Maximize Tool Life in Micromilling

by Jay Ball, Product Manager, Seco Tools LLC

Micromilling compresses the challenges of maintaining tool life into a tiny working scale at which the cutting edges of these tools measure about the same size as the grain of the grinding wheels used to produce them. Fortunately, micromilling typically does not create the harmonic vibrations and chatter that often cause problems with long standard-size tools or during heavy conventional roughing operations. Instead, the basics of maximum micromilling tool life start long before the cut even begins, with criteria as fundamental – and as diverse – as tool design and selection, toolholding and material properties.

A well-balanced, rigid toolholder and the proper selection of machine tool feeds and speeds go a long way toward maximized micromilling tool life, as does starting with the right machine tool – and the proper cutter for the job – in the first place. Tool geometry improves surface finish and boosts tool life, if the tool geometry matches the specific workpiece features at hand. Additionally, coatings provide a thermal barrier to protect the tool from wear when it machines extremely hard materials, and the alloy chosen for the tool substrate makes a critical difference in terms of toughness, which helps promote a greater bond between substrate and coating.

First and foremost, of course, alloy selection translates to greater tool life when the properties of the tool match the hardness and abrasiveness of the part material. Substrate hardness, particularly in carbide tools, heads the list of criteria related to tool life. Hardened tool steels run between 48 and 65 HRc on end mills and slightly lower on inserts. A tool needs greater hardness than the material it cuts, and some materials present two forms of hardness, one on the surface and the other below it. A material with a hard surface and a hard, abrasive particle, for example, presents a special challenge where tool selection is concerned because it cuts like a harder material than its specifications indicate.

Along with tool-selection criteria, shops also must monitor such parameters as toolholder collet-bore cleanliness, machine stability and consistent operating temperature to ensure best results. A poorly cleaned, neglected toolholder can harbor chips and grit that cut tool life when they cause wear or interfere with secure clamping. For that matter, a subpar toolholder chosen for its price alone and not for its clamping capabilities will cause higher runout – and high TIR is the enemy of tool life. Runout of only about 0.0004" will cut tool life in half, so drastic toolholder economizing rarely yields the performance necessary for optimal tool life.

In addition to tool setups, shops need to make the right choices in programming their equipment for micromilling. Today's machine tools offer feeds and speeds that were considered impossibly high in the past, so it's understandable that shops want to take advantage of such full capabilities and run as fast as possible. But the intricate geometries of many micro machining workpieces – complex mold structures, for example – require slower speeds to keep up with the feed rates necessary for proper chip formation.

If machine speed exceeds the point of proper chip formation, the tool rubs instead of cuts the workpiece, pushing the material around and roughing up the surface. Rubbing produces a visibly inferior surface finish and makes for a difficult finish cut. With a proper chip load, the tool produces a consistent finish that offers 50% more surface integrity, which reduces the amount of additional work required to achieve completed parts. (Check out a previously run Seco blog that goes into greater depth on the importance of maintaining constant chip load.)

To maximize tool life and workpiece quality, therefore, shops need to let feed rates determine machining speeds. As a general rule, a machine tool should maintain its programmed feed rate for 80% of the expected cycle time. When speeds and feeds prove unsustainable, reduced RPM and feed rate create a sustainable working pace.

To set up micromilling projects for success, shops can rely on toolmaker data to help them optimize RPMs, feed rates and chip production. Seco Tools develops and distributes cutting data that represent optimal combinations of angle of engagement, feed rate, stepover, surface footage value and material type, providing baselines that enable shops to achieve micromilling part production success – and longer tool life – without all the guesswork.