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  • limit switches on mill

    I am finishing up the conversion of my mill to CNC. It has the Centroid Acorn controller. Until now I have been doing a simple home without homing switches or limit switches. My question is do you all think it is better to use the mechanical switches that were on the mill or should I mount induction limit switches? I have the induction switches already. They are very good quaility three wire NPN switches. You can see both switches in the picture.
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    Location: The Black Forest in Germany

    How to become a millionaire: Start out with 10 million and take up machining as a hobby!

  • #2
    I would go inductive. The existing mechanical ones have had “some” life used up.
    Ive just bought inductive for my CNC build, as the last one has mechanical ones and they have been prone to failing/ swarf ingress

    Dave
    Just south of Sudspumpwater UK

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    • #3
      You say limit switches. But just what are they going to be expected to do?

      Prevent over travel?

      Provide a zero point? If so, with what accuracy? How will that zero point be used? And how often?

      Others?

      I have always been suspicious of the use of mechanical switches (micro switches) for limits or set points on machinery. First the carriage is going to approach such a switch at high speed. The system must be able to stop that motion before the mechanical switch is damaged or moved. And remember that there is momentum involved: the table of a milling machine alone is heavy. A mechanical switch must have some provision for over travel after the contacts close or open.

      But then, if the switch is going to be used to establish a zero point, then it must repeat with the accuracy that this requires.

      I have worked with many "limit" switches, mostly mechanical and optical. There were always problems, especially if the usage was frequent and the environment not exceptionally clean. An inductive switch may easily provide the needed over travel, but it's accuracy may depend on a number of factors. It could easily change operating point with a change in Voltage or even temperature. And a ferrous chip could easily alter it's characteristics.

      Paul A.
      SE Texas

      And if you look REAL close at an analog signal,
      You will find that it has discrete steps.

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      • #4
        For Home position , Some of the better CNC machines use two switches , the first reduces speed to slow to reduce inertia effects and allow a controlled approach for repeatability

        Rich
        Green Bay, WI

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        • #5
          To get the accuracy you will desire for anything repeatable, you use the home or limit switch to detect range. It trips, the mill or lathe backs off, then slowly re-engages and trips it a second time. The next step which the Acorn supports is if you are using a servo, is that it will now move the motor until the Z index is tripped on the servo encoder. This gives you the industrial quality accuracy you desire. So the accuracy of the switch itself needs to be pretty good but it's not responsible for the actual accuracy.
          My mill is open loop steppers, so my homing switches are very high quality to make up for it.

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          • #6
            When I put together my Acorn-powered lathe, I sprung for some fairly expensive sealed/enclosed limit switches, I think Omrons? I'd have to check. Used they were like $60 to $80 each.

            BUT... when I was first setting up the machine- you have to essentially fine-tune the pulses-per-rev to get exact travels- I was using a digital caliper style bar. The kind of thing people use as an inexpensive quill readout, or on a tailstock, etc.

            With those switches, it would repeat it's zero point precisely, each time.. It would always return exactly to 0.0000".

            Again, that's with an external, standalone digital readout- NOT the controller's DRO numbers on the display.

            Yeah, they were expensive, but they're water and oil proof, factory rated at multiple millions of cycles, extremely accurate, and with the wheel-tipped plunger, could be easily mounted in multiple angles and orientations.

            Barring using servos with a reference-mark in their encoder, personally I'd prefer to have the most accurate switches I could put on the machine, even if they did cost quite a bit more.

            Doc.
            Doc's Machine. (Probably not what you expect.)

            Comment


            • #7
              It's worth noting that on Haas VMC's, well at least on their "Office Mill", and I'm sure on all of their mills, they use proximity switches which can't be as accurate as a good Metrol limit switch. They use the index pulse on the servo encoder for accuracy.
              And for tuning the steps per rev, I was going crazy until I used Occam's Razor, machined a part, measured the part, and used the delta between desired and actual! Now good to within .001.

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              • #8
                I’d use the inductive pick ups you have ( after testing) all you need is 24v on them, we had IP aka inductive proximity switches that could pick sub millimetre distances on machines in work, the fed into plc,s, test them out
                mark

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                • #9
                  Thank you for all the replies Gentlemen and you too Mark! I hooked up the homing switch that was on the machine just to see how it would repeat. It is repeating back to home with up to 3tenths of an inch. For me that seems quite good. I am using the DRO on the machine to compare to the computer screen of the Acorn control. The Acorn Dro. is always going to the same spot. The DRO is vary by 3 tenths. Do you think I will get better results with a proximity sensor? I would like to use the switches as they are already mounted and the wiring is already there. It would simplify the installation a little bit.
                  Location: The Black Forest in Germany

                  How to become a millionaire: Start out with 10 million and take up machining as a hobby!

                  Comment


                  • #10
                    Go for it.

                    And good luck, happy chips!
                    Paul A.
                    SE Texas

                    And if you look REAL close at an analog signal,
                    You will find that it has discrete steps.

                    Comment


                    • #11
                      Use which ever you prefer. But understand that the "home position" is just to give a start point for the machine and everything after that is just relative to it. It does NOT matter if that switch repeats to +/- 1/16". You ONLY need to rehome if you crash the machine and it loses zero. It's not there to send the machine back to to swap in a new part. And if you crash, well you have ruined your part anyway and need to start over.

                      So choose which ever limit switches you like. It really doesn't matter.
                      If you think you understand what is going on, you haven't been paying attention.

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                      • #12
                        The old Bridgeport EZ Trak mills and EZ Path lathes will move to limit switch home positions after they boot.

                        They do this very very slowly, it is best to leave the machines at the limits when you turn them off or there will be a slow homing move the next day.

                        The new lathe that I have been using for a year has no homing switches, when restarted it has to be taught that it is a lathe.
                        Leaving it in turret location #1, tool # 1 at a nice round position such as X 2" X Z 10", write yourself a note and simply enter these positions when you restart it.

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                        • #13
                          If this was a lathe on the X axis, you most certainly would want to use the index pulse as your home/limit after the home switch touches so your X offsets will remain consistent. Z axis isn't a big deal. Without the pulse index, you'll have to take a light skim cut and measure the diameter to calibrate the X axis.

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                          • #14
                            Forgive my ignorance. If I was making more than one of the same part and I had to shut the machine down in between parts so there fore re-home the machine upon start up again I would then have to set part zero again also, correct?
                            Location: The Black Forest in Germany

                            How to become a millionaire: Start out with 10 million and take up machining as a hobby!

                            Comment


                            • #15
                              Originally posted by Black Forest View Post
                              Forgive my ignorance. If I was making more than one of the same part and I had to shut the machine down in between parts so there fore re-home the machine upon start up again I would then have to set part zero again also, correct?
                              If your home switches are perfectly accurate or using the encoder pulse in conjunction with home switches, AND the Acorn saved the part setup("don't know if it does, I never tried it) you wouldn't have to re-zero the part.

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