Just can't stop that chatter

Chatter can be caused by many factors from poor insufficiently supported fixturing to the wrong speed and feed selected. Chatter can also come from many areas while machining mainly part chatter or tool chatter. Identifying the cause and location is important in determining the proper corrective action. Chatter can be destructive and in some cases dangerous.

The science of chatter roots from the accumulation and increased intensity of a wave. This article deals with chatter that is due to the inability to support the part in a sufficient manner and nothing else can be done. All other root causes have been tested and found to not be fixable for some mechanical reason.

As the part rotates a wave of a particular frequency begins to build due to a consistent rpm. As the part spins the wave grows larger and larger reinforcing itself every revolution until it breaks the insert, damages the part or worst of all becomes a danger to the operator and those around  him as it becomes destructive and throws the part from the machine.

The first solution is to look at how you are holding and supporting the part and prove beyond a shadow of a doubt that nothing else can be done. If you look at the part in the picture, you will see that it is a large, flat ring and to worsen the situation it is split in half, held together by the force of 8 wrapping soft jaws. Jack stands are also added at the ID of the part for additional support but chatter remains damaging a required surface finish.

A experienced operator knows that speeding up the RPM’s, called outrunning the chatter is an effective solution in contrast to an inexperienced operator who will slow down the RPM’s to break up the wave. Experience also tells an operator that a combination of slowing down and speeding up the spindle by varying the spindle override works very well by not letting a wave complete enough turns to show up in the finish.

The following is a custom macro that automatically varies the RPM up and down by a varied amount through parameters passed in to a modal macro. It very easy for an operator to use this.  function because all he has to do is replace the G01 command with G66 and the 9000 level program number and then run the finish pass. It does not requir the programmer to do anything.

MACRO PROGRAM:

O9330;

#541 = .05; (X distance to travel before increasing or decreasing spindle)

#542 = #4640; (X axis travel distance – check this parameter for your controls)

#543 = #2554; (place holder for spindle speed)

#544 = #543 X #541; (change factor)

WHILE [#542 LT #5640] DO 1;

#542 = #542 + #541;

#2554 = #2554 + #544; (increase spindle by 5 %) 

G01 X- [#541];

#2554 = #2554 – #544  - #544; (slow down spindle)

G01 X- [#541];

END 1; (loop back until finished)

M99;

PROGRAM CALL: 

N1255 G0 X134.75;

N1256 Z0.0;

N1257 G66 P9330 X106.255 F.01;

N1258 G67; (turn off modal command)

N1259 (continue with program) …

The fluctuating factors can easily be changes in the 9000 level program by changing their common variable values or you may read the section in the operators manual on how to pass these values from the G66 line.

This is a very effective way to control stubborn chatter. Simply, the operator could stand there varying the spindle override switch if this cut completes in a reasonable amount of time. The cut in the pictures took several hours so this solution was not practical.