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Cut-off Questions and a Good Tip

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  • Cut-off Questions and a Good Tip

    I needed to cut-off a piece of work that I'd turned. The part has a "flat" back that is the full diameter of the workpiece at the cut-off point. The stock is 8" dia. 6061 aluminum and the workpiece is 7" in dia. at the cut-off. I decided this would be a good time to try my new cut-off and grooving tool. The tool uses a MGTN-3 carbide (C5 PVD TiN coated)insert, which is 0.120 wide with 0 degree lead - so it's essentially flat in the direction of the cut. I used about a 400 sfm cutting speed (at the start).

    When I first started the cut, I could only get a foil-thin chip and had so much chatter - actually very high frequency vibration- that I figured I must not have set the tool high enough to get proper clearance. Oh yeah, I'm also trying the "upside down" tool bit trick that someone talked about a few days ago (and yes, the workpiece is turning the correct direction). The amount of tool-bit force required to get even this chip seemed excessive. Perhaps the tool was actuall too high, which would result in an almost positive cutting angle. A

    nyway, after fiddling around I was able to get a marginaly better cut - i.e I was getting more metal. However, the insert tended to gum up with a big slug of aluminum pretty quickly and the vibration continued.

    So finally to the point or at least the questions: Is the PVD TiN coating wrong for aluminum, or is the gumming and vibration just a characteristic of the soft material and taking a relatively wide cut with a blunt (what looks to me like a dull nosed) insert?

    As I got (finally) got deeper into the cut I seemed to get better and better results. part of this may have been dur to a better cutting angle as the diamerter got smaller. But, a large part of the improvement was due to something that may be useful to everyone - in addition to using the "upside down" cutting technique, I also started directig a hard stream of air into the cut groove just behind the tool to help clear the chips. Enough air goes around the insert holder that it dramatically reduces the chips that stay in the groove and "wrap around" to jam up on the top of the bit. Hope this works for some of the rest of you

  • #2
    On a very deep parting operation like this on a manual machine it is often a good idea to make two grooves side by side so that you have 1 wide groove to allow chips to clear without binding in the grove - this is important if surface finish is a priority! You have to work the parting tool on both sides of the grove (one pass on left, then right) as you progress towards the center.

    BTW, neutral inserts are not the best choice they are more of a grooving tool than for actual parting. For most parting operations you want no nub on the workpiece and by using a RH insert it cuts the nub off the parted piece leaving a smooth and shiney surface. On the other hand, if the piece you are parting is scrap - you would want a LH insert. If you are running the parting tool upside down, these would be reversed.

    Normally high pressure coolant directed from top and bottom is used on CNC machines in a single groove to clear chips. If set properly the chips should come out as small rolls of metal with a non-chipbreaker insert (not recommended for deep grooves BTW).

    ***very important*** parting inserts MUST be dead center to work properly use the steel ruler trick to insure it is centered otherwise they rub if set high or do not cut properly if low. Neutral inserts are the most critical.

    The chatter you got was because you did not have enough infeed or rigidity - it would not be as bad with RH or LH inserts. Use coolant.

    [This message has been edited by Thrud (edited 12-09-2004).]


    • #3
      Thanks for the input Thrud, some very useful info there. Your point about the insert needing to be exactly centered is well taken - I believe that is probably the root cause of the vibration problems I experienced. I don't think it was rigidity or feed rate. As the cut got deeper and I had to extend the tool holder, the chatter actually lessened. Although I agree that a very light feed rate would cause chatter, in this case faster feed rate - i.e. more pressure actually would increase the vibration. A "medium" amount of pressure actually seemed to minimize the problem.

      Also, you mentioned using coolant. I don't have a massive volume flood system, so I don't know how well that might work, but when I've used a medium volume coolant flow on aluminum it has caused the chips to stick together even worse and simply ball up around the toolbit. Thats' why I tried the compressed air and it seemed to work really well (other than lots of flying chips).

      AS far as surface finish on the cut-off, your points are well taken. In this case I was leaving some material on the "backside" anyway as I need to flip the piece to do additional work on that side. I can certainly see where the wider space would improve the chip exit path, but I'm not so sure how that long, thin cut-off blade (insert holder) would respond to the asymetric cutting forces when you are only cutting on one side. Also, does anyone have any idea what kind of insert will fit an MCIH 26-3 style blade that would have a RH or LH lead cut instead of the neutral cut that the MGTN-3 inserts give?


      • #4
        Let's see, you are parting off a piece that is 7" in diameter. So your parting tool must be out 3.5" plus a little. That is long ways for parting. I think that is where you trouble is coming from. I would start the job with the p/t sticking out only half of what you need and when you get as far as you can then move it out as far as you need to.

        Paul G.
        Paul G.


        • #5
          I started with the cut-off tool extended about 1" and kept extending it about a half inch more at a time just to feel it out. To be honest, even though it was working better as I cut deeper, I chickened out at aout 2 1/4" of extension. This was by far the bulk of the metal to be moved. Since I didn't care about the surface finish of the back, I just used a hacksaw for the rest of the parting - under lathe power down to about the last 1/2" and then I REALLY chickened out, stopped the lathe and cut the last little bit by hand.

          BTW, Stanley makes a very nice high tension hacksaw that has an amazing full 5" deep throat that works great for this.


          • #6
            MGTL-3 inserts would be left handed
            MGTR-3 inserts would be right handed

            the 3 usually stands for a 3mm wide insert

            I use a 1mm cut-off blade and almost always neutral inserts as I do more o-ring grooves than actual cut off . I use a narrow blade because some of the material I have worked with is several hundred dollars per inch - I do not have a deflection problem.


            NEVER use the hacksaw in the cut-off groove "under lathe power" unless you really do want to loose some body parts! THAT is a strict NO-NO! Don't do that again - it is extremely unsafe! AND NEVER SUPPORT THE CUT-OFF PIECE WITH THE TAILSTOCK EITHER!

            [This message has been edited by Thrud (edited 12-11-2004).]


            • #7
              As usual, excellent info Thrud. I early-on considered that if the hacksaw blade jammed in the groove, bad things could happen. All I could think to do was keep the lathe speed low, use a wave cut blade so the cut has some clearance for the blade and take my time. I'll reconsider next time. BTW, I actually was taught this method in a high school shop class - circa 1965 - but on a MUCH smaller diameter workpiece. (This piece is only 1.25" thick so the tailstock wasn't in use.)

              So, for pieces this large and larger, what IS the best method for parting off? Just get a longer cut-off tool and plunge on in? And where do you stop the cutoff operation, I certainly don't want a 7" dia. chunk of metal spinning off the lathe at 220 rpm! I guess if I had been gutsier with the cutoff tool extension, I could have taken it down to that last half inch like I did with the saw.

              Or do you just take it to the bandsaw (assuming you have one with a deep enough throat) and leave enough material to put it back on the lathe for a clean-up facing cut?


              • #8
                That is a deep parting off operation first off and Thrud and the others pretty much covered the best ways to do this. I saw that you mentioned you had tried coolant and it made it gum up more, I agree coolant is not your best option. A really thick cutting oil applied with a paint brush will keep those chips from sticking so much. Personally I would take the part to a band saw, it is way less stressful and will take less time.


                • #9

                  Mochinist has stated what most would do. Some projects might require the workpiece held in the chuck and the end cut removed - then finished. It all depends on the job dynamics - you do what you have to, but you do it safely so no one gets hurt.

                  The important thing is - it is better to slow it down an err on the side of caution than to run it too fast and panic.


                  • #10
                    Thruds right about never supporting the cuttoff piece with the tailstock, I learned that one the other day, good thing I was going slow, and it was a small part.
                    The part bent, DUH, and thank god for the flat belt on my southbend. Would of killed my bison chuck.