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Face milling cutter uneven surface

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  • #31
    And here are the inserts that I used and the cutter head


    • #32
      Regarding the step between passes, I have looked at that in detail again. There is a step between pass 1 and pass 2, but there is no step between pass 2 and pass 3. So the step issue is inconclusive at this time. Maybe that is a workholding issue. I will do another cut and check that again. There is also a thickness difference of 0.15 mm between one end of the workpiece and the other, and I think I know why that happened. After tighteing the piece in the vise over a pair of parallels, I should have given the workpiece a knock with a mallet so that it sits well on the parallels, I haven't done that.


      • #33
        I have also checked the tram of the mill. the mill is off by 0.03 mm in the Y direction (measured at two extreme points of the table). It should have been off quite a bit to explain the steps. But the step issue is inconclusive now, I need to repeat that cut after properly holding the workpiece.

        But the tram of the mill is WAY OFF in the X direction. It's too much off to really read out the figure. The dial makes almost half a turn. Luckily, I can tram out the X direction by just tilting the head. I would not be able to easily tram out the Y direction and would have to use shims.


        • #34
          Yes, the work needs to be "seated down" and a tap with a lead hammer may help with that. I do not think that will affect your "step", because the top surface is established by the cut.

          The sideways (X) tram of the head should not make a step in the surface by itself. The tram issue will make the middle of the cut low, but the edges should be the same all across. So even if if you overlap the cuts, there will be a low place between the cuts, and if you do not overlap, the edges will be OK, but the surface will not be flat.

          A step suggests that the knee and head are not at 90 degrees to each other..... the "Y" tram is off.

          Assuming the problem is in the "Y" tram:

          The natural tendency of ANY knee mill (I have two smaller ones) is to have the outer edge of the knee sag. The weight of knee and work tends to pull the knee down and it will wear in that direction. Sometimes the knee is set slightly tilted up by the factory in order to compensate and make the knee become better trammed as it wears. Of course eventually it wears farther and sags down.

          You seem to have the "strip-type" gib, just a strip of metal that is adjusted by screws to be just snug enough to hold the knee up, but not tight enough to cause it to bind and not move (I have those also). They are not quite as good as the tapered gib, but they work.

          Three possible ways to help with that come to mind, assuming that the head is difficult to adjust, or has no practical adjustment. (and that the gobs are in fact adjusted correctly)

          1) Sometimes using the "lock" on the vertical (Z) movement will pull the table up. I do not know what that extra nut is for, it could be a "lock", although I doubt it. Usually the "lock" is an extra gib screw with a handle so it can be easily tightened and loosened.

          2) It may be possible to use the "Z" adjustment to push the table up into alignment, assuming it is sagging. You lock the knee, and then adjust the knee "Z" up very slightly in order to take out the slack and tilt the knee up.

          3) You can take the first pass, then if you know the size of the "step", you can adjust the knee just enough to take out the step for the second pass. Again on the third pass. The top and bottom of the work will not be parallel, but the top surface will be as flat as you want to get it. You can "sneak up on" the right adjustment if necessary. The requirement is that you know how big the step is, which you can measure after each pass.
          Last edited by J Tiers; 03-12-2019, 02:48 PM. Reason: added comment assuming gibs are already adjusted
          CNC machines only go through the motions.

          Ideas expressed may be mine, or from anyone else in the universe.
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          • #35
            For a piece that large you're better off not using a vise but rather to directly hold the part to the table. You can add possible bowing upwards from the jaw pressure to your list of possible issues. And not that it's not seated to the parallels. That it's actually bowing upwards due to the pressure being mostly in the lower portion of the plate. When hundreths of a mm count EVERYTHING acts like elastic bands and styrofoam....

            I'd have to see more of it but from your pictures that single oddball stud and nut is not your gib adjustment. That part looks to be some other function. Like perhaps the slide lock. I'm betting dollars to donuts that the four allen socket head screws with lock nuts are your actual gib adjustments. At least those are the classically correct number and placement for gib adjusters. And not checking for play over 10 years of use might well mean that there has been some slight wear which is allowing the table to nod outwards a touch.

            In a thread a while back I posted the pictures shown below on using a magnetic gauge and dial indicator to span a dovetail slide and check for free play in the dovetail. If you have not done anything to the dovetail gibs in 10 years you may want to start there. Even with good oiling things wear or shift a bit. The way shown isn't the only way but I think it's a nice easy option that requires no additional tooling.

            There will be some wiggle of the needle. But what you're looking for that is bad is where the needle "clicks" from one reading to the other and stays at each end of the "click"

            Note too that this first picture is measuring in the right axis of the dial gauge so as to avoid any play in the lead screws being mixed in and masking any unwanted movement of the dovetail.

            So here's the "right way";

            And here's the wrong way that I set up at first then realized that there was WAY too much movement... then I realized it was reading the play in the lead screw and nut.... Note the dial gauge is in line with the lead screw (bad) and in the picture above it's at right angles to the lead screw (good).

            Chilliwack BC, Canada


            • #36
              That cutter will happily cut 0.001" finishing cuts on aluminium. Those pics in #30 show up the out of tram in the X beautifully and that should be fixed first, even if it is irrelevant to the underlying problem.
              If possible, try the Y tram test at differing heights of the knee, and, if the knee can be locked, also with it locked and unlocked. Try having the mag base on the centre of the bed with the dti resting on the main body that the head sits on, and lifting/pushing the bed rearwards with the Y axis locked. Any play in the knee might show up.
              Check for play in the spindle bearings and in the quill separately.
              Last edited by old mart; 03-12-2019, 02:35 PM.


              • #37
                I have a similar machine, and I do get some sag in the knee if it's not locked. Do you have the manual for the machine? It should tell you how to lock the knee.

                Have you checked the head bolts to make sure they are tight? The head will sag if the top ones are loose.

                You may as well make sure the ram is tight as well. I doubt it would sag much if loose, but I would check it anyway.


                • #38
                  From the appearance of the cut surface I conclude your tram is way off in X as well as Y. Only the leading edge of the cutter is marking the part. With perfect tram in X the trailing edge of the cutter should leave marks creating a cross hatch pattern. Your cut looks like you used a very high feed rate. Either that or you have one insert set low or maybe a chip welded to an insert.

                  The force required at high feed rates can cause deflection. The results of deflection are similar to the machine being out of tram.

                  If you are machining rolled plate you can expect the material to warp as you machine away the surface. That also could be contributing to your problems. Tooling plate (Mic6) is Blanchard ground from cast plate and is used where distortion can not be tolerated.


                  • #39
                    Minimize the steps size use a smaller 20 mm diameter cutter or correct the error on the tram. or use a 40 mm cutter and cut across the plate on the "Y" travel


                    • #40
                      I bought the machine about 11 years ago, but it has been used very, very little. 99% of the time it sat there in the shop, covered with rust protectant. The only tramming i did was right after I bought it, because I had tilted the head 90° to the side (to lower the center of gravity so that I can safely move it around), and apparently I did a lousy job.

                      It makes sense to do the work directly the table as opposed to the vise. And once I cut out a step on the left and right of the workpiece for the grabbers, I will be able to hold it on the table, while leaving the surface completely available for the face cutter.
                      Last edited by taydin; 03-12-2019, 05:57 PM.


                      • #41
                        By the way, this is how I checked the tram on X and Y (just for illustration, not my mill)