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When parting on the lathe, why?

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  • #31
    Originally posted by Mcgyver View Post

    Id say its the opposite. Short parting its easy with ground hss bits like Jerry posted, its deeper heavy cuts that really benefit from the insert's side clearance geometry.
    Totally agree. The insert tools that are in the pics will do close to 2", and do it MUCH easier and with less drama than any HSS tool I have used.

    Yes Yes Yes, it is the side clearance, coupled with the groove that folds the chip inwards. The Insert tools are my go-to for anything bigger. I use the smaller ones because they fit in the 4-way toolpost I have

    My problem is that the tools seem to be obsolete and I cannot determine what inserts are now available to fit them, if any.... and that I need to make a tool for replacing the inserts, which I have not yet done, partly because I have never seen the tools specifically for these inserts. I never had to replace them but now one needs it.
    1601

    Keep eye on ball.
    Hashim Khan

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    • #32
      A lot of the tools are 2 pins, on a handle. one pin againt the blad, the other against back of insert , push handle forward it ejects.
      putting them in without a tool , get an aluminium shim against a workpiece and against insert, well below cutting edge, so it won't fracture the tip. Push in gently with cross slide.

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      • #33
        Thanks. Yes I have seen them, but have not developed dimensions etc for these.
        1601

        Keep eye on ball.
        Hashim Khan

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        • #34
          Why does it happen? Your lathe is probably flexing a bit as you are cutting. The cutting tool is pushed back against the compound and the carriage, perhaps even riding up a bit. There is a lot of horizontal force at play. Then, when the remaining metal gets thin enough, that remaining metal can no longer hold the tool back and the force drives the tool forward in a quick lunge that breaks through into the hollow center in only part of a revolution. The part breaks off and you are left with that partial ring still on it.

          What can you do? I can make some suggestions.

          A larger rake angle on the tool can help draw it into the work so it will not be forced back as far. No guarantees here as with a lathe that allows flexing or that has a lot of backlash in the cross feed or compound screws it may draw it too far in and you will have other problems.

          You can grind the nose of the cutoff tool at an angle so that there is a point on the right side where the parting actually happens. That will leave a conical ring on the stock in the chuck and a much cleaner part. The problem with this is that angle can also deflect the cutoff tool as you feed it in and you can get a dished surface. If you do this, keep the angle small.

          Remove the compound from the cross slide and mount the cutoff tool in a holder that is mounted directly to the cross slide. This eliminates half the reason for the looseness and flexibility in the lathe. And half the backlash in the screws. And don't use a quick change tool holder to mount it. Make a holder from a single piece of CI or steel with as little of the inside drilled out as possible. Lots of mass in the holder. Just one hole for mounting it with a stud. And it should have a large base; lots of area to sit on: FLAT AREA. Yea, yea, I know: probably not going to happen. I probably wouldn't do it either (my QC tool post is a lot sturdier than most), but it WOULD help.

          Buy a really expensive cutoff tool. It may not work, but you will have a real right to bitch and will know who to direct that complaining to.
          Paul A.
          SE Texas

          Make it fit.
          You can't win and there IS a penalty for trying!

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          • #35
            That sounds like a good trick. I too have just tried to eyeball it. I will definitely give this a try.

            Thanks.



            Originally posted by BCRider View Post

            If it varies between setups for you I'd say that is due to not having the blade dead on square to the spindle axis. With cutoff blades there's no side clearance so if the blade is not dead square to the axis of the spindle and/or dead parallel to the cross slide bed travel it will try to flex as you go deeper.

            A trick I've been using for about a year now is to bring the parting blade up against the front surface of one of the chuck jaws with the tool post hold down loose. I press the blade against the jaw and snug the hold down screw. It's not perfect because I've never dialed in that face with a part held tight. But it's proven itself to be "close enough".

            Prior to using this trick I'd eyeball the blade against a machined line on the cross slide. But since I added a big chip tray to the front of the cross slide I've lost my visual alignment lines. That's when I came up with the front of the jaw trick. Give it a try. It may not totally cure the dishing but it should minimize it. And if it doesn't cure it for you perhaps you'll get a feel for how much hold away at one side or the other you need to get things square/parallel.
            Paul A.
            SE Texas

            Make it fit.
            You can't win and there IS a penalty for trying!

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            • #36
              Aligning the blade is the most crucial aspect involved in parting. A tall thin blade gets its strength from the fact that all the force is directed straight down through the thickest plane. If the force is applied from the side where it is thinnest it will deflect and bend. Once it bends it will bind and even snap.
              At the end of the project, there is a profound difference between spare parts and extra parts.

              Location: SF East Bay.

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              • #37
                I align my tools by using a 1-2-3 or 2-4-6 block between the face of the chuck and the tool before tightening the tool down.
                Last edited by Ridgerunner; 02-29-2020, 02:51 PM. Reason: correct block sizes

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                • #38
                  Without dismissing the need for accurate alignment, the use of the insert type tools allows some error because of the wider portion at the actual edge vs the "shank". That can be good for deeper cutoff depths, because it is more critical to get the right angle the deeper you have to cut.
                  1601

                  Keep eye on ball.
                  Hashim Khan

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                  • #39
                    This admission is from a rank amateur just learning. I was getting convex face surfaces on my new imported lathe. Tried all kinds of things but to no avail. Then notice I was not tightening down much on the the carriage lock. It must have moved a tiny bit under tool pressure. My early tools didn't help as they were sort of a joke anyway. Much improved flat faces after I made sure the carriage was actually LOCKED. Duh!
                    S E Michigan

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                    • #40
                      This Kennametal insert tool is routinely used to part off 2 1/2" steel. It takes right, neutral, or left inserts. Neutral and right inserts in the second picture. I have some lefts somewhere lol.

                      Click image for larger version

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                      Click image for larger version

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                      • #41
                        That right hand insert is what I was talking about when I said to grind an angle on the nose. That right edge will cut through first and there will still be a substantial amount of metal under the center and left side. And the angle is slight, as I suggested.



                        Originally posted by Ridgerunner View Post
                        This Kennametal insert tool is routinely used to part off 2 1/2" steel. It takes right, neutral, or left inserts. Neutral and right inserts in the second picture. I have some lefts somewhere lol.

                        Click image for larger version

Name:	IMG_1168 resized.jpg
Views:	76
Size:	58.1 KB
ID:	1858790

                        Click image for larger version

Name:	IMG_1169 resized.jpg
Views:	75
Size:	54.4 KB
ID:	1858791
                        Paul A.
                        SE Texas

                        Make it fit.
                        You can't win and there IS a penalty for trying!

                        Comment


                        • #42
                          Just peel it off with a pair of pliers, like opening a can of Spam.

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