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Homebrew CNC: What about limit switches?

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  • Homebrew CNC: What about limit switches?

    We're working on a homebrew CNC setup, which is very much an educational experience for all parties involved. We're getting close to a very early testing stage, and it's time to start thinking about mounting some limit switches.

    Can I just use some off-the-shelf leaf-type microswitches? I know at some point I'll need more robust and better protected switches- the plan for this machine does include coolant at some point. But for the time being, can I just use some common micros?

    I'm still very new at this, so pardon the ignorance. Doesn't the usual stepper-based homebrew setup use a "home" position, marked by the limit switches, as a base/zero? In which case, the ultimate position is at least somewhat determined by the accuracy/repeatability of the switches themselves?

    Or are the switches basically just "don't go past this point" indicators?

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

  • #2
    For limits you can use anything that works, for homing you need something with very repeatable break and make points if you intend to reference your machine position from them and use tool offsets.
    Most systems have separate inputs for home & limits, limits will stop your machine if you run a job which would otherwise "hit the buffers"
    Regards,
    Nick
    If you benefit from the Dunning-Kruger Effect you may not even know it ;-)

    Comment


    • #3
      On servo systems the limit switch acts as a stop point. From there the system backs up the servo motor to the marker on the servo's encoder to accurately establish home or "reference" position with high repeatability.

      With a stepper usually there aren't encoders so the system would depend on the accuracy of the limit switch which might not be so accurate.

      An issue could be when running a part held in a vise that wasn't finished the last time the machine was run. Will your current "homing" position be the exact same spot within the machine's accuracy as previously? You might be able to get around this by using some sort of edge finding device to reestablish position off the part itself or the vise. Then the accuracy of home would be within the accuracy of your edge finding device.

      For testing it seems any limit switch would be suitable.

      Comment


      • #4
        My bridgeport interact has home and limit switches that are standard ( good quality )industrial microswitches inside a housing so whats good for them...
        There's a home switch a little off the limit switch for the axis. And it has limit switches at either end wired in series that the software should never hit once homed so they can be hard limits.
        The other thing that everyone knows already probably is to wire them NC, so if a wire goes bad then the cnc detects the switch as depressed all the time, rather than NO and making contact when its engaged, in which case if you have a wiring fault you find out when a axis goes past its limits and all hell breaks loose.

        Comment


        • #5
          Most of the machines I've ever worked on used proximity swiched, you know the kind with 3 wires, and a led on back to indicate operation, durable, accurate but require the appropriate power input, available NO, NC, mostly screw barrel, muck water and spit proof
          Mark

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          • #6
            Hiya Doc, I think a truly accurate homing system is VERY important. Sure does make it nice when you're setting up the tool table. Just use the homing point as tool #0 and reference all the others (I have 17 now) from that. As mentioned, it's also very nice when you have to restart the system.

            I do it on my little CNC lathe by using a micro-switch at one end of the axis travel which switches on an opto switch that's triggered by a slotted disc on the back end of the double-shaft stepper motor. That gives a homing point that is accurate to the same spot on the stepper motor's rotation every time. When you consider that it's a 200 step/rev motor geared 2.5/1, it's pretty darn accurate!

            Mine has relatively low-powered steppers so I passed on the limit switches due to lack of available inputs. If it crashes (so far it hasn't, knock on wood) it will just hit mechanical stops, stall the stepper and skip some steps. A quick re-homing routine, fixing the errant g-code that caused the crash & I'm back in business. Sounds good, huh?
            Last edited by DICKEYBIRD; 03-17-2016, 09:48 AM.
            Milton

            "Accuracy is the sum total of your compensating mistakes."

            "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

            Comment


            • #7
              Chinese proximity switches are cheap, accurate, and fairly resistant to chips and coolant. But bulky. If a chip shield is needed a thin piece of pexiglass may suffice.

              I didn't go with the leaf switches because of the coolant issue, though you can build some sort of housing to shield from coolant & chips.

              Optical would work too, but again would probably require a shield.

              All have been used successfully, just depends on what you have to work with and how creative you are at mounting them.

              Comment


              • #8
                Originally posted by DICKEYBIRD View Post
                Hiya Doc, I think a truly accurate homing system is VERY important. Sure does make it nice when you're setting up the tool table. Just use the homing point as tool #0 and reference all the others (I have 17 now) from that. As mentioned, it's also very nice when you have to restart the system.

                I do it on my little CNC lathe by using a micro-switch at one end of the axis travel which switches on an opto switch that's triggered by a slotted disc on the back end of the double-shaft stepper motor. That gives a homing point that is accurate to the same spot on the stepper motor's rotation every time. When you consider that it's a 200 step/rev motor geared 2.5/1, it's pretty darn accurate!
                Can you share part numbers and schematic/drawing?

                Thanks

                Comment


                • #9
                  Note that there are also water proof micro switches. Might be cheaper than the inductive sensors.

                  Comment


                  • #10
                    Hi Doc Nickel,
                    A few things to consider in what type of switching method to use.
                    Micro Switches work on the principal of the contacts changing state when the internal spring is deflected enough to flick the contacts to change of state. If there is pitting of the contacts due to the switching current and amps the repeatability of the switch point will differ. There also can be differences in the switch point due to the tension of the internal spring.
                    Limit Switches are a better option as the contacts are mounted onto the actuating bar and the NC contacts are forced apart. A snap-action Limit Switch is the better type as they are very repeatable, the better manufacturers of Limit Switches will have a table that shows the amount of travel of the actuator, switch point, over travel and at what point it will change state. They have various heads that bolt on for the different switching applications.
                    Inductive Proximity Switches are good as well as it works on the eddy current principal so a good repeatable switching point can be obtained. The method of setup is important though, always mount the unit so the switching action is across the sensing face not moving towards the face. A embeddable type unit is better for repeatability and make sure the switching target is to the manufacturers recommendations. Constant switching distance also makes for repeatable switching.
                    Most Limit and Inductive Switches are IP65 rated so coolant isn’t a problem but swarf can be for Inductive Switches as it can cause unwanted false switching.
                    Keith_W.

                    Comment


                    • #11
                      Originally posted by elf View Post
                      Can you share part numbers and schematic/drawing?

                      Thanks
                      I used one of these I got from from ebay a long time ago.


                      That one is NLA but I see others are plentiful now...looks like about the same thing. Pretty cheap; less than $2 if you're willing to wait. http://www.ebay.com/itm/Opto-optical...0AAOxy5rpSTmFn

                      As far as a schematic, I'm pretty worthless when it comes to that sort of thing. I just put a little roller tipped micro-switch from my junk box in series with the signal wire from the opto widget, fed that into an open pin on my CNC4PC C-11 B.O.B. then went into the Mach config. screen & checked the appropriate boxes. When you press The "Set Home X (or Z)" button on the lathe screen it makes the axis slowly move until it trips the micro switch/opto sensor & reverses til' it sees the signal again...done.

                      Sorry I couldn't be more specific...it's been several years ago that I did this. (...and it still works, yay!)
                      Milton

                      "Accuracy is the sum total of your compensating mistakes."

                      "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                      Comment


                      • #12
                        Sunx has a range of Slot Sensors as well http://www.panasonicsensors.com/comp...=slot%20sensor
                        There in different mounting arrangements and either NPN or PNP outputs.
                        I have a few and will be using one to make a speed display for my Lathe.

                        Keith_W.

                        Comment


                        • #13
                          Originally posted by Doc Nickel View Post
                          We're working on a homebrew CNC setup, which is very much an educational experience for all parties involved. We're getting close to a very early testing stage, and it's time to start thinking about mounting some limit switches.

                          I'm still very new at this, so pardon the ignorance. Doesn't the usual stepper-based homebrew setup use a "home" position, marked by the limit switches, as a base/zero? In which case, the ultimate position is at least somewhat determined by the accuracy/repeatability of the switches themselves?

                          Doc.

                          Doc, if it's just testing you want to do, you may not need the switches. In fact, I don't have any on my CNC and don't plan to add them. The forces aren't great enough to cause any damage if a limit is exceeded; you just hear the sound of missing steps (have to reset things, of course).

                          Mach3 does have a home feature, but it will run fine without. I always set my work coordinates on G54 (55, 56, etc. if using more than one offset). The G-code you load will be machining based on your work coordinates, not the machine coordinates. On shut down, I simply position to X0, Y0 and it's right there when I fire things back up again.

                          About the only thing I can see myself using the limits to set the machine coordinates for would be in order to define soft limits. However, the soft limits in Mach don't impress me, so I wouldn't use them anyway. As far as homing to reset or check things prior to machining, I think you can swing an indicator on a known location, or similar, to pick up and verify the numbers just as quick or quicker than running the home sequence. Always more of a sure thing to verify against your actual part, rather than a switch.
                          George
                          Traverse City, MI

                          Comment


                          • #14
                            Thanks guys!

                            What we've got is we lucked out and found a prebuilt (albeit homebrew) controller that another home-shop type was selling, after converting a machine back to manual. It will run on Mach3, and includes prewired connections for limit switches. This conversion is an early effort for all parties involved, so we expect any number of screw-ups. I'd like to put in the limits if nothing else, just to hopefully cut down on the number of potential crashes.

                            Now, if one doesn't use limit switches on a homebrew setup like this, or doesn't use them to "home", how does one generally go about doing that? Telling the controller where the tool is by touching the workpiece?

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

                            Comment


                            • #15
                              Originally posted by Doc Nickel View Post

                              Now, if one doesn't use limit switches on a homebrew setup like this, or doesn't use them to "home", how does one generally go about doing that? Telling the controller where the tool is by touching the workpiece?

                              Doc.

                              Yes. However, I don't touch the workpiece; too much chance for a crunch. I use a 1" jo block and jog down until I can slide it under the tool with slight resistance. Then, I select the Z readout in Mach (making sure it's set to the work coordinate I'm going to use - G54 99% of the time). Then I type in 1.000 and hit enter. 1.000 will appear in the window and Mach knows that this tool is 1 inch above the work.

                              Note: I have an R8 spindle without repeatable toolholders, so I set each tool when changing and have nothing set in the tool tables. If you are using the tool tables, be sure that the correct tool is called up when setting.
                              George
                              Traverse City, MI

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