Constant Chip Loads Let Your Micro Machining Tools Live Longer

As part features get smaller, the challenges loom large for manufacturers, particularly those in the mold and die industry. After all, when the cutting tools can be as small as a tenth of a millimeter in diameter, a stiff breeze might cause them to break. But even when micro machining processes are perfectly stable, they may be suboptimal in terms of tool life.

Advancements in coatings, carbide substrates and cutting geometries, such as those Seco and Niagara Cutter have incorporated into its latest solid end mills, have resulted in up to 40 to 50 percent longer tool life, but even the best tool will run into problems with dirty collet bores, ambient temperature changes or unstable machine foundations. Even a perfectly balanced tool will instantly run out of balance if its temperature drops a single degree. But perhaps the most important aspect of the micro-milling process to consider is chip load consistency.

It’s far less easy to measure chip load in micro machining, given that average chip thickness will likely be tenths of a thousandth of an inch. Unfortunately, many manufacturers approach micro machining as something best solved with high-rpm spindle speeds, a cutting strategy that can actually impact chip formation in a bad way.

Good chip formation requires a balance between speeds and feeds, but this can be deceptive at the micro level. Even the fastest machines fail to accelerate or decelerate fast enough to keep up with spindle speeds. The results are poorly formed chips, a subpar surface finish and sharply diminished tool life. The solution? Speed up to slow down.

It may sound like an oxymoron – how can you go fast by machining slower? – but after figuring out the average functional feedrate, a shop can then adjust the spindle speed down to the appropriate rpm. Even when depths of cut are as small as 0.001", the same mechanical actions, the same heat and pressure, is required to cut the chip instead of just pushing it around.

For example, a cutting program set to 40,000 rpm at 50 ipm is well within the range of advanced machine tools, but because of the short micro-milling cutting paths, it simply can’t feed that fast. Instead, the actual average speed is 25 ipm, and a reduction in spindle speed to 20,000 rpm reestablishes equilibrium and a constant chip load.

In developing these processes, trial and error is one option, but naturally, tooling OEMs are best equipped to help with these calculations. Given the huge range of materials and tools in use in today’s manufacturing industry, manufacturers will typically be the best resource for cutting data for unfamiliar materials.

Whether it’s components for lifesaving medical devices or the scored grip on the bottom of a tube of lip balm, manufacturers now need to generate the smallest of part features, often with molds. And as these molds continue to shrink in size, tooling manufacturers like Seco and Niagara Cutter will provide the techniques and tool geometries needed for the smallest cuts – and the biggest productivity boosts.

Cutting Tool Coating Chemistry 101

Manufacturers continually push the envelope in terms of materials and processes, which means that their tooling suppliers must do the same. As a result, inserts and solid tools alike now appear with a wide range of coatings in a muted rainbow of golds, blacks, silvers and blues. And each has its own complex grade designation and chemical formula. No matter how complex these coatings appear, however, one doesn’t need a degree in chemical engineering to understand how these materials work.

It helps that coatings are relatively new in metal cutting, with the first coated cemented carbide tools appearing in the 1970s. On the other hand, the main processes used to produce these coatings – physical vapor deposition (PVD) and chemical vapor deposition (CVD) – have been known for centuries. PVD is a low-temperature process that creates a thin coating that protects and strengthens sharp edges, whereas CVD requires higher temperatures for a chemical reaction that leaves a thick, wear-resistant coating.

However, the only key difference between PVD and CVD involves how the coating material is initially vaporized in a process called sublimation, in which a material passes from the solid state to the gas state with no intermediate liquid stage. Typically, this material is titanium, which reacts with nitrogen and/or carbon during the coating process to produce titanium nitride (TiN), titanium carbide (TiC), or titanium carbonitrides (TiCN). These innovations served as the foundation for most modern coatings.

Today, many tools and inserts feature titanium aluminum nitride (TiAlN), which reacts with high temperatures to form a layer of aluminum oxide (Al2O3), which adds further hardness to the coating. Other coatings have introduced ceramics like silicon nitride (SiN), which particularly excel in high-temperature applications. And rather than use heat to force the formation of Al2O3, manufacturers have coated tools in it directly for the highest level of crater and chemical resistance at high speed and high temperatures.

While understanding the principles behind cutting tool coatings is simple, however, the finer details get increasingly complicated. For example, the Duratomic® coating developed by Seco involves the molecular manipulation of coating components to produce a layer of 100% α-Al2O3, a stable crystalline structure similar to that found in sapphires and rubies. Duratomic is also one of many coatings that use multiple processes and materials to form layers of Al2O3 and various titanium compounds.

Luckily, tool manufacturers like Seco make it easy to use printed catalogs or online tools to find the right coating for a given material and application. To learn more, visit secotools.com or speak with your local Seco distributor.