Another reason micro hard milling is growing in popularity these days is that micro cutters are being offered in more expansive ranges of cutter reach lengths, provide longer cutting lives and generate near perfect surface finishes. And cutters are manufactured to exacting tolerances. For example, some cutting tool manufacturers hold diameter tolerances on their micro cutting tools to +0/- 0.0004” and to negligible amounts of tool runout, even in the shanks of the tools.

The smaller the tools, the more precisely they have to be manufactured. Flutes are tiny to begin with on micro cutters, and chip loads are slight. Any errors in the tool will be magnified when running at speeds in excess of 32,000 rpm for instance, and that will jeopardize part tolerances.
Micro tool geometries can differ substantially between micro cutter ranges, which could include square and ball end versions. Some are for non-ferrous or soft applications involving materials like aluminum, plastics and copper. Square-end tools are single-flute for optimized chip evacuation, while ball end versions have two flutes; and, geometries are extremely free cutting with very high positive rake angles. These are paired with a special carbide substrate and coating developed from aluminum cutters.

Other mid-range micro cutters have geometries developed for softer steels, such as stainless steels and Inconels. This group includes cutters with 4-degree rake angles and geometries that are not as free cutting as the non-ferrous tools, yet not as neutral as those for heat-treated steels. They also have their own unique carbide substrate/coating combination.

Still more micro tools apply only to hard/heat-treated tool steels, and again involve completely different carbide substrates, coatings and geometries. Geometries for the square end tools are neutral, with 0-rake and 0-helix. Ball end cutters do have some helix, but still 0-rake angles. A very specialized carbide substrate can be used for some heat-treated steels of micro tools. It is a much harder substrate best suited for heat-treated steels.

During operation, most of the principles that apply to larger diameter cutters also work for micro tools. As with bigger tools, a certain percentage of a micro tool’s diameter is used as a guideline for determining proper depths-of-cut for getting the longest cutter life and fastest cutting speeds possible. Although, shops may want to consider slightly higher percentages for feed-per-tooth amounts when running smaller cutters.

When it comes to micro machining, many shops often make two critical mistakes. One, they try to accomplish the process on machines with high enough spindle speed capability, but that lack the necessary control and accuracy for the task. The other is using subpar toolholding. Holders must be balanced and appropriate for the intended machine tool spindle–whether it’s 40,000 rpm or 100,000 rpm. The key is also extremely accurate clamping that provides as close to zero runout as possible. Toolholding systems such shrink-fit, hydraulic and certain collet types are most prevalent in micro machining applications.