Cornering Chatter in a Concave Radius

     Chatter – chatter – chatter – not an uncommon problem in CNC machining.  It can be especially challenging to correct chatter when milling an internal concave radius.

     We’ve discussed chatter and means of eliminating chatter in this column and in my many training classes.  Who remembers our definition of chatter?  Chatter is a harmonic imbalance between the tool and the workpiece.

     A person can talk to himself but in machining the tool doesn’t chatter by itself.  When the tool and the material are in contact with each other in a smooth and balanced condition there is no chatter.  If they are not balanced there is chatter.

     Take a look at this part; an end mill is used to machine the internal shape.  The straight line cut along the internal wall shows good results but chatter occurs as the cutter encounters the inner radius.  The chatter is shown as swirling lines at this radius feature.

     Chatter on the radius but not on the straight cuts.  How can this be corrected?

     There are three things to consider when combating chatter when using an end mill - the style of end mill, the tool holding method and the feed rate.

 End mill style

    An end mill has a number of cutting edges.  When making a straight cut the contact area between these teeth and the part material at any one moment is relatively small.  When the cutter enters a radius move the contact area increases dramatically.  The part “wraps around” the cutter, increasing the contact resulting in more chatter.

     The harmonics of the cut are made worse if the edges of the cutter hit the part at exactly the same time each revolution.

     Think of chatter as a “tuning fork”.  If something is continually vibrated at exactly the same pitch it starts to vibrate.  When the end mill’s multiple cutting edges are hitting the part at exactly the same time the result can be chatter on the radius of the part.

     With face milling cutters, this problem was addressed with “differential” teeth spacing.  To counter this “tuning fork” effect, tool companies have designed face milling cutters with teeth that are not evenly spaced around the cutter. The net result is that the teeth are not striking the part at a uniform rate - not setting up a harmonic effect.

     Now this concept has been applied to end mills; cutters with uneven teeth spacing are readily available.  The angle of the tool flute has also been modified from tooth to tooth. One flute of the end mill may have a 30˚ spiral angle and the next flute may have a 28˚ angle.  By changing the flute spacing and flute angles we are overcoming the inherent harmonics of the cutting action.

 Tool holder style

     Some time ago, here in Shop Talk, we talked about end mill holders and hydraulic milling chucks.  One reader pointed out that I had omitted to mention the “shrink-fit” type holders.  This is a relatively new method of holding an end mill but it has gained wide acceptance as a superior method of holding an end mill.

     The shrink fit concept has the effect of making the tool and holder behave as a single solid unit. This rigid holding method can assist in the battle against chatter.

 Feed Rate

     The feed rate of the cutting action is a known factor in dealing with chatter.  The feed rate should be reduced when the cutter enters the radius.

     There is a formula for this reduction.

Modified IPM = [(part radius - cutter radius) x linear feed IPM] / part radius

     Be sure to use an accurate feed rate when milling the straight line move and calculate the correct reduction for the radius.  The feed rate for each application may need additional adjustment as required.  This is a simple and easy calculation to assist in solving the chatter problem.

     When working in machining, we will always deal with chatter.  Keep in mind all factors that affect both the cutting tool and the material.  For tricky issues of the concave radius, consider at the tool, holder and feed rate to help eliminate chatter.