Chatter

[gabeless]

Hello all,

I am keeping a glossary of HSM terms and I have a question,

What is Chatter?

How do we define Chatter?

I was hoping for some references as well, so i can read more about it.

thank you

gabeless

[hardmill]

Bad surface finish, usually a result of too much spindle speed.


PEACE

[Rekd]

Chatter is when the tool or material 'bounces' or 'resonates' while cutting. It's caused by lack of chip-load in most cases. It leaves bad finishes, beats up the cutter, and sounds nasty.

It can usually be eliminated by a) shorter tools, b)heavier chip loads, c)higher RPMs, d)more sturdy stock/setup

'Rekd teh School of hard knocks

[gabeless]

So chatter is caused by the Milling machine and not the actually part.

The reason for my asking is I have a thin walled monolithic structure which I want to vibrate rapidily, however I believe that this is not called chatter due to the source. The source being the thin wall feature and not the machine and the tool.

I use a tuned carbide 1/2 4 flute straight cutter with 3 in over hang. So I guess I should have explained my better the first time. What are your guys thoughts on this.

Thanks again
gabeless

[Rekd]

So chatter is caused by the Milling machine and not the actually part.

Not true. Usually the setup, speeds/feeds, and tooling are to blame. Most chatter can be eliminated with heavier chiploads coupled with lower sfpm, reguardless of what machine it's on.

The reason for my asking is I have a thin walled monolithic structure which I want to vibrate rapidily, however I believe that this is not called chatter due to the source. The source being the thin wall feature and not the machine and the tool. I use a tuned carbide 1/2 4 flute straight cutter with 3 in over hang. So I guess I should have explained my better the first time. What are your guys thoughts on this.

The thin wall feature should work well, after you get your speeds/feeds set.

Don't be shocked to notice that on finish cuts, you could end up at 1500rpm at 15ipm with that 1/2" EM.

HTH

'Rekd

[creative_mind]

How about this? Heh heh.

Main Entry:1chatter
Pronunciation:*chad.*(r), -at*-
Function:verb
Inflected Form:-ed/-ing/-s
Etymology:Middle English chatteren, of imitative origin

intransitive verb

3 a : to make the sound of or as if of rapidly repeated noisy contacts (as of the teeth of one who is extremely cold or frightened) *master's teeth chattered with horror Donn Byrne* *chatter like castanets in a Spanish dance* *skis will chatter in a turn if they are edged too much Operations in Snow & Extreme Cold (U.S. War Dept.)* *machine guns chattering Philip Wylie* b of a cutting tool : to vibrate rapidly in the action of cutting so as to form ridges or nicks *the plane chattered along the edge of the plank* c : to operate or perform with any irregularity that causes rapid intermittent noise or vibration *the motor chattered in reverse*

[camminc]

Hello, this is Randy. You ask a very good question. This is a long writing to explain, so hang on. What I read from your question is – is it chatter, if so is it the cutter or the part that is causing the chatter of your machining operation? So, the best way to look at this is to determine the frequency of the problem occurring. Is it from the cutter or is it from the part or is it both? This is pretty simple to solve but you will need the proper equipment to derive the answer. What is required to be known are the natural frequency of the cutter at the tool tip and the natural frequency of the part being machined at it’s intervals of machining.

I will try and break it down to understand. Let’s look at chatter from physical aspects first. Everything upon this earth has matter, which has a natural frequency. Ever seen those commercials of a glass breaking with the tone of a person singing? The glass breaks because the frequency of the singer reaches the natural frequency of the glass and shatters it due to matching the natural frequency of the glass, resonance, causing the glass to resonate at it’s natural frequency. You can’t hear it because it is to high a frequency. The glass shakes from this frequency, which is chatter by it’s own natural frequency. How about those guys driving next to you and your car shakes from their sound system in their car? Those are low frequency vibrations, which carry high amplitude, causing your car to shake. This gives you an idea of the force of frequency vibrations.

This is the same property in machining. This is called the Tooth Pass Frequency. The cutter has a natural frequency and the part has a natural frequency, period. The cutter natural frequency does not change but the part frequency will, due to material being machined from it. The more mass of the material the lower the frequency, the less mass of material the higher. Therefore, the more machined of a part say as to make it more thin walled, the higher the natural frequency it will produce. Now, to determine, which is causing the problem. Certain frequencies of the cutter, machine and part could cause resonance, which I do not fully understand. You can be advised from experts in the field at my site.

The easiest way to determine if your cutter or part is causing a problem is to do an impact test. Special equipment is required but you can use this equipment on all your machines, on tools, on parts on the machines themselves. You will need to see my site for these products. Called the MetalMax System, from MLI.

Once you know the natural frequency of the cutter at the tool tip you will be able to determine the proper rpm to run chatter free of the cutter on your machine, the higher you reach the number 1x the deeper you can go. Natural Frequency x 60 divided by number of teeth will be your optimum rpm and depth of cut, divide this by 2 for the next rpm, 3, 4, etc. if your machine tool can not reach a certain rpm. Once you reach the 5x on this calculation it will probably do no good to match this tooth pass frequency. This equipment will also tell you the flexibility of the cutter being examined to tell you the depth of cut, which you can to optimize. It will also tell you the natural frequency of the part to determine which cutter to use by way of frequency analysis.

Please see: http://members.cox.net/camminc/

[camminc]

Chatter can be caused by the cutter or the part depending upon frequencies. They could both be causing one to chatter more. One needs to determine the frequency of the vibration to eliminate the problem. The machine should only cause forced vibration by way of it's dynamic characteristics, which is usually far lower a frequency than the part or cutter. Yet a combination of frequency from the cutter could cause resonance of the machine tool in some situations, which I do not fully understand. You will need to see MLI on this condition, at my website. Forced vibration of the machine tool is usually due to high feed rates, to deep a cut that overloads the axis of a machine tool. Chatter and forced vibration are two different animals and they produce two different frequencies. Forced vibration occurs at spindle speed frequencies, chatter occurs at cutter or part natural frequencies. Unbalance also occurs at spindle speed frequency, which makes it not viable to chatter conditions. Spindle speed frequency is: RPM / 60. Compare. See: http://members.cox.net/camminc/

[HuFlungDung]

I think resonance is due to the "frequency generator" (the frequencor ) matching step with the natural frequency of the "frequency receiver" (the frequencee)

This can be visualized as the peaks and troughs of the two vibrations adding together with greater and greater amplitudes being reached, until something gives: usually, your ears, or else your hair starts on fire

[camminc]

Chatter Questions and Answers:

gabeless - What is Chatter? Answer: Chatter is caused by the cutter, holder, spindle or machine self-exciting, causing very high chip thickness variations as each tooth passes during a cut (also referred to as regeneration of waviness). It continues to grow until the chip thickness is exceeded and the cutter jumps out of the cut. Spindle speed out of phase with the cutter natural frequency is a determining factor. Spindle speed can be adjusted to maximize conditions, usually higher spindle speeds are required than what is expected to put in phase conditions for chatter to vanish, enabling higher depths of cut both axially and radially.

gabeless - I was hoping for some references. Answer: There are many articles on the internet explaining chatter. Do a search for Scott Smith, Jiri Tlusty, Tom Delio, Harmonizier and see: http://mmsonline.com/dp/zones/index....HSM07&zone=HSM

Rekd - Most chatter can be eliminated with heavier chiploads coupled with lower sfpm, reguardless of what machine it's on. Answer: This method is referred to as process dampening. What it does is to not give the cutter time to chatter due to low rpm and more time engaged into the cut, therefore the cutter can not resonate at it natural frequency to cause chatter. The problem with this is that it is very slow going. Many times, in HSM'ing you will have a machine with more rpm to work with to find the so called sweet spot to eliminate chatter and then go faster and deeper in a cut. Remember if you know the natural frequency of the cutter at the tool tip you can take that number x 60 / number of teeth to get the highest rpm that this cutter would like to run. This will be the optimum spindle speed for that cutter. You can see this site for a part that has been thin wall machined with this process: http://www.mfg-labs.com

There are many methods used to eliminate chatter for milling such as off set teeth to break up the harmonics - tooth pass frequency. Too much primary clearance on a cutter can cause chatter prematurely also. There are tunable boring bars used in turning which enable the operator to dampen out vibration or chatter during a cut when long reaches are required. This method cannot be used during milling because the cutter is rotating which would then cause too much unbalance. The Harmonizer can be used, an audio frequency analyzer software program, that enables a user to find the chatter frequency during a cut then calculate a suitable rpm to run the cutter at and of course the impact test method which uses a loop back system of accelerometer and impact tool to measure natural frequency and flexibility of the cutter at the tool tip to then calculate correct spindle speed rpm to stay out of chatter. You can see this product and video - called MetalMax System at: http://members.cox.net/camminc/

[Egar Twins]

or just try and see what works and what doesn't...experience can play a big part where science is confusing