Manufacturers of tooling for CNC routers offer a bewildering array of choices. Once you get past basic issues like choosing upcut (maximum chip evacuation), downcut (clean top surface edge), compression (clean top and bottom cut edges but less chip evacuation), and V-carve (for 3D shaping), you still have to sort out geometry, flutes, sizes, metallurgy, plus feed and speed rates.

As a new CNC user myself, I confess to being quite confused, and then came coatings. There is literally a rainbow of tooling coatings available with just as many claims as colors. And while most tooling manufacturers do offer various coatings for woodworking tooling, a few actually suggest that coatings are not so important for wood applications.

Trying to sort all of this out, we reached out to tooling companies and experts we know and trust to try to give us information CNC router users need to make intelligent choices for tooling purchases. Warning: The experts don’t all agree on everything, but we’ll still try to sort the sizzle from the steak.

Why coatings?
Let’s start with Mark Alster, national sales manager for Leitz. He offers a straightforward explanation of why you should consider coated tools. “Tool coatings or surface treatments are designed to accomplish three performance enhancing attributes,” said Alster: 

• Increase surface lubricity (smoothness) to resist build-up of material that would otherwise trap heat (pitch, resin, glue).
• Increase surface hardness and durability to extend tool life or slow/minimize body erosion.
• Improve cutting/friction heat deflection, to slow thermal corrosive dulling of the cutting edge. 

On that last point Alster cautions that the first step in heat management is not the coating but selection of correct chip load through feed and speed settings.

According to Alster, coated tools offer a “combination of increased cutting-edge hardness, and improved heat resilience to extend tool service life, while providing improved cut quality and greater process efficiency.”

Battling heat
Experts at Amana emphasize the role that coatings play in battling heat in machining. “To truly appreciate the value of applying scientific coatings to a cutting tool, we need to first appreciate what the #1 threat to overall tool-life is in any carbide tool. Heat,” according to Amana.

“To be more specific, any machining process, due to the spindle RPM and friction on the tool’s cutting edge, almost 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,” said Amana. “Combined, this is known as “hot corrosion,” and it can be essentially described as a microscopic attack of the cobalt binder contained within the tungsten carbide matrix at the surface of the cutting edge.”

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.

“The results of a proven scientific coating can be dramatic, often resulting in impressive gains in useable RPMs, faster traverse rates, and/or impressive gains in tool-life before the workpiece’s surface finish begins to degrade,” according to Amana. “A full 40% or more improvement in all these fields is common, all of which is directly attributable to an appropriate ceramic coating.”

What are coatings?
So, now that we understand what coatings are supposed to do, let’s explore what exactly they are. 

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,” said Alster. “Caution, thinner is usually better, as coatings can round and dull the cutting edge as well.” Coatings can range from relatively common Titanium Nitride (TiN) to super hard and wear resistant Amorphous Diamond options such as DLC or Diamond-Like Carbon Coating.

Rainbow of choices
That brings us to all the different coating colors and what different kinds of coatings are supposed to do.

“Not too long ago, you had two choices in router tooling, which were no coating whatsoever or perhaps a brilliant gold coating referred to as TiN (Titanium Nitride),” according to Amana. “That has developed in the last few years to more options, such as a dull gold Zirconium Nitride, to bronze-colored coatings, to nearly black coatings, to variegated hues of blue/red. All of them have certain characteristics that either lend themselves to a particular workpiece material, discourage their use, or leave the end-user unimpressed.”

To further complicate things, certain coatings seem to work better in certain materials. 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.

“The compatibility of certain coatings varies depending on the workpiece material,” said Amana. “Certain heritage coatings — once held in high esteem for many years in the metal-working theater — may have little-to-no impact on many modern work materials or might actually work against the end-user. Case in point might be the extremely common AlTiN, that, while wonderful in low to high carbon steels, exotics, or cast iron, would possibly create issues while cutting aluminum.”

How long do coatings last?
Knowing that coatings are very thin, only a few microns, how long do they last and what about resharpening tooling?

“Most coating bonds are very durable, as the coating is effectively etched or impregnated into the tools’ somewhat porous base carbide or steel material,” said Alster from Leitz.

“Certain nanocomposite coatings and DLC (Diamond Like Carbon) can exhibit an attractive variegated hue, a product of the particular “recipe” of this coating or that, while in the crucible chamber,” according to Amana. “That coating might not be any thicker than 2-3 microns, but the hardness and lubricity might be beyond impressive, and will retain those features until the tool has eventually been used long enough so that the heat inevitably gets into the tungsten carbide matrix.”

Some tooling users actually use the coating as an indication of tool life. “Many shops count on the brilliant gold color of a Titanium Nitride coated router or drill, NOT for strict performance values so much but because the absence of that gold hue on a specific portion of the tool can indicate either an edge failure, or the inevitable wear resulting in the need to swap out to a new tool,” according to Amana. “Old eyes and dim lighting in certain shops make the ability to see a chipped tool nicely visible, which is a kind of benefit.”

Different coatings will last different times in different materials. Certain woods might deposit tars and resins on cutting tools that obscure the coatings but don’t remove them. Judicious cleaning can sometimes restore the tool.

Can they be resharpened?
A key question is whether coated tooling can be resharpened and, if so, does the coating lose its value?

“Regrinding/sharpening processes will 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,” said Alster. “If a tool can be OD ground only, the coating will remain on the face of the cutting edge and provide the greatest secondary performance. If the tool must be face or flute ground (scalloped roughing cutter) the coated peripheral edges will still provide a harder cutting edge at the outermost point.”

Some dedicated users of coated tooling focus on the extended tool life and simply replace the tooling rather than resharpen it. Southeast Tool claims their carbide tools with a proprietary coating will run 30 to 40 percent longer without issues. 

No 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 sharpening shop. On the other hand, more sophisticated sharpening operations today like Ekstrom-Carlson sometimes offer recoating services. And we did talk to a couple of tooling companies that suggest that it’s preferable to use uncoated tools in woodworking and resharpen or move up to diamond tooling for maximum tool life and performance.

Sizzle or steak?
With all the marketing emphasis on coatings and colors, it’s no surprise some people might be convinced it’s more hype than science. But clearly coated tooling is popular with many users.

“Our selection of PVD coatings is driven by the demands of our clients, rather than a marketing team,” said a spokesman for Amana. “Our end-users know far more about their processes than we do, and if they feel they are getting added value with ZrN or a Spektra coated tool, then we will do everything we can to make that coating available at a fair price. If a coated tool lasts up to 2.5 times longer than an uncoated tool, we’ll sell fewer tools, yes, but we’ll have the client’s best interests looked after.”

Mark Alster at Leitz has a similar take. “Performance coatings can greatly enhance tool performance and value, so much so that Leitz has significantly invested in in-house coating processes and equipment to maximize the performance value,” he said. “Many of our products are now only available in coated form because we recognize the significantly improved performance value to the customer.”