Author Topic: Chatter - what is it and how to counteract it  (Read 295 times)

eskoilola

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Chatter - what is it and how to counteract it
« on: June 14, 2018, 09:47:17 PM »
Thanks to vtsteam on the idea of this thread.
I searched this forum with keyword "chatter" and the response to that search was quite thin. Seems like everybody in this forum has a very rigid machine and everybody knows how to deal with the chatter. Well, I don't.

I started this metal hobby quite recently, jus 8 years ago. My first lathe was a Russian tabletop thing. It had no ways but a central very rigid tube over which the saddle was riding. With that lathe I learned something about chatter. Here is a picture and description of an identical lathe.

My first mistake was to have too long stick outs combined with high speeds and deep cuts. All which can cause chatter individually , yet in a combination.

So what is this chatter anyway?

To my understanding it is a resonance. Resonance means that Your work piece, the tool and the holding system form together a bell which rings when banged. The ringing frequency is the frequency You are going to enjoy Your chatter with. It may also be some harmonic of that frequency. Harmonic means a multiple of that frequency. Usually, i think, it is a more complex system of vibrating components singing in a choir. Nevertheless it is a resonance since it is self amplifying and usually has a fairly constant pitch.

With that Russian lathe I learned some tricks to defeat chatter. As I am actually an electronics guy I was immediately thinking of oscillating electronic circuits and how to make them not to oscillate. With that I am pretty good. So could some of that knowledge be used to defeat chatter in metal working? The answer turned to be: Yes.

In order to make an electronic circuit to oscillate three things are necessary. First there needs to be amplification greater than one. Secondly there needs to be a resonant circuit. It may be a coil or it may be the ever present delay on an electronic circuit. The third thing needed is a feedback from output back to input.

Now, what are the analog three components in a lathe:
- amplification is the force that pulls the cutting tool tangentially when pushed into the work piece. If the tangential pull has a relation with the push then we have amplification.
- in a lathe there are many resonant circuits. Any hardened component has a strong tendency to ring. This is probably the reason why lathe frames are made out of cast iron and church bells are not. To my knowledge cast iron is not very ringy.
- the feedback is naturally the tool holder and the cutting tool itself.

So what do I do when I have an oscillating amplifier which is not supposed to be oscillating ?

First, if i can, i bring down the amplification as much as is practical considering the usability of the circuit. My analogy of amplification in a lathe is rigidity. The more rigid the setup is the less You have amplification and therefore the chances of chatter get smaller. In this context rigidity includes all components participating in the cutting operation including chuck, bed, saddle, tool holder and whatever. Some things can be made super rigid, others just are what they are like a boring bar. So minimize stick outs and make the machining in an order which maintains maximal rigidity of the work piece. Sometimes this is easier said than done.

If my amplifier is still oscillating or if the amplification cannot be brought down I try to find out the resonant circuits and eliminate those as much as possible. My analog in a typical lathe would be the work piece itself, the cutting tool and the cutting tool holder. I usually keep the compound locked and the tool holder in a place that is as near as possible of the pivoting point of the compound. Making stick outs smaller has an effect here as well as short pieces have a higher resonant frequency which is easier to dampen. One trick widely used in electronics is to include another resonant circuit which counteracts the original resonance. This works in a lathe as well. I found out that snipping a vice grip in the boring bar shaft dampens the chatter. I found out that the best dampening was achieved by having some rubber in between the shaft and the vice grip jaws. Sometimes just touching the bar with a finger stopped the chatter.

The third thing is the feedback. In an electronic circuit an unintended feedback usually circulates through power supply or stray capacitances etc. In a lathe the feedback is directly related to the cutting edge itself. Having sharper tool and making lighter cuts with slower speed decreases feedback and counteracts chatter. Turning the tool upside down changes the feedback phase and can eliminate chatter all together if You are lucky. I have my parting tool upside down and this has been defeating chatter quite well on parting operations.

So, what kind of experiences You have on chatter?
How do You defeat chatter?

Offline WeldingRod

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Re: Chatter - what is it and how to counteract it
« Reply #1 on: June 14, 2018, 11:06:47 PM »
Let me add some thoughts:
Chatter is very much like washboard road; mass spring system, some damping, and the vibration is both driven by the road AND modifies the road.
With that analogy, you realize for poorly damped systems in forced vibration, you get a large response, but only near resonance.  Change the speed, get off resonance, things settle down.  Well damped systems, however, have a broad frequency range where they oscillate (but they do it at lower amplitude).

Solutions:
Continuously vary the cutting speed so that oscillations don't have time to build up at a specific frequency.
Add an impact damper; smooth bar in a pretty close fit hole, allowed to rattle.  I'd have to dig up some papers to find the clearance.
Add an oiled damper; similar to above, larger clearance, oil in the gap.
Stop, shave off the bumps, and try again.
Change your depth of cut and feed.  Depth of cut increases the likelihood of chatter, I thing heavier feed reduces it.  Not sure...
Change from a two lip tool to a three+ lip tool.  Core drills absolutely ROCK at opening holes!!! 

And for the obligatory sarcasm:  in the electronics world, when we were really frustrated in a lab, we occasionally practiced resistor sacrifice.  Didn't help anything other than a small relief of stress ;-)  beating something on the anvil might be the mechanical analog.  Or twisting the head off.a 1/4-20 bolt.  I've actually sent young engineers out to the shop to do it, so they realize how bbq easy.it is to do!

Sent from my SAMSUNG-SM-G891A using Tapatalk


Offline Joules

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Re: Chatter - what is it and how to counteract it
« Reply #2 on: June 15, 2018, 01:02:04 AM »
Depends where the chatter is.  Sometimes you just accept a little if itís only cosmetic and doesnít affect the part.  Sharp tooling, and changing speed.  My biggest issues with chatter is normally using a boring bar, so you have this long tuning fork dragging along this hard material.  Wish someone would invent a transducer that could ring the tool to counteract the harmonic.  Then again something else on the lathe would start singing in itís place.  You are conducting a badly behaved orchestra unless you run a tight ship.
« Last Edit: June 15, 2018, 03:30:04 AM by Joules »
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline seadog

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Re: Chatter - what is it and how to counteract it
« Reply #3 on: June 15, 2018, 03:39:50 AM »
AvE came up with an interesting solution last year -