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  • Help needed in design of a pin cutoff machine

    Hello all

    Lately I've been toying the idea of building a small cutoff machine that is mainly used to cut ejector pins used in molds to proper length. It uses a Norton 150x1x32 mm cutoff disc turned by a 250 W 3000 RPM 3-phase motor and the spindle is going to have tapered bearings. The pins diameters are 20 mm maximum.

    The pins are round, uniform in diameter and at the other end have a bigger diameter base. The base diameter and thickness grows as the pin diameter goes up. This is what an ejector pin looks like: http://ppunch.com/_images/punches/economy.gif

    In the current design the pins are held in a small V-slot that has an opening in the middle of it for the cutoff disc to go to, so the pin and the cutoff part are held in place very good during cutting.

    The length is adjusted by an adjustable backstop, nothing special here. I'm also planning on putting a coolant system on this one, so the pin won't overheat. It is mainly to control the dust and to ensure the pins length doesn't change, as the current cutting machine will heat up the pin very well above 300 Celsius and it is hard to keep the length in control.

    I have some issues that I would like to hear options/ideas/criticism for, as there are things I'm not so familiar with and some things I can't wrap my head around.

    Available is electric and pneumatic (up to 0.7 MPa) power to be used.

    1. Dust protection for the spindle bearings? I've been thinking of those rotary seals, the ones that are rubber and are pressed in to a hole and have a spring loaded lip on them. Are these good for this application or is there a better solution? I'm thinking that these would also keep the coolant away from the bearings.

    2. Connection between spindle an motor. These two can't be inline or otherwise cutting off long pins would be impossible, so they have to connected with a belt. Is a basic V-belt good or should it be something else? Haven't done belt drives before, so can't say what works and what not.

    3. In its current state the pin to be cutoff is held stationary, so the cutoff wheel has to pass through the whole diameter. I've been trying to figure out a way to get the pin spinning during cutoff, so the cutting would be shorter and easier. I just don't get how to fix the pin in place against the backstop and still have it rotating. And would have to be some sort of quick clamp, as usually there are tens or a hundred pins to cut at a time.

    4. If the pin is rotating, then the V-clamp would have to be different too, as it would have to allow the pins rotation but still hold it in for the cutting. What if the V-clamp would consist of small shielded bearings, two under the pin and one in the movable clamp to form the 3-point contact?

    The reason I'm designing this is because I'm fed up with the current cutoff machine at work, it can take half days work to cut 50 pins to length and even then it can be +-0.1 mm if all the planets do not line up. And the fuss is even greater if one wants to cut XX mm long pieces from a rod, as there is way to do it. And the maximum length to be cutoff is about 60 mm, so cutting a longlong pin has to be done in steps...did I meantion tedious, frustrating and not effective?

    Thank you for your ideas and help, everything is welcome!
    Amount of experience is in direct proportion to the value of broken equipment.

  • #2
    Would something designed like an arrow cutoff saw work?
    http://www.cabelas.com/catalog/large...oductId=744739

    Completely adjustable and fairly economical to build.

    GST

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    • #3
      Originally posted by gstprecision View Post
      Would something designed like an arrow cutoff saw work?
      http://www.cabelas.com/catalog/large...oductId=744739

      Completely adjustable and fairly economical to build.

      GST
      Maybe, but I'm going with the design in progress, just wanting to refine it better. The machine in the link doesn't address all of my issues and looks like the motor and is inline, so I'm quite sure it comes in front of the cutoff piece.
      Amount of experience is in direct proportion to the value of broken equipment.

      Comment


      • #4
        You might be interested in this concept.

        http://www.asmfab.com/Spin-Rolls.aspx

        Gene

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        • #5
          Originally posted by easymike29 View Post
          You might be interested in this concept.

          http://www.asmfab.com/Spin-Rolls.aspx

          Gene
          That indeed looks like a nice concept Would not be so hard to incorporate such a mechanism, just needs wheels with exactly the same diameters on both halves and probably also preferred to be hardened. Not so hard to make, I have access to a cylindrical grinder.

          At least that would solve the problem of holding & rotating the workpiece regardless of the diameter. Just would need to make it as double to hold the cutoff piece also.
          Amount of experience is in direct proportion to the value of broken equipment.

          Comment


          • #6
            Originally posted by Jaakko Fagerlund View Post
            That indeed looks like a nice concept Would not be so hard to incorporate such a mechanism, just needs wheels with exactly the same diameters on both halves and probably also preferred to be hardened. Not so hard to make, I have access to a cylindrical grinder.
            Tip: The moveable wheel must be "canted" a couple of degrees to cause the part to move in the direction of the stop. I cannot remember which way it must be canted. Also, the driving rolls must be made from the hardest material you can find. D2,CPM10V, HSS etc. It would also be better if the moveable roll is spring loaded.

            Comment


            • #7
              Sounds like a job for a screw machine or turret lathe. Mount an angle grinder with a cut off wheel on the cross slide.

              Comment


              • #8
                Originally posted by Rustybolt View Post
                Sounds like a job for a screw machine or turret lathe. Mount an angle grinder with a cut off wheel on the cross slide.
                The thing is that every pin can be different length. And this has to be a tabletop machine, the place is already filled with big machines

                I'm thinking of having an automated backstop, just input the length wanted and the machine moves to backstop where it should be.

                Would like to hear about the belt and bearings protection a little more, as these are the areas I haven't done much.
                Amount of experience is in direct proportion to the value of broken equipment.

                Comment


                • #9
                  You need a good grinding spindle design for your machine. There are quite a few of these around, but an easily accessible one is the Quorn/Bonelle tool and cutter grinder spindle. This uses angular contact rather than tapered bearings, but it has a proper pre-load arrangement and a fairly simple labyrinth seal, which would be preferable to a spring loaded rubber seal for a fairly high speed spindle. This is a belt drive spindle and most people have used round section belting, which can be joined with a bit of heat. Have a look at the Quorn site on Yahoo Groups for drawings of this type of spindle.

                  The backstop arrangement cold be automated with a little ballscrew arrangement. I am not sure about your tolerances for length of ejector pins, but I would guess fairly tight. You could put together the control for your automation with a bit of LinuxCNC HAL and inbuilt PLC. Should be fairly simple.
                  Bill

                  Comment


                  • #10
                    willmac, thanks for the Quorn spindle suggestion, I'll take a look on it for ideas

                    I was earlier thinking about the ballscrew and a stepper connected to it to move the backstop, just needs some thinking to prevent dust from enterin the screw and nut. And I'm probably putting on an Arduino to control the thing, fairly simple to setup an LCD, keypad and a couple of wires to the stepper driver board, which can be quite low powered device. All this fits nicely in a small electrical cabinet, which the machine will have, so makes for a nice compact and standalone machine.
                    Amount of experience is in direct proportion to the value of broken equipment.

                    Comment


                    • #11
                      why dont you use a thiner disc and run it faster?

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                      • #12
                        Originally posted by dian View Post
                        why dont you use a thiner disc and run it faster?
                        Because there is no need. Even the 1 mm thick disc can break during operation, so don't want thinner, especially when cutting off 20 mm ejector pin. It can be run faster, the limit is 80 m/s and at the moment the 3000 RPM motor provides about 23 m/s. This is one of the reasons I want a belt drive, so I can also push the RPM of the spindle up without buying a VFD.

                        And one very good reason to stay with these 1 mm thick discs is because we have them by the boatload.
                        Amount of experience is in direct proportion to the value of broken equipment.

                        Comment


                        • #13
                          Just spitballing:

                          Small bench top lathe: Unimat, Sherline, Taig, 7X12, etc.

                          Modify a three jaw chuck with one or more notches in the jaws for the larger diameter heads. The pins could be inserted with pressure against either the top or the bottom of the pin's head, depending on where you want to measure the length from. Three jaw is OK as precise centering is not needed for cut-off.

                          Make a cutoff attachment to mount on the cross slide. Either abrasive or small circular saw blade. You could either use the normal in-feed screw to advance and retract it or make a lever operated slide for faster action.

                          Protect the slides from the abrasive dust with covers. A vacuum hose near the cutting area would help a lot.

                          The length could be adjusted with the lead screw: add a calibrated hand wheel if it does not have one. Add a scale to allow you to quickly dial in particular sizes. A table with the scale number and the hand wheel number for each size would be handy.

                          Seems to give you everything you want. I would think you could do a pin a minute or better with this set-up. And with an added scale and calibrated hand wheel, set-up would be fast and accurate.

                          If your reference for the length is the far end of the pin, you could use an adjustable stop in the spindle to fix the pin's positon instead of using the notch in the chuck jaws. The jaws would still need to be notched for the shortest pin length and largest head possible, but those notches would not need to be quite so precisely cut.

                          Used small lathes can be had for a few hundred dollars on E-bay or other web sites.
                          Last edited by Paul Alciatore; 05-11-2013, 02:07 PM.
                          Paul A.

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

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                          • #14
                            In the corner of our toolroom we have an ancient Dronsfield Eagle manual surface grinder (that was old when Gods dog was a puppy) fitted with the same elastic wheel you listed, that we use *just* for cutting ejector pins to length. Very simple, super sturdy, accurate & quick.
                            There is one on e-bay right now, currently at £31 and won't go for more than £150-£200 tops. If I were in your place I would buy it and use it and then think about the spin grinding bit afterwards.

                            http://www.ebay.co.uk/itm/Victa-Eagl...item35c761ca0e

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                            • #15
                              Originally posted by Jaakko Fagerlund View Post

                              And I'm probably putting on an Arduino to control the thing, fairly simple to setup an LCD, keypad and a couple of wires to the stepper driver board, which can be quite low powered device. All this fits nicely in a small electrical cabinet, which the machine will have, so makes for a nice compact and standalone machine.
                              Arduino is fine and i understand the advantage of small and low powered, however LinuxCNC gives you a lot of really useful tools from which you can build your machine control.

                              The inbuilt software PLC should be capable of handling all the sequential control requirements, which might include collet open, feed to stop, collet close, feed grinding wheel, retract, reposition backstop etc....

                              The G code interpreter might be useful, but you may consider it OTT for your machine.

                              The Hardware Abstraction Layer allows you to glue together software/hardware components to create your specific machine. Components include PID, step controls, encoders etc.
                              Bill

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