Machining strategies for difficult materials

Toolpath strategies for efficient milling should consider much more than part geometry. Cuts should be selected that will not cause chatter, stall the spindle, break the teeth or tool shank or separate the tool from the spindle. A good toolpath strategy should also impart specified surface finishes and achieve acceptable metal-removal rates and tool life.

Typically, tool life has been considered independently of the toolpath. Tabular data listing recommended chip loads and surface speeds for various tool and workpiece material combinations—typically developed through measurements during a large number of turning tests—are widely available.

Tool wear mechanisms such as diffusion and oxidation are strongly temperature-dependent, and temperatures in the cutting zone are often quite high. The temperature rises in the shear plane, where the majority of chip deformation occurs. Then, as the chip slides along the rake face of the tool with high pressure, the resulting frictional forces raise the temperature even more.

When the temperature gets high enough to activate thermal wear mechanisms, a “thermal barrier” is crossed and tool wear increases rapidly. However, milling is substantially different than the turning typically performed in tests to create the data. Toolpath choice can change wear conditions dramatically.

For some combinations of tool and workpiece material, tool life is not a critical factor. Solid-carbide tools, for example, can tolerate the melting temperature of aluminum. For that reason, permissible surface speed when endmilling aluminum is almost unlimited.

In other workpiece materials, such as titanium and nickel-base alloys, tool life is decisive. These materials produce much higher cutting temperatures than aluminum, and they are chemically active with respect to the tool materials. As a result, these difficult-to-machine workpiece materials are typically machined at low surface speeds and low mrr.

However, because milling cuts are inherently interrupted, they provide an opportunity to change the game. If the radial DOC is selected to be a small fraction of the tool diameter, say 10 percent, then each individual tooth is only cutting for a short time. Before the temperature can rise to the thermal barrier, the tooth is out of the cut. The tooth has time to cool before it re-enters the cut on the next revolution.

All images courtesy of S. Smith