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  STAINLESS STEEL MACHINING

 

The three characteristics of stainless steel that have the greatest influence on machinability are its relatively high mechanical properties (including yield strength), its high work-hardening rate, and its ductility, which explains the material’s tendency to form a built-up edge during machining.

            For example, the chips removed in machining exert high pressures on the nose of the tool, and therefore tend to weld fast, producing what machinists call a “bug.” This causes the tool to run hot, slows down the job and interferes seriously with the finish.

            The austenitic stainless steels (300 Series) are not only troublesome because of “bugging” and chip disposal, but they work harden so that the tool, in passing over the work, will harden the surface and thus interfere with the next cut. The only remedy for this is to reduce the speed, increase the cut somewhat, if possible, and keep cutting. The tool must not be allowed to dwell on the work.

            The best mechanical method for chip control is to grind the tools with a fairly steep top rake, or lip angle. Tools with a 5o to 10o angle will generate less heat and be freer and cleaner cutting. Generous chip curlers or chip breakers are also a decided advantage. It is also helpful to stone the top of the tool smooth as an aid to skidding the chips. For “general purpose” drilling, twist drill makers produce a drill for drilling stainless steel. It has a shorter flute and overall length than regular drills and is therefore heavier and stronger. As sold from stock this type of drills generally pointed with an included angle of 140o.

            Where close tolerance and fine finish are necessary, it is advisable to use a shave tool with a light cut and fast speed. This tool should be sharply ground and stoned. Running at high speed and taking a light cut (0.002/0.008”) produces an excellent finish and holds to extremely close tolerance.

            Sulfur-based cutting fluids have been recognized for years for their ability to cool and prevent seizing. As a result, properly blended sulfur-base fluids have become the standard cutting fluid for machining all types of stainless steels. Here is a handy rule-of-thumb to use regarding the mixture, if the chips are welding to the tool, keep adding sulfur-base oil; if the tools are failing by rapid abrasion, add more paraffin-base oil.

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Last modified: November 29, 2001