Coating of tools increases surface hardness and durability to extend their life and minimize body erosion.

Manufacturers of tooling, especially for CNC routers, offer a wide range of choices. There are basic issues like choosing upcut, downcut, compression and V-carve. There are choices to be made for geometry, flutes, sizes, metallurgy, feed and speed rates. And then you have coated tools!

There is a rainbow of tooling coatings available, with just as many claims as colours. Leitz maintains that coatings can greatly enhance tool performance and has invested in in-house coating processes and equipment.

But why you should consider coated tools? Surface treatments for tools are designed to accomplish three performance enhancing attributes:

•        Increase surface smoothness to resist build-up of material (pitch, resin, glue) that would trap heat.

•        Increase surface hardness and durability to extend tool life or minimize body erosion.

•        Improve cutting/ friction heat deflection, to slow thermal corrosive dulling of the cutting edge.

Coated tools are said to offer a combination of increased cutting-edge hardness, improved heat resilience to extend tool service life, improved cut quality and greater process efficiency.

Heat corrosion

Besides good manufacturing practices to select correct feed and speed rates, coatings are designed to help boost the prevention of damage to tooling from heat.

Heat is the biggest threat to overall tool-life in any carbide tool.

Any machining process, due to the spindle RPM and friction on the tool’s cutting edge, invariably generates impressive quantities of heat. Heat also encourages other aspects associated with breaking an otherwise keen cutting edge down, including chemical reactions and mechanical stressors.

‘Hot corrosion’ can be described as a microscopic attack of the cobalt binder contained within the tungsten carbide matrix at the surface of the cutting edge.

But the results of a proven scientific coating can be dramatic, often resulting in impressive gains in useable RPMs and faster traverse rates. A full 40% improvement in all these fields is common, all of which is directly attributable to an appropriate ceramic coating.

Coating combos

Coatings on CNC tooling are made up of combinations of chemicals and elements that are applied using either a process called physical vapor deposition (PVD) or chemical vapor deposition (CVD). This is usually done by the manufacturer, but some tooling sharpeners also offer coating services.

Coatings are deposited in layers until the desired compounds and thickness have been achieved. They can range from relatively common Titanium Nitride (TiN) to super-hard and wear-resistant amorphous diamond options such as DLC (diamond-like carbon) coating.

From the brilliant gold (TiN) and dull gold ZrN (Zirconium Nitride) coatings to bronze-coloured coatings, to nearly black, to various hues of blue and/or red, they all have certain characteristics that either lend themselves to a particular workpiece material or discourage their incorrect use.

Certain coatings seem to work better in certain materials. The compatibility of certain coatings varies depending on the workpiece material.

If you are cutting a lot of melamine coated particleboard, you might want a different coating than if you are cutting plywood or solid wood. Or the AlTiN coating that is good for high carbon steels or cast iron might not suit cutting of aluminium.

Nearly all coatings are very thin (measurable only in microns) but are very durable because the coating is effectively etched or impregnated into the tools’ somewhat porous base carbide or steel material. Some nano-composite coatings and DLC may come in various hues, depending on the “recipe” of the coating.

Re-sharpen or no?

Different coatings last different times in different materials. Certain woods might deposit tars and resins on cutting tools that obscure the coatings, but mere cleaning can sometimes restore the tool. But can coated tooling be re-sharpened?

The re-grinding or re-sharpening processes can remove the coating from at least one cutting edge (face or relief), but the remaining coated angle will still provide some degree of performance enhancement.

Some dedicated users of coated tooling focus on the extended tool life – they simply replace the tooling, rather than re-sharpen it! No re-sharpening also means users don’t have to worry about changing tooling parameters in their software after a bit size inevitably changes when it comes back from the re-sharpening shop.

With all the marketing emphasis on coatings and colours, some operators might be convinced it is more hype than science. But clearly coated tooling is getting increasingly popular with many users worldwide.

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