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Surface Finish Question

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  • Surface Finish Question

    2 or 3 nites ago I was facing off a 1.5" round steel bar (of unknown heritage) and getting a very rough finish (HSS tool bit, power infeed). After getting it faced and center drilled, I cranked the RPM up some and took a couple of facing cuts with power out feed. That improved the finish somewhat for about the inner half of the part tho still a little rough, then it VERY ABRUPTLY changed to really smooth. Now I'm aware that all materials will have optimum cutting parameters, but I'm puzzled as to why the transition from rough to smooth was not just a gradual change. The crossfeed seems to be working normal. I made 2 or 3 cuts, all w/same result. Any thoughts/ideas?
    Lynn (Huntsville, AL)

  • #2
    Could it be harder in the center than at the surface, or vice versa?


    • #3
      can't remember if it was 2 or 3 nights ago? Gee, and I was worrying about my addling mind

      More than cutting speed, it sounds to me like chip removal must have changed as a result of the change in SFM. Try increasing your rake angle a little bit for a cleaner finish.



      • #4
        From the sound off it you were facing from the center out? It would be worse in the middle area and then at some point it cuts smooth. If you measure this diameter and then calculate the SFM you you will find that this is the magic number for THAT material.

        This is the very reason I got a VFD so I could double the speed of my lathe - the finish on small parts was unacceptable at 2800RPM. I still get a small tit of bad surface finish in the middle at 6000rpm that I cannot get rid of, but most people would never notice anyway. I need 20,000 rpm for that...

        It is a interesting way to determine base cutting rates. Other factors for carbide include too slow and too fast. The last one is difficult to do with modern carbides pushing the 2,000SFM mark. It should be noted that pushing the speed will improve surface finish if you get tearing.


        • #5
          Albert, in my case I consider it pretty good to remember in which year something happens, especially in Dec or Jan.

          I'd always notice a poor finish w/in about 3/16" of center when facing, but I usually see a more gradual transition. In this case the roughness, which looked like 'tearing' as Thrud described it, extended out about .5 from ctr. I'd also tuned around w/feed rate and that had no noticable effect.

          Mine is an old Leblond (gearhead) that only goes up to 500 rpm. I've wondered how well it would tolerate higher speeds. Of course it is nice to have 25 rpm for threading, etc.
          Lynn (Huntsville, AL)


          • #6
            Anyone heard of the "Whittaker Ring"? I remember that we were told, years ago, when facing off there is sometimes a change from coarse to smooth finish...and this presumably is Whittakers ring. I must admit that I have never found a use for this piece of information.... until now.


            • #7
              Whittaker's ring. I like it. That's what I got, Whittaker's ring!
              Who's Whittaker?
              Lynn (Huntsville, AL)