings. I only ever got into this zone once when a lump of EN36, impatience and a Colchester Triumph 2000 all came together. ‘Blue Chip’ manufacturing….

One can go down rabbit holes on this… 🙂

https://www.sciencedirect.com/science/article/abs/pii/S0020740319319472

https://iopscience.iop.org/article/10.1088/1757-899X/63/1/012016/pdf

All the best.

 

 

[SillyOldDufferModerator @sillyoldduffer]

What causes vibration can be hard to pin down, and it may be due to a combination.   On a lathe the list would include  periodic noise from the motor, belts, bearings, gears plus vibration due to a projecting tool mounted on a less than rigid tool post, maybe on a saddle with loose gibs that’s being traversed via a worn half-nut by a bent lead screw or a wonky rack.

Lathes have resonances despite being rigidly made from a material than absorbs vibration (cast-iron), .   Banding could be as simple as an unfortunate combination of cutter, RPM, depth of cut and material.   My lathe sometimes creates faint bands, mostly in EN3 and soft Aluminium, both sticky alloys.  I think the stickiness vibrates the cutter and the job, like a bow scraping violin strings.  The cure is to alter RPM up or down slightly,  easy with a VFD.   I’m fairly confident the cause is resonance because the bands don’t relate to the rack, fine-feed gearing or anything else mechanical.

One of my failed projects attempted to identify vibrations by attaching a microphone to the machine and logging the audio file.  Then the file was analysed on a computer by applying a Fast Fourier Transform, a mathematical process than converts signals in the time domain (mixed up audio), into the frequency  domain, that is the individual frequencies of each source.  Knowing the frequency of a vibration makes it easier to identify the cause.    Failed as I recall because I couldn’t get meaningful data out of the FFT, with me misunderstanding how to apply FFT to an audio stream being a strong candidate!   Ought to have another go!

My 1920’s lathe book mentions a mechanical device that does much the same.  It consisted of a frame supporting several pendulums of different lengths, each responding to a different frequency.   Plonked on the lathe, which was put to cutting metal:  if the lathe vibrated, one or more pendulums would start swinging and reveal the frequency.  As described in the book, done when a new lathe was being developed.  When a resonance was detected, the bed or whatever would be altered to reduce it.  But I think the technique should work on a worn or maladjusted machine and detect cutter / material / work-holding problems too.

The hardware is rather simple – a laptop, or RaspberryPi.    All I have to do is get my act together, oh dear, we’re doomed…

Dave

 

[old martParticipant @oldmart]

We have round insert lathe tools from 3mm to 14mm diameter and they are all much more prone to chatter than anything else, even the aluminium grades although they are better. The aluminium grades can be used for very light cuts on steel where you don’t want to rip into the work.

[Simon CollierParticipant @simoncollier74340]

First I’ll check the oil level in the saddle as the window is cloudy. The digital readout scale is in the way so I’ll have to remove at least its cover. I’ll dig out the book and see how hard it is to dismantle the saddle feed gears. I hate working on machines but once in a while it is necessary. I have a second lathe so projects won’t stop. Thanks for replies.

[SillyOldDuffer]

On SillyOldDuffer Said:

Fast Fourier Transform

That’s very clever. I’d like to see a sensible guide on that some day.

I did a quick search for digital motor stethoscopes but couldn’t find any. That would make for a sweet product , a bluetooth stethoscope with a spectrum analyzer.

 

[Simon Collier]

On Simon Collier Said:

First I’ll check the oil level in the saddle as the window is cloudy. The digital readout scale is in the way so I’ll have to remove at least its cover. I’ll dig out the book and see how hard it is to dismantle the saddle feed gears. I hate working on machines but once in a while it is necessary. I have a second lathe so projects won’t stop. Thanks for replies.

Dismantling ‘saddle-feed gears’ seems a bit extreme to me – aren’t enclosed gears running in an oil bath likely to run more smoothly over time, not less..?

Notwithstanding that speeds and feeds are those which you know work for you..

…the first question I’d ask myself is ‘what has (have I) changed (recently)’ – i.e. things like has the toolbit been out of it’s holder for a visit to the stone?

If so (pardon me, but in order to illustrate a point only), is it correctly formed, clean, tight, not rocking in it’s holder, etc.

Is the same true of the holder/toolpost/topslide interface? ..and so on back to the ‘chuck-jaw-to-work-piece-interface’..

Is the material a piece from a bar that you have used before, or something from a job-lot recently acquired?

Has a drive belt ‘taken a set’ over the winter?

It is so often (almost always – in my experience) something really straightforward & simple, rather than obscure and in the internals of the machine..

[Howard LewisParticipant @howardlewis46836]

To add another possibility.

Is it possible that the changewheels are too deeply meshed, and any slight eccentricity is varying the instantaneous speed of the leadscrew / power shaft?

I always run a piece of paper through the mesh, to give about 0.075mm backash, as the banjo is locked into place.

Howard

[Michael GilliganParticipant @michaelgilligan61133]

Just to emphasise Dave’s point about chatter:

Back when I worked at BAe Army Weapons they had a quite sophisticated multi-axis machine that produced nicely detailed tubes with thin walls [well they would, wouldn’t they] … and it was producing chatter marks.

The cure in that instance was to carefully adjust the flood coolant so that it mechanically damped the vibrations.

MichaelG.

[Simon CollierParticipant @simoncollier74340]

Nothing has changed, no exotic material, happened on brass and 316. Change gear mesh is OK. What I have not been doing is oiling the rack and drive gears up under the front of the apron. Also the area is filthy and I am going to do a big cleanup and change the oil in the head, gearbox and saddle. The Hercus lathe gets pampered but I treat the Chinese lathe as a workhorse and now realise I have been neglecting it, or at least the bits that are out of sight.
Oh, and no winter lay off here in Sydney.

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