Tag: Chatter

Chatter- It’s Not the Machine

Posted on by

“If you have a flat tire on a car, you fix the tire, you don’t look at the tire balance, vibration of the motor, or the paint job…”

Thoughtful follow up to our post on Vibration, Harmonics and Chatter   from William R. Shaffer,  VP at Conicity Technologies.

Conicity provides specialized edge prep solutions for our industry.

Here’s what Bill had to say:

So should I look at the engine mounts, the shocks and struts, the paint job, or should I just fix the tire?
So should I look at the engine mounts, the shocks and struts, the paint job, or should I just fix the tire?

“Perhaps there is some chatter that is a function of machine harmonics, but I have not seen that as of yet. My feeling is that the machines are designed and built with the high level of technology, much higher than the technology that is associated with the design of the cutting tool.

“Cutting tools used to drive the machine tool industry to higher levels of capability because the capability of the tool always exceeded the machine.  Spindle speeds and rigidity often lagged tool designs and machine tools were not able to take full advantage of tool capability.   That was then. Not today.

“Bottom line, machines have surpassed the tool capability and tools really have not made and significant breakthroughs to push the machine building community.

“I deal with chatter on a daily basis for our customers.

“We have successfully cured severe levels of chatter in metalcutting by addressing the micro-geometry of the cutting tool.

“At the end of the day, vibration starts at the tool because perhaps the tool geometry being suspect, friction, perhaps feed rates combined with tool design, but it starts with the tool.

“If the machine hums and vibrates when it not running a workpiece, then you do have a problem.   If you have a flat tire on a car, you fix the tire, you don’t look at the  tire balance, vibration of the motor, or the paint job…

“So the question I would like to explore is “what do the people that make the machine tools feel about their machines having inherent vibration and harmonics that create issues in machining?”

“What do the folks who make the machines have to say about this?

“Is the picture that many may have in their minds that “the machine has it’s own set of harmonics, so does the material that is being processed, perhaps even the tool,” valid, or not?”

Your comments please…

The Chatter About Chatter

Posted on by

 
Chatter costs money when you reduce the productivity of the machine by slowing it down  to make the vibration go away.

Chatter.

Conventional wisdom states that there are two kinds of chatter- Forced and Self Induced. Some shop guys like to think its caused by the material.
Forced chatter is a result of alternating cutting forces  that result from
1) Interrupted cuts (milling);
2) Machine vibrations such as out of balance motors,  spindles, gear or shaft irregularities, bad couplings or bearings, (Loose motor mounts and weakened or stretched) couplings;
3) Load on tool / workpiece changing as a result of acceleration or decceleration;
4) Vibrations being transmitted through the machine and foundation from other equipment.
If forced vibration is what you have, confirming the integrity of  the machine tool and its power train is a critical first step. Reducing the feed per revolution is one way to determine if it is the variation in the cutting process that is forcing the vibration. Changing the SFM or RPM’s by at least 25% is also something to try (increase or decrease!)
Self excited chatter is induced by a change in the cutting forces themselves, and is where I place the chatter that may be caused by the material.  Self excited vibrations can be distinguished from forced vibrations in the machining system  because self excited vibrations stop when the cutting does. Forced vibrations are not dependent on the cutting process, and so continue even when the tool is not in the cut. Self excited chatter can be caused by:
1) Change in forces needed to cut caused by differences in the material-  Material characteristics (such as workhardening or microstructural differences) that result in variation in chip thickness.
2)  Unstable built up edge (BUE) forming then breaking off causing variation in the cutting forces
3)  insufficient Stiffness of the workpiece,  spindle,  tool   and tool holding (think deflection and too much length).
To eliminate self excited chatter decrease the length of the tool in the cut,  shorten the tool holder, or substitute more rigid tooling and support materials (a carbide boring bar  deflects less and can make three or more times heavier a cut  than one made of steel for example) .
Still think it’s the material? Here’s my Metallurgist’s tip:  Look and see if you have  changing build up edge conditions on the tool that exhibits chatter. If  the self excited chatter is due to material such as an unstable built up edge (BUE)  forming,  try increasing the RPMs / SFM. Spindle speeds that are too slow allow workpiece material to weld to the tool edge  (pressure weld) and build up.This creates higher forces until it sloughs off.  Then forces go back to normal, and build up again until…
Higher RPMs help to keep BUE stable and  under control. And  they allow you to run faster cycle times, contributing to profits.
Bottom line: Chatter doesn’t always mean you need to slow down.
Photo credit.