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16806Re: [multimachine] Re: Large lathe for flame spray operations

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  • David G. LeVine
    Feb 3, 2013
      On 02/03/2013 10:16 AM, Dr. Alexis O'Neill wrote:
      We need to finish a 316 stainless steel thermal sprayed coating on a carbon steel plate, well, several, but this is the sample, and we can't seem to get the job done. We've done everything up to the first sealing correctly, the technicians think, and this type of coating has a hardness of 81 HRb only, not very hard. Yet the machine shop, using a fly cutter, burned up 7 cutting blades trying to finish the sample, which was only 63cm by 30cm on 1" carbon steel, which makes no sense. I have samples on plates and cylinders sent to me from our suppliers in England and Asia and I have been assured by everyone there that fly cutting the stainless steel is no problem, but we can't seem to get it done. We have taken samples to 2 machine shops here, one of which has around 15 CNCs and plenty of experience, but they said it was too hard a finish. One of my engineers brought me one of the small cutting blades, I don't know what to call them in English, about 3/4 " square and 3/8" thick, and it indeed was blacken with the heat during cutting. The edge was smoothed completely. They ruined 7.

      Hmmm...  Okay, let's go back to basics.

      Stainless is NASTY to machine, it work hardens.  If your feed is too slow, you won't be able to machine it, ever.

      If your SFM is too high, stuff burns right up, so generally it is large chip load, no fine cuts and slower than... 

      I can assume this is what you are doing, but if not, don't feel too bad, I have seen a huge number of burned out tools from too light a cut on SS.

      Another well known issue is machine rigidity for exactly the same reasons.  The machine flexes a little (or is a little loose), the bit rides on top of the affected layer and it can't ever get back.

      Some experiments:


      Try fly cutting as slow as your machines can go (on the order of 35-70 SFPM, no faster!) and cutting completely through the stainless in one pass.  It will destroy the test piece, don't worry.  Watch the cutter, if it gets hot STOP!  You are running way too fast, cut the RPMs in half, the chips should NEVER turn blue, at worst brown or straw colored.  Eventually you will find a feed and speed which work.  For a 100mm diameter fly cut, I would try 30 RPM or less, I don't know the feed or depth of cut, but I would guess it would be pretty high and the chip load would be pushing the machine's limits.

      You also need to look at tooling, TiAlN coatings (the purple ones) or TiCN or one of the new SilliconNitride/Aluminum Titanium Nitride  coated tools may be a big win in dry cutting.  Read complete details in this Cutting Tool Engineering article.

      Now, take another test sample and prove you can fly cut ALL the stainless off without destroying the tooling.  Once you can do this, try leaving a VERY thin layer of stainless.  My guess is that you will find the ripples will be horrible because the machine is not rigid enough, or the tool will start to burn up.  Slow down and try again.  After a short while it should be obvious that feed and speed are critical and you will probably find a few combos which allows you to cut the coating without getting molten chips.  After that, it is just fine tuning.  Remember, using coolant to find the range which works and then going dry and getting the new parameters is not a bad idea. 

      This chart:

      may be of help.

      You may also find an FDA approved cutting fluid (like soda water) which will transfer heat out of the cut, my best guess would be to find a food grade surfactant (detergent) to use in the water, if only to pull heat out of the cut.  Cool the tool bit and it might just last much longer.

      Does this help at all?

      P.S.  There  is no room or the chart would also be in the FILES area.

      Dave  8{)


      "Among the many misdeeds of British rule in India, history will look upon the Act of depriving a whole nation of arms as the blackest."

      Mohandus Ghandi, An Autobiography, Page 446.
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