How Process Damping Impacts Milling Stability

Because the tool-toolholder-spindle-machine is not rigid, the tool vibrates in response to the cutting force and chatter can occur. Chatter exhibits large forces and poor surface finish, so it should be avoided. You can use models to select spindle speed-axial depth combinations that avoid chatter, while meeting design tolerances. These milling stability maps, which separate stable spindle speed-axial depth combinations from those that produce chatter, are generated using a measurement of the tool-tip vibration response and a cutting force model. The stability map in Figure 1 exhibits large stable zones at higher spindle speeds (cutting speeds). When tool wear constrains the maximum cutting speed, however, lower spindle speeds must be selected. At low spindle speeds, process damping can increase the allowable axial depth without chatter.

To describe the physical mechanism for process damping, consider a tool moving on a sine wave while shearing away the chip (see Figure 2). Four locations are identified: 1) the clearance angle, g, between the flank face of the tool and the work surface tangent is equal to the nominal relief angle for the tool; 2) g is significantly decreased and can become negative (which leads to interference between the tool’s relief face and surface); 3) g is again equal to the nominal relief angle; and 4) g is significantly larger than the nominal value.

At Points 1 and 3 in Figure 2, the clearance angle is equal to the nominal value, so there is no effect due to cutting on the sinusoidal path. However, at Point 2, the clearance angle is small (or negative) and the thrust force in the surface normal direction, n, is increased. At Point 4, on the other hand, the clearance angle is larger than the nominal and the thrust force is decreased. Because the change in force caused by the sinusoidal path is 90 degrees out of phase with the displacement and has the opposite sign from velocity, it is a viscous damping force (i.e., a force that is proportional to velocity, like a shock absorber). Given the preceding description, the process damping force in the surface normal direction depends on the tool velocity, axial depth of cut, cutting speed and a constant, which is included in the cutting force model.

When including process damping, the stability map changes at low spindle speeds. The stability boundary is increased, as shown in Figure 3.

The force and vibration are shown for three spindle speed-axial depth combinations in Figure 4. All have an axial depth of 3 mm, but the spindle speed varies. At 9,000 rpm, the cut is stable. This is expected because the point is within the stable gap in the stability map near 9,000 rpm. At 7,500 rpm, however, chatter is observed. This is again expected because the point is located above the stability boundary. At 250 rpm, the cut is again stable due to process damping.

Process damping is influenced by the cutting edge design. Tool designers now add features on the relief face of the cutting edge to encourage interference with the workpiece surface and provide energy dissipation that reduces the vibration and increases stability.