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  • Taper when turning question

    I was turning down some 416SS, TMNG insert of questionable origin, .050" deep .008" feed ,chips are coming off great and all over the shop. When I checked the OD it was within a half thou over 3" which is pretty good. As I got close to the final diam. I switched over to a HSS tool bit to get a better finish. Now as I checked the diam. it was tapered by .0015 to .002" over the same length.

    OK, is it the different tool shape,
    Different tool pressure during the cut IE: depth of cut
    type of tool material, cutting differently

    or something else. Is this normal or just annoying.
    The shortest distance between two points is a circle of infinite diameter.

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

    Southwestern Ontario. Canada

  • #2
    Fatter at the free end? I'd say that there's more pressure from the HSS tool. Maybe a trifle dull? Wrong angles for the type of material? Build up on the cutting corner making it act dull and pushing the stock away from the tool? If this last is the case some cutting fluid might aid with stopping any buildup.

    A good test to see if the tool is putting pressure on the stock to bend it away is to draw the carriage back without retracting the cutter. If it cuts a spiral path then it wasn't cutting clean on the way in and there was pressure pushing the stock and cutter apart. And when that occurs the part can often end up with some taper.
    Chilliwack BC, Canada

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    • #3
      I would suspect the problem could go both ways. All of the usual suspects could be causing it- tool sharpness, relief angles, height of the cutter, build-up on the cutting edge, poor tool choice for the material, etc.

      You might try a two collar test, and pay attention to what happens with spring passes. Maybe mount a freshly sharpened cutter for a 'clean up' pass, and carefully monitor or control all the plays on the machine, just to know that it isn't something ambiguous with the machine.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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      • #4
        Makes me wonder if the HSS isn't getting dull in the middle of the cut. Stainless would do that.

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        • #5
          I did stone the tool before cutting but that isn't a guarantee that it was sharp enough. 416 SS is a free cutting variety, easy to turn.
          The shortest distance between two points is a circle of infinite diameter.

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

          Southwestern Ontario. Canada

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          • #6
            This is a case where those aluminum-cutting sharp carbide inserts are really handy and work better than HSS. You can take off just a thousandth or less if needed and leave a good finish, and they stay sharp through the entire cut so you shouldn't have any taper.

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            • #7
              I have TNMG in aluminium grades which give 6 extremely sharp edges, ideal for finishing Stainless. I also have them in a grade for stainless which is a useful halfway house between the general steel grades and the aluminium grades.
              Last edited by old mart; 01-14-2020, 01:45 PM.

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              • #8
                Originally posted by loose nut View Post
                I did stone the tool before cutting but that isn't a guarantee that it was sharp enough. 416 SS is a free cutting variety, easy to turn.
                Nose radius or perhaps not the right top rake for SS? How many diameters worth of stick out on the stub you'd rough turned?

                Depending on what I'm turning I run into the same thing now and then. And it's often a combination of things. Although in your case you had good results with the insert and only ran into trouble with the HSS. So I'm guessing it's something about the angles or shape of the HSS tool. Perhaps not enough back or side rake on the top side? The grooves in the inserts are not easy to see clearly all the time and can mask some fairly aggressive rake angles that aid the cut. Angles that are far more obvious if they are present or not on the flats ground on an HSS shape. Also just the roughness of the finish on the top side of the tool from the grinder might make it more "sticky" to the chip coming off and cause build up. Perhaps try stoning the top face to a nice smoothness as a trial?

                That's a lot of things in one paragraph. A bit of early morning free association I'm afraid....
                Chilliwack BC, Canada

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                • #9
                  whats the length, diameter and how held? speed correct? unless something is going horribly wrong, you're not dulling the hss appreciable in a finish cut with 416. Unless its an unusual grind, a normal hss should produce a lot less cutting force. Off the cuff, the roughing was a lot higher force, it would take so much off, then the work deflects. Lighter loads on finishing deflect the work less and is a truer picture of bed/tailstock alignment. Lots could flush that theory though....like any sense how the lathe usually turns for that size of work/set up?
                  .

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                  • #10
                    The lathe usually cut fine with little taper but that seems to depend a lot on the material. Some get a bit of taper and some don't.

                    416 cuts like butter more lick ordinary steel then any type of SS.
                    The shortest distance between two points is a circle of infinite diameter.

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

                    Southwestern Ontario. Canada

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