Problems with face cutting on lathe, wavy pattern

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Bryanbdp

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I have an Enco lathe that is giving me a weird wavy pattern when I face a piece of bronze bushing stock.
I have tried different spindle speeds, feed speeds and cutters.
It almost seems like the pattern follows the turning of the cross screw, even though I have snugged up the gibbs tight and locked down the compound slide.
I just installed the new tool post, but it seems quite rigid.
Any suggestions as to what is causing this and how to fix it?
Does it have to do anything to do with the bronze material?
Thanks,
Bryan

lathe photo.jpg
bushing photo.jpg
 

goldstar31

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As you seem to have tried most alternatives, I'd guess at a slack feed screw/ nut.

Maybe putting a dial mike at the back of the saddle would show the amount of slackness there

Only a guess from a distance


Norm
 

XD351

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Are you using the power cross feed ? If you are then it might be something with the drive that is doing it , if you are doing it manually try auto feed also try locking the saddle as well . I think Norman may b on the money with the cross feed screw nut having too much end play . Do you get this with steel or aluminium ? Bronze can be funny stuff to machine depending on what is in it , what cutting tools are you using and what tool geometry ?
 

petertha

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I'm guessing your lathe has a power feed rod that drives the carriage & cross feed? When you see stripes its a hint that its cyclic. If you are seeing a similar stripey pattern in longitudinal power feed turning, then I would suspect something in the power feed drive line. It could be bowed power feed rod, worn worm gear assembly, possibly junk in the powerfeed keyway slot or the worm gear key is loosening up. f the longitudinal is fine & only on the cross feed, that that narrows things down to lead screw, lead screw nut, possibly bearing assembly or the gears that are transferring motion up from the apron. If you can hand feed rotate the wheel as consistent as possible & you don't get stripes then I would suspect something in the power feed.
 

goldstar31

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What we tend to forget that is that ' we are always cutting a thread' in lathe work.

The finest shiny surface is 'still' a thread. and so is the roughest!

Regards

N
 

XD351

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The concentric rings are extremely regular which to me would indicate an intermittent feed with a regular pause , like feeding one turn then stopping to reset ones grip in the feed handle . That is why i asked if it was manual or auto feed and if it is the same for other materials using the same tools. If you can do the same facing cut in aluminium or steel using the same feed and cutting tools and get a good finish i would be looking at tool type and geometry . Bronze , brass and copper have many different types and some machine better than others although in your photo the finish looks ok . I’m guessing that you are facing from the outside into the centre , if so try a very light cut from centre out - sometimes this will improve the finish .
We can only offer suggestions from what we see in the picture and it will be up to you to go through these to find the solution .
 

goldstar31

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I've been carefully following the excellent resume of XD351's thoughts and am prompted to ask whether there is a piece of swarf on one of the gears causing the cut to 'miss' regularly.

Clearly, it is a most interesting and thought provoking post

Regards
N
 

TonyM

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They look pretty uniform in distance to me. You could do a quick measure of the pitch to see if it coincides with one full turn of the cross slide. Could be a slight bend in the cross side screw
 

HWF

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I have a Grizzly G4003g and there have been reports of finish issues related to motor vibration. As I recall the issues had to do with the single phase Chinese motors. A variety of solutions were offered including changing of the motor and isolation mounting of the motor. I replaced my motor with a 3 phase and a VFD.
 

petertha

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That's a good point too HWF. I changed the belt on my lathe motor & noticed a kind of superimposed cyclic looking finish over & above the cut I had not seen before. The motor is mounted on a kind of a hinge plate to accommodate belt size/wear. You have to get the belt tension right but more importantly have to lock the plate in position so it doesn't 'bounce' which is exactly what is happening a a certain frequency. Amazingly that can transfer to the work. Mine has kind of a threaded post with a lock nut. In this case (and I assume yours) if it was cutting without incident before & now has a problem, should be easier to isolate. You hear about the bad motors or distorted sheaves on new Asian lathes & that takes more work to investigate because all you know is you have a problem 'somewhere' right off the bat.
 

Bryanbdp

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Thank you for all the replies. I am starting to think it is the material itself, and the wrong type of cutter. It is 660 bronze, and I guess it has a tendency to pull the bit into the part. That does seem to be what's happening. It loads up, then releases.
I'm going to try a 0 rake tool, and see if that helps...
 

petertha

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I didn't have any issues straight turning or boring bronze with a sharp single point uncoated carbide insert cutter used for aluminum - which was a bit counter intuitive rake angle wise. But drilling was another matter. It progressively wants to grab the larger the diameter so it wanted the same dubbed tip geometry as drilling brass.
 

BaronJ

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Hi Guys,

The pattern appears to be very regular and not a spiral. So I agree with Tony on this one. A slightly bent feed screw would also be my guess. If that is the case feeding by hand would produce larger or smaller spacing in the pattern.
 

banjoT1

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Before I post my first message on this forum about the topic I would like to make the comment that now retired and getting set up to resume personal projects [suspended while ‘’life got in the way’’], I have a great deal of respect for the pool of knowledge available here and the veteran folks that have spent a lifetime learning their trade and craft of metalworking – so thank you for taking your time to help foster folks like me.

Although my background is in the furniture design and manufacturing area, and education, from first-hand experience I’ve come to learn there are many basic concepts of materials processing, engineering, assembly, troubleshooting – to name only a few – that are easily transferable to almost any other activity.

To keep my message brief but relevant, troubleshooting as I se it often requires a person to quickly think on their feet by sorting through a vast mental library of successes and failures to conclude what might be the most likely and simplest gremlin in the mix.

I lack the knowledge to chirp in about feed, feed rates, or other likely mechanical problems because at this point I am apprenticing on this forum; however, I agree with XD351 when he says that [paraphrased] ‘’here are possibilities – now you figure it out’’. Also, I believe that HWF offers a very interesting possibility that, because the root cause is virtually invisible, such problems of vibration can easily be overlooked or not even considered at all.

So, what is ‘’transferable’’ [as mentioned above] is the term ‘’resonant frequency’’ or otherwise described pretty much as the frequency at which an object will naturally and inherently vibrate due to its physical properties and certain energy acting upon it. As a stringed instrument player and instrument set-up tech, I’ve had several years in learning about energy/sound/vibration transmission and many of the problems unwanted vibration can create with stringed instruments.

With regards to the second photo showing the ‘’weird wavy pattern’’ I am not necessarily stating that vibration is the culprit but only that it could be a very quick troubleshooting option – that is, try the simplest and least expensive possibility first; then, if not that is a cause, go to the second possibility.

A loose motor, sloppy belt, line surges, lathe connected to the same circuit as a fluorescent light, worn bearing, incorrectly sharpened or positioned cutting tool, wooden floor, floppy metal parts (such as a lathe back panel), lathe with very little mass, etc., etc., could all be contributing factors conducive – each as a singular vibration or when combined as vibration ‘’harmonics’’ – i.e.,, multiple overlapping vibrations, or ‘’beats’’ (also described as ‘’wolf tones’’).

Just saying…… that if Bryanbdo’s ‘’weird wavy pattern’’ is due to vibration sources as explained above, then the face of the bronze turning I a real and actual recording of that vibration in the same way as we know a musical recording to be.

Hopefully this posting may contribute to the forum, to the learnin’ process, and to identify machining problems not easily seen.
 

BaronJ

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Hi Bryce,

Thank you for your comments. I completely understand what you are saying and don't in any way disagree. The pattern is very regular and a vibratory resonance is a possible cause. The only time I've seen something similar is a visible but an unmeasurable pattern on a turned rod. That turned out to be a drive belt that was in the process of disintegrating.
 

John Antliff

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I've seen this type of problem occur when the end thrust nuts have come loose on the lathe mandrel. With a tough material like bronze the use of a dull or incorrect tool form can cause repetitive marks to appear due to cycles of rub and cut as the tool tip pressure rises and releases. I have also seen vibrations being transmitted to the mandrel when using a variable (VFD) drive, certain speeds settings can induce harmonic cycles (beat frequencies) which can be alleviated by changing the speed setting slightly. Good luck with the investigation.
 

Eoin

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This thread has been an interesting read and reminded me of a similar incident with a new Lightburn production lathe back in the 1970s.

I was the Machine Shop Foreman (civilian) in an Australian Army workshop when the new lathe was installed and became the "personal" lathe of our top machinist.
First thing that he did was to turn some test pieces and he soon discovered that he was getting a wavy pattern on finish cuts.
The suppliers were called and they sent out a couple of men with electronic test gear who eventually, despite the evidence, proclaimed the lathe to be 100%, and, in truth the pattern didn't mean anything from a production viewpoint, but it didn't look nice!!

Further talks with the suppliers resulted in the "legendary" old hand arriving without any gear other than a wooden dowel about 12 inches long and with well rounded ends.

He started the machine, had a listen via the dowel, varied the speeds, listened some more then declared that a particular gear was the cause.
Next day a team arrived, stripped the head and changed the suggested gear....and never another wavy pattern.

Magic!!
 

Dubi

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Thank you for all the replies. I am starting to think it is the material itself, and the wrong type of cutter. It is 660 bronze, and I guess it has a tendency to pull the bit into the part. That does seem to be what's happening. It loads up, then releases.
I'm going to try a 0 rake tool, and see if that helps...
Not sure if this will help or hinder. I was machining some Delrin plates and had a similar pattern to yours. I had been machining manually but changed to auto on the slowest feed I could and reduced the speed to about 350 rpm. They came out perfect.
 

john_reese

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The material and the tool can be interacting to produce the pattern. Some bronzes require a very sharp tool to cut them. If you are using carbide inserts with a honed edge that might be the problem. As the tool is fed into the work it tries to penetrate the metal but doesn't, deflecting the tool and machine components. At some point the tool pressure will be high enough to overcome the cutting resistance. At that point the tool penetrates the work, releasing all the elastic energy stored in the deflected components and it takes a big bite. The cutting forces build up again and the scenario repeats.

The solution appears to me is to change tools. A very sharp HSS tool should work or a carbide insert designed for aluminum with a sharp edge and polished surface.
 

john_reese

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If changing the tooling does not work then investigate problems with the machine.
 
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