Dealing with chatter

Many sounds categorize the atmosphere of a machine shop. Whether it is the whir of a grinder or the buzz of a tool cutting the part, one sound specific to a machine shop that is preventable is chatter--that annoying squeal or squeak from a machine telling you 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 is it fixed?

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? There are several approaches to take when correcting chatter. With 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	The flat on the bar is not indicating parallel, an incor- rect tool rake angle results in chatter. Make sure to indicate the flat on the boring bar.
2. Part setting	The part is not correctly sup- ported in the chuck, chatter may be the 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.
During machining
4. Overextended bar	The boring bar is extended exces sively in terms of length to diameter ratio. In this case consider using an antivibration bar.
5. Depth of cut	Try reducing the depth of cut to reduce chatter.
6. Feedrate	Both high feedrates and low feedrates may cause chatter. The first inclination with chatter is to slow down the feed. Try increasing the ipr first. An increased feed bends the bar, removing the tendency to chatter.
7. Chipbreaker style	The style of chipbreaker may be causing excessive force, try changing to a positive style chipbreaker.
8. Bar support	If the bar is not correctly supported in the turret, use an extended bush or sleeve.
9. Surface speed	If the cutting surface speed is not harmonized between the tool and the part, material chatter occurs. The first inclination with chatter is to slow down the speed. Try speeding it up first.
10. Cutting edge too sharp	When using a new insert the sharp edge may need to be slightly honed to create a smooth cutting action.
Advanced
11. Tool pressure	To reduce tool pressure, change the nose radius of the cutting insert. A sharper insert nose radius has a lower cutting force. This is often a last resort solution because changing the nose radius of the tool requires changing the part program, cutter compensation, and feedrates.
Chatter can be a complex situation. Remember the definition and tailor the solution to correct the problem. When the tool and part are in harmony, chatter is eliminated. Steve Rose is a manufacturing consultant and president of Rose Training Systems, Solon, OH, which also offers Internet web site development. Mr Rose can be reached by phone 440-542-3066; fax 440-542-3006; e-mail shoptalk@cnc-training.com; or on the web at http://www.cnc-training.com Copyright Rose Training Systems Inc, 1998

Originally published in the December 1998 issue
of Tooling & Production.
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