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  • Quality of finish

    When i machine on a cold roll shaft on my 15x40 lathe i can get a nice finish if i take heavy cuts 40 or 50 thou but when i take a smaller 10 and under cut i seem to get a rough finish, what could i do to fix that? I am using carbide on a 1 1/2 shaft running a 1000 rpm on the heavy cut i get small blued chips but on smaller cut i get ribbon Thanks Denis

  • #2
    Look up the Cut vs Feed chart for the particular carbide insert being used and you will find there is a window of working for a particular insert. To light a cut and it will not work well or give a good finish.

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    • #3
      Some types of materials won't give you a nice finish with a small cut unless you crank up the rpms.
      The shortest distance between two points is a circle of infinite diameter.

      Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

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      • #4
        Or go to a very sharp HSS cutter for the final.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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        • #5
          Or just change the carbide to a finishing grade. Multiple tool holders on Quick Change make this easy.
          Last edited by lakeside53; 02-20-2013, 01:49 AM.

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          • #6
            Originally posted by Deny1950 View Post
            When i machine on a cold roll shaft on my 15x40 lathe i can get a nice finish if i take heavy cuts 40 or 50 thou but when i take a smaller 10 and under cut i seem to get a rough finish, what could i do to fix that? I am using carbide on a 1 1/2 shaft running a 1000 rpm on the heavy cut i get small blued chips but on smaller cut i get ribbon Thanks Denis
            This is only ≈ 400 fpm.

            If you have the spindle speeds available, try 600-800 fpm.

            Dave

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            • #7
              Getting ribbon from an insert tells that the chip breaker isn't functioning, meaning that the depth of cut is not enough for that insert and/or feed rate is too slow.
              Amount of experience is in direct proportion to the value of broken equipment.

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              • #8
                I am a beginner compared to most of the guys on this forum so take what I say in that light.

                I have found that I have to take a deeper depth of cut with a high feed per revolution to get a good surface finish. Also I find it easier to "hit" my target diameter if I am not timid in my cuts. I was turning using Walter WNMG with a nose radius of .8 and getting bad finishes. Pixman on here told me to crank up the RPMs and feedrate. So I went from my 800 rpms to 2000 and took a 1.5mm cut per rev and now I can shave in the finish and find it easier to hit my diameter.
                How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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                • #9
                  Recently I had some major stock removal to do so I had time to experiment. Finish started improving as the cuttings turned straw and continued to improve as they turned blue to black.
                  Byron Boucher
                  Burnet, TX

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                  • #10
                    Ok, I guess most non-carbide users are not aware of this, but...

                    If you can get a good finish by cutting .020"
                    but you get a bad finish when cutting .010"
                    then try to adjust the amount taken per pass
                    so you end up taking incriments of .020" when
                    you get close to your finish pass.
                    So if you need a finish dia of 1.000",
                    then before your last cut, your piece should
                    measure 1.020". Take that last .020" cut and
                    you are right on final size.

                    This concept scares the crap out of many
                    new machinists, but it is how it is done
                    when using carbide tooling.
                    Standard operating procedure making
                    real parts for real money.

                    --Doozer
                    DZER

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