Coatings fight rust on toolholders: the combination of damp air and iron or steel adds up to a dirty result that is common in manufacturing--rust. (Workholding/Toolholding).
Stainless steel is resistant to rust, but since toolholders are not considered premium tools, the investment is not often justified. Total prevention is basically out of the question. Maintenance becomes the focus. With advanced cutting tool technologies and the use of high-pressure through-the-spindle coolant, it is not uncommon to find cutting tools operating for up to six months without a tool change. Combine this with the increased use of more environmentally friendly water-soluble coolants and lubricants, and rust prevention becomes a full time job.
Toolholders will last a long time if they are cleaned and handled properly. That includes frequent cleaning with rust inhibitor solutions, as well as oiling when placed in storage. However, if the toolholder sits idle in a spindle for extended periods of time, it will rust. Removal can become difficult and damaging to mating surfaces of the spindle and toolholder.
For the most part, manufacturing today takes more care in handling the newer HSK tool holders, but V-flange, Acme, and traditional designs are basically "beat up."
So what can be done to inhibit the corrosion?
In most cases, coatings are the answer. But the overall success of these coatings varies widely, according to Lee Flick, director of manufacturing and engineering, T.M. Smith Tool Int'l Corp. (Mount Clemens, MI). For example, for years, both hot and cold applied black oxide has been one of the universally accepted corrosion inhibiting toolholder coatings. Because black oxide is an acid bath process, the coating inherently has a tendency to stick to itself. As a result, final grinding of the finished mating surfaces after coating is necessary, which defeats the purpose.
Various types of chrome coatings are also used. The added benefits of using chrome are the lubricity and wear properties that the coating provides. However, harsh environmental humidity bath tests that T.M. Smith has conducted reveal that the rust inhibiting properties are really no better than black oxide. Newer versions of chrome coatings include an added polymer coating, similar to what is being used on rubber injection molds. This coating is applied with a dipping process. Similar harsh environmental tests have shown that the polymer chrome coated tool holders are a big improvement and exhibit no perceivable rust over a 10 week testing period.
The newest tool holder coating involves the use of chemically applied electroless nickel. Initial tests have shown this to be the best rust inhibitor coating, compared with black oxide or chrome. Additional testing is still underway to finitely determine the wear characteristics of the coating as well.
All of the aforementioned coatings provide some rust inhibitor benefit. The more the benefit, the greater the added cost. In terms of percentages, black oxide typically adds a 2% premium, chrome coatings with or without the added polymer range from 10% - 15%, and nickel coatings adds 20%.
Rusty tool holders are a problem and always will be a problem for manufacturing systems all over the world. There's no magic to solving the problem. It's a matter of the right combination of prevention and inhibition and the value placed on it. T.M. Smith Tool Int'l Corp.
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|Publication:||Modern Applications News|
|Date:||Dec 1, 2002|
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