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The truth about TiN.

The truth about TiN

In a recent discussion with engineers at Balzers Tool Coating and Niagara Cutter, North Tonawanda, NY, Tooling & Production found some secrets of TiN-coated tools. Number one, don't expect the hard coating to solve problems caused by poor tools or poor applications. TiN coatings will not convert a poor tool to a good tool. Similarly, TiN coatings will not change a poor application to a good application when a tool is improperly applied. Roger Bollier, president of Balzers, says that titanium-nitride coatings have been misapplied, only to give coatings a bad name.

According to Russ Wickins, vice president of field engineering, Niagara Cutter, "Chances are, the customer is going to pick his toughest job as the place to `try' TiN coating. And, while getting ready to accept an order for tool coating might have most sales representatives rubbing their hands in anticipation, accepting your customer's worst machining problem as the initial TiN coating candidate may create more problems than it solves."

Balzers feels that at least 30% of the time, a customer with tooling problems is using the wrong tool. Coatings usually can't solve that problem. Better to try TiN on an on-going successful operation.

A tool with edge chipping is another poor prospect for TiN coating. The remedy is to change tool geometry and upgrade basic tool material.

Again, TiN coating probably won't prevent tool breakage, unless the breakage is caused by chips packing in the flute. While TiN lubricity can help move chips out of the flutes, the user might also prevent this condition by selecting a drill with a more aggressive flute helix angle. Here's where you'll likely see the increases in tool life and productivity.

What good is TiN? Here's where it helps best: In cuts on low- to medium-carbon steels, stainless steel, aluminum, and fine-grain CPM tool steels; it can reduce chip welding, crater wear, and flank wear; and it can improve poor workpiece finish - solving problems of galling or torn, rough finishes. It can double or triple tool life and offer more consistent performance - especially important for HSS and carbide drills, taps, end mills, and other tools used in automated equipment. But the biggest potential cost savings are in increased productivity.

Proper application is vital. Boost feeds when using TiN-coated tools. Speeds may be faster, too, but the main benefits come from heavier feeds. And don't be afraid. PVD coating leaves compressive internal stresses, so that the tools can take greater shock for interrupted cuts.

The PVD coating process provides tools with harder, thinner, denser edges with sharper geometry (compared to tools processed by the chemical vapor deposition or CVD method). These characteristics may not be optimum for all applications, so, again, make sure you want these features. The TiN characteristics (hardness to 80 Rc) are certainly good for tools with positive rake.

Balzers offers TiN coating applied by PVD (physical vapor deposition), which operates at temperatures below 950 F. Thus, the process does not affect HSS substrate material hardness or dimensional size. CVD, on the other hand, operates at temperatures up to 2000 F - not suitable for coating HSS tools. CVD is an older process and requires less tool preparation than PVD, according to Balzers. (Mr Bollier says it doesn't require the special cleaning expertise of PVD operations.)

Balzers says CVD can reduce the strength of carbide up to 65% in some cases, requiring carbide-tool builders to compensate for coatings by changing the carbide formultion or tool geometry. The new lower-temperature plasma and altered CVD processes tackle this problem, but still operate at temperatures too high for coating high-speed steel.

Too much of a good thing? Improved chip flow allowed by the lubricity of TiN coatings works fine for end mills or other tools that have interrupted cuts. But, for twist drills over 1/2" dia, it can create a problem. In some cases, chips slide freely up the flutes without breaking. Even doubling the feed may not do the job. A redesign of the tool, or a better tool selection may be required. Also, in machining centers, increasing performance of a single tool within a group of tools may not increase the process production rate.

New coating

Balzers is testing a new coating said to be even more effective than TiN. It's a special layered Titanium carbonitride or TiCN. It will provide HSS tools that approach the performance of carbide tools, according to Roger Bollier. It will be have a lower coefficient of friction than TiN, and will provide even longer tool life, perhaps doubling the life of today's HSS coated tools. You can learn more about this new coating at Balzers' presentation at the IMTS-90 Technical Conference, Monday, September 10, at McCormick Place.

Toward permanent tools

While writing the PVD story, the T&P editorial office received a phone call from Paul Wierman, a vice president of Electro-edge, Los Angeles, CA. He announced a new electrochemical-conditioning process that can make HSS, cobalt, and carbide tools last three to four times longer than untreated tools - all without a coating. In fact, there's no reason to believe that coatings wouldn't grant another two to three times the life, but he hasn't tried that, yet, and cutting-tool manufacturers are probably glad that he hasn't!

The new process takes the stress out of sharpened or resharpened tools, provides further sharpening, and leaves cutting surfaces with a matte finish. It works especially well with drills and end mills, and may be helpful with taps. It works with precision-ground inserts, but has not been researched with molded types.

The user can buy equipment to do the job, and the process costs less than coating. Get more information by calling Mr Wierman at 818-882-4951.
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Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Title Annotation:tool coatings
Publication:Tooling & Production
Date:Sep 1, 1990
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