Managing multiple milling constraints

Milling is a complex process bound by multiple constraints. Successful milling requires selection of cutting parameters that respect the constraints. Presented here are explanations about six of those constraints.

Bending moment. The bending moment equals the cutting force times the distance from the cutting zone to the toolholder/spindle interface. If the bending moment is too great, the shank of a slender tool may break or separation can occur where the toolholder face contacts the spindle, allowing the toolholder to pull out of the spindle. That’s especially problematic for HSK holders because they derive their stiffness from the face contact. The bending moment can be reduced by shortening the tool or by reducing the cutting force, which requires reducing the axial DOC, radial WOC or the chip load. For most tools, the chatter limit is reached long before a shank breaks.

Chatter. Chatter generation is a limiting factor when milling, particularly when the objective is to have the metal-removal rate as high as possible. Chatter arises due to insufficient dynamic stiffness, which, in turn, is caused by long slender tools, small-diameter spindles (due to the DN limitation, where D is the diameter of the main bearing bore, and N is the spindle speed), thin workpieces (although a good machining strategy can usually avoid this) and the low-damping characteristics common in mechanical systems. Strategies to avoid chatter without changing the machine or tooling configuration include selecting the cutting conditions based on a stability lobe diagram, reducing the axial DOC and/or radial WOC and choosing a low spindle speed to enhance the process damping effect. Strategies that change the machine and tooling configuration include adjusting the tool length (usually shorter is better, but not always) and applying tools with nonproportionally spaced teeth.

Tool/workpiece wear mechanisms. The strength of the cutting edge limits the chip load, and this limit is usually specified by the manufacturer. Edge strength does not limit the axial DOC because, as the tool cuts deeper, there is more cutting edge to carry the load. Several tool wear mechanisms, including diffusion and oxidation, are strongly temperature-dependent, and this kind of tool wear places a surface speed limit, which is a spindle speed limit, on milling. Similarly, some tools exhibit thermal softening at high temperatures, increasing the abrasion and attrition wear mechanisms, again limiting spindle speed. Strategies to avoid this limit include lowering the spindle speed, changing the tool material, adding a coating or using a small radial WOC.