Metal Cutting Tools get a New Coat
Men (and women) have been cutting metal in machine shops since the late 19th century. They started with crude ideas and methods which have progressed to the current “high tech” technologies of today. The constant during these progressive years has been the cutting tool itself. The actual product that contacts the raw metal, cuts and removes chips from the raw stock, creates the finished work piece. Cutting Tools have evolved over the years from high carbon steel, to high speed steel, to cobalt, to carbide, and for specific applications, diamond and ceramics. You can see an example of cutting tools at www.kodiakcuttingtools.com .
In the late 1970’s a company named
Balzers started applying a very thin film to cutting tools, thus improving their overall life, in some cases, by 400%. This process is called “surface treatment” or “tool coating” and is used widely in today’s machine shops and manufacturing facilities.
There are several processes used to apply surface treatments, but the (2) main types used for cutting tools are PVD (Physical Vapor Deposition) and CVD (Chemical Vapor Deposition) and these are readily available from special coating facilities located throughout the world. My goal with this article is to share with the reader some common PVD and CVD choices, and reasons to utilize each.
The right surface treatment on cutting tools can increase tool life, decrease cycle time and enhance surface finishes. However, choosing the right coating can seem overwhelming at best. Each one has some distinct advantages and disadvantages depending on your individual set of machining in the machining process.
Coatings have several characteristics to be aware of:
Microhardness HV
Abrasion Resistance
Friction Coefficient
Max Service Temperature
Hardness
A high surface hardness from your coating is one of the best ways to increase tool life. Coating hardness is measured in units called HV, (Hardness Vickers). TiN is the start point with a hardness of about 2300 HV, followed by TiCN at 3000 HV, TiALN at 3500 HV and AlTiN ringing in at about 4000HV. Tools coated with these coating would work best in ferrous and non ferrous materials, such as stainless steel, carbon steels and aerospace materials like Inconel and the like. With a surface hardness near 10,000 Vickers, CVD diamond coatings applied to tools show up to 20 times improved tool life over no coatings at all. This is the coating of choice for production work in non-ferrous materials, especially aluminum and graphite.
Friction Coefficient
The lower, the better. A low coefficient of friction causes helps the tool slice through the raw stock with increased efficiency. This helps reduce heat at the cutting edge thus increase tool life. This slicker surface lets chips slide off the face of the tool, thus eliminating “built up edge”, or the “welding” of chips to the cutting edge. Also, it helps to eliminate the re-cutting of chips because they can evacuate themselves from the work area. Re-cutting chips in itself will ruin a cutting edge very quickly.
Abrasion Resistance
This is the ability of the coating to protect the tool from breaking down due to abrasion. Materials, such as Aluminum, glass filled plastics and Graphite may not be hard, they are extremely abrasive. As such you need to choose a coating that will be able to stand up to the abrasion of a particular application. Diamond is very abrasion resistant, but if you run a diamond coated tool in steel it will chip the diamond coating off as soon as the tool touches the work piece. I write this to show the tradeoffs associated with choosing a coating.
Max Service Temperature
Is the maximum temperature that a coating can stand up to. Generally Tin = 1,000 Deg F, TiCN = 750 Deg F, TiALN = 1,470 Deg F, Diamond = 1,100 Deg F. However, there is another key component you need to be aware of. That is the oxidation temperature. Coatings such as AlTiN and TiAlN do not become effective until the hit a certain temperature. The properties of TiAlN coatings make them ideal for high temperature cutting operations in many materials. When exposed to high cutting temperatures, TiAlN forms a hard aluminum oxide layer with low thermal conductivity and high chemical stability. As cutting temperatures increase, TiAlN and AlTiN insulate the tool and place the heat into the chips. This allows for higher speeds and feeds and longer tool life.
Common Coatings
• Titanium Nitride (TiN)
Microhardness = 2,300HV
Coefficient of Friction = 0.4
Coating Thickness = 1-4 microns
Max Working Temp = 1,100 Deg F.
Color = Gold
General purpose PVD coating that increases hardness and wear resistance. Mainly used on HSS tools for cutting steels and aluminum.
• Titanium Carbo-Nitride (TiCN)
Microhardness = 3,000HV
Coefficient of Friction = 0.4
Coating Thickness = 1-4 microns
Max Working Temp = 750 Deg F.
Color = Blue/Gray
General purpose PVD coating that increases hardness and wear resistance. Mainly used on HSS, Cobalt and Carbide tools for cutting steels and aluminum.
• Titanium Aluminum Nitride (TiAlN)
Microhardness = 3,000-3,500HV
Coefficient of Friction = 0.4
Coating Thickness = 1-3 microns
Max Working Temp = 1,470 Deg F.
Color = Purple/Gray
High Performance PVD coating that increases hardness and wear resistance. Mainly used on Cobalt and Carbide tools for cutting exotic materials, and dry machining steels. This coating is also used in the ever evolving world of HSM, or high speed machining.
• Chromium Nitride (CrN)
This coating works very well in copper and other “sticky” materials. Generally it has the slickest surface which won’t allow materials such as copper to stick to. This coating is used on taps also.
• Diamond
High performance coating used for cutting graphite, aluminum and glass filled resins. As mentioned above diamond coatings should not be used while machining steels. The shock will destroy the coating.
Choosing your coatings:
The best way to choose a coating is through trial and error. That sounds like a lousy way to do it, but how else will you know? You can start with your tool distributor’s recommendation, or go to companies like
Kodiak Cutting Tools for recommendations. Also, tool coater websites will be helpful like
Vergason,
Balzers, and
Northeast Coating. That’s only a starting point though. 9 times out of 10 it will get you very close and you’ll be happy with the results. Experimentation will take you the rest of the way.
As you can see there are a number of coatings you can apply to industrial cutting tools to increase their performance and shorten your cycle time. By using the coatings properly you will have a faster cycle time, less machine down time, better surface finish, less wear and tear on your machine spindle, and overall a more productive environment.
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