Stopping 'idle' chatter: How to find harmonic imbalance between tool, workpiece.

Author:Rose, Steve
Position:Shop talk

Many sounds categorize the atmosphere of a machine shop, from the whir of a grinder, to the buzz of a tool cutting the part. One preventable sound specific to a machine shop is chatter--that annoying squeal or squeak from a machine. When you hear it, you know something is wrong.

Not only do you hear chatter but you can see it in the imperfect finish on the part. Everyone experiences it, but how do we fix it?

What is chatter?

Chatter is a harmonic imbalance that occurs between the tool and the workpiece because the part and the tool are bouncing against each other.

Remember it takes two to tango, so keep in mind that chatter can be caused by the tool bouncing against the part, the part bouncing against the tool, or both. It is not always easy to determine why chatter is happening, or which piece is the culprit.

How is chatter corrected?

With some careful setup and investigation, the problem can be isolated and fixed. Here is a list of items to keep in mind when correcting chatter.

May Cause Chatter Should Fix Chatter Before Machining 1. Tool setting If the flat on the bar is not indicating parallel, an incorrect tool rake angle results in chatter. Make sure to indi- cate the flat on your boring bar. 2. Part setting If the part is not correctly support- ed in the chuck, chatter may be a problem. Tap the part lightly with a wrench. If the part rings like a bell it is not supported correctly. 3. Final Part Shape Perform boring operation prior to turning the outside diameter of a part to decrease the risk of chatter. [ILLUSTRATION OMITTED] During Machining 4. Overextended Bar The boring bar is extended exces- sively in terms of length-to-diameter ratio. In...

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