Tips on machining vol.2
Suppressing chatter in long-workpiece machining on turning center
－How to cut long and thin workpieces－

Chatter generated when a long workpiece is machined on a turning center is one of the common problems facing many machinists. The cause of chatter varies. Although in some cases it is difficult to identify the cause and find a solution, chatter is largely due to thrust force (a force that is applied on the workpiece in a direction perpendicular to the surface), which causes the workpiece to be bent. The bent workpiece then tries to return to its original shape. The repetition of bending and returning to the original shape generates chatter. In other words, the factors contributing to chatter are “thrust force” and “workpiece deflection (bending).” This indicates chatter can be suppressed by reducing the thrust force or preventing the deflection of a workpiece on which thrust force has been applied.

There are two ways to reduce the thrust force: one is reducing the thrust force itself, and the other is generating the countervailing force to offset the thrust force.
Thrust force in general can be reduced by changing cutting conditions (e.g. lower the rotational speed of workpiece, reduce the depth of cut) or by changing the tool tip and holder specifications (e.g. widen the rake angle (breaker)).

In O.D. machining, it is important to set the cutting tool position higher than the workpiece center in advance, as the tool tip is lowered by the main component force (a force applied in the workpiece rotational direction), which causes the practical rake angle to be narrowed.
In the case of generating the countervailing force, a workpiece needs to be machined on a turning center equipped with the upper and lower turrets, under the cutting condition that thrust force is applied equally to both turrets. This technique has already been proved to be effective.

To prevent deflection of a workpiece on which thrust force has been applied, the workpiece needs to be machined with the use of a steady rest or a tool path that ensures workpiece rigidity. It is also effective to increase the tool feedrate to enable the workpiece to be machined while maintaining its bent state.

Resonance is also one of the contributing factors to chatter. In general, resonance depends on natural frequency of a workpiece, and resonance-induced chatter can be suppressed by changing the workpiece rotational speed during machining.

These are the brief outline of the causes and measures for chatter. The important point for suppressing chatter during machining of a long workpiece is to control thrust force and workpiece deflection. The use of IoT and AI is expected to grow in the machine tool field, so it may not be long before the new machining technology that achieves automatic chatter control comes into practical use.

We at DMG MORI have accumulated a wide range of solutions for customers’ chatter-related challenges through our long-term experience in customer support.
If you wish to learn more about chatter, from the basic mechanism of chatter to tips and tricks for suppressing chatter, please feel free to consult with our experienced machining engineers.

• Consult with DMG MORI engineers