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Home Switch Repeatability

Discussion in 'CNC IN THE HOME SHOP' started by TomS, Oct 2, 2016.

  1. TomS

    TomS Active User H-M Supporter-Premium

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    Just curious. What is a reasonable expectation for home switch repeatability? Checked my X axis with an indicator and it bounces around a bit (~ +/- .010"). I set the homing speed to 20% from 35% and no change in repeatability. I read on the internet that lever type switches are not repeatable but wanted to check with the resident experts.

    Tom S.
     
  2. JimDawson

    JimDawson Global Moderator Staff Member Director

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    To get better accuracy you want a slow make/ break rather than a snap action. +/- 0.010 is not too bad for a snap action. Lever switches work better with a cam actuator. You are best to use a two step process to home. Move to switch until it changes state, then very very slowly move off of the switch until it changes state again.

    On my router I have slow make/break switches. It homes consistently at 0.000 measured with a dial indicator, but it comes off of the switches at 10 pulses/sec with a resolution of 25400 pulses/inch (about 0.023 IPM) in the second homing stage.

    If you want real accuracy get these guys, 5 micron repeatability, they also make a 1 micron switch.
    https://www.amazon.com/gp/product/B...rd_wg=JKhXL&psc=1&refRID=JJTSV8EG9C1V9BVP6QC3
     
    Last edited: Oct 3, 2016
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  3. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    The Metrol switch looks like an interesting alternative to the typical microswitch and ramp. It is still vulnerable to swarf though

    I designed and installed an optical homing system on my Tormach mill and have .0001" repeatability. The OEM microswitch and ramp could vary by as much as .003" which is why I made the modification.
     
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  4. John Hasler

    John Hasler United States Active User H-M Supporter-Premium

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    Use an optical interrupter.
     
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  5. TomS

    TomS Active User H-M Supporter-Premium

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    I get what you are saying. I'll slow the feed rate way down and see what I get. My setup uses slow make/break switches (pictures attached). I'd be happy with +/- .005".

    Tom S.

    X axis
    20150323_103336.jpg

    Y axis
    20150319_135317.jpg

    Z axis
    20150323_103402.jpg
     
  6. JimDawson

    JimDawson Global Moderator Staff Member Director

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    That looks like a good setup, just the way I would do it. :encourage:

    But..........Now I get to climb up on my soapbox :grin:

    I see no reason to have home switches on a mill at all. The only exception to this would be on the Z axis to insure a parking position for an automatic tool changer that requires the Z to be in a specific position.

    IMHO the ''home position'' should be a variable ''parking position'', settable on the fly. All coordinates should be relative to the work 0,0,0 position.

    To accommodate a power fail or other catastrophe that causes a zero loss, a datum point can be set up somewhere within the work envelope of the machine. When using a vice, I normally set the datum to the vice surfaces, and in most cases this is also the work zero. When work is bolted directly to the table, I know the location of the fixture or work edges or the location of some feature on the work, or in the rare case that it is not possible to do that, will set a datum by bolting a small piece of scrap to the table somewhere and just move over it and spot a small divot at a known location.

    The idea of having two coordinate systems (machine and work) on a machine is totally crazy. When setting up a job, you don't set the job to the machine coordinates, you set the job on the table then set the machine to the work. Now for a manual tool change on a knee mill, you normally need to move the table (and the work) to a location that allows clearance to make the tool change. On your mill with a mile of Z travel all you really need to do is move Z up to a parking position that gives you enough clearance to do the tool change.

    Once an operation or job is complete, then the axes move to the convenient parking location that allows access to the work for measuring, removal, etc. But this is still relative to the work coordinates.

    Now on my router, this is a slightly different situation, since my work is normally aligned to the machine, the top, right corner of the table, I use that as the ''home position'', but that can still change if needed to accommodate the work. But this is still the work coordinates and only coincidently the machine coordinates.

    OK, I'll climb down off of my soapbox now ;)
     
    Last edited: Oct 3, 2016
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  7. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    Tormach's Path Pilot requires homing the three axes before work. Personally, I like the ability to accurately and repeatably rehome the machine should an e-stop occur or should I have to shut the machine down as in overnight to continue the next day. There are a number of jobs that I have done where the workpiece reference disappears once machining begins. I did build an external reference into the table which would allow me to re-obtain an accurate reference but Mach 3 wouldn't reset my G55, G56, etc. work offsets to the updated reference coordinates. Path Pilot goes one step further in that if an external reference is used, it has soft stops which prevent the machine from traveling past that reference point.

    When I installed the optical homing I left the OEM homing intact. This was done partially as a backup if the optical system failed but also because the homing switches also served as limit switches for the extremes of travel. Path Pilot has eliminated the need for second feature with its soft stops as it very neatly prohibits movement past the soft stop. Even with a G0 move at 135 ipm, it anticipates the stop and the software will stop exactly at the soft limit. Since switching to Path Pilot, my OEM limit switches never are activated.

    Bottom line, yes I could function without the homing but it sure is convenient to have. If you are going to use homing though, IMO it should be as accurate as your work requires. Otherwise there is no use for it.
     
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  8. JimDawson

    JimDawson Global Moderator Staff Member Director

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    :+1:
     
  9. jbolt

    jbolt United States Active User H-M Supporter-Premium

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    Other advantages of a Home position in Mach3 are:

    1) If you setup the machine parameters Mach3 will display your work position on the table. Useful to verify the code is correct.
    2) If you setup soft limits Mach3 will tell you if the tool path exceeds the travel limits.
    3) If you use multiple work offsets for multiple fixtures/vises.
     
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  10. cs900

    cs900 United States maker of chips Active Member

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    another nice feature of having home/limit switches is that if you keep your vice on your table you can use the surfaces on your vice at your work coordinates. Home your machine then indicate the edges you want to use as your reference and record the machine coordinates for that location. You can now just punch in those coordinates for your G54 when you restart your machine without the need to indicate anything. I actually use the left side of my vice as my G54, and the right side is my G55.

    ZK9vQtt.png
     
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  11. JimDawson

    JimDawson Global Moderator Staff Member Director

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    OK, you guys have convinced me that there MIGHT be SOME use for hard home limits :grin: But I still like my floating home position:p ;)
     
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  12. TomS

    TomS Active User H-M Supporter-Premium

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    Wow! Lot's of good advice. Thanks guys. I too like to ability to shut down at the end of the day then be able to restart the next morning, home the mill, then key in the work offsets. And, if for some reason I run up against a limit switch re-homing gets me back to my starting point/work offset. Items 1 and two that jbolt mentioned are one's I use to make sure I haven't set myself up for a crash.

    All that being said I went back and reduced my homing speed to 5% as Jim suggested. The X axis now repeats within +/- .0005". I'm happy with that. The Z axis repeats within +/- .001" with a little help from me. Evidently my gib is a bit on the tight side so if I put some body weight on the head I can pull it down a few thousanths. I'll readjust the gib and that should cure it. The Y axis is another story. For some unknown reason the Y comes up against the limit, stops, but does not reverse. If I hit the jog button (opposite of the homing direction) it trips the limit. Not sure what's causing this but I just did a rewire of my limit switches from individual inputs to wired in series. The switches worked fine before the rewire. I'm going to wire the Y switch to a separate input and see what happens. If this doesn't work out could it be noise? My limit switch wiring is not shielded but has been working flawlessly for nearly two years. I can rewire the circuit with shielded cable if necessary but would like to rule out other causes first. How about a bad limit switch? Anyone have these symptoms and traced it to the switch? Could it be Mach3? I had a homing problem when I first got my machine running where it would blast through the switches and crash when I attempted to home the machine. I reloaded Mach and the problem went away.

    I've got a few things to work through but am always open to criticism and different ideas.

    Tom S.
     
  13. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    Tom,


    The fact that the y axis homing stops indicates that the switch is tripping.

    You might try temporarily switching the x and y wiring to see if the problem stays with the y axis or moves to the x axis. That should tell you if it is a switch/wiring problem or Mach 3 or the controller.
     
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  14. TomS

    TomS Active User H-M Supporter-Premium

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    I'm out of town for the next 8 days but will do the wire swap when I get back.

    Thanks,

    Tom S.
     
  15. Boswell

    Boswell United States Hobby Machinist since 2010 H-M Supporter-Premium

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    just to weigh in on the "To Home or not To Home" conversation. I am with Jim on this one. I can and have "Homed" my mil and even used work offsets but 99% of the time I do not Home the mill but just Zero on the appropriate place for the job I am doing. Most of the time I am doing one-off parts anyway. I even feel confident to turn off my Mill at night and come in the next day without re-zeroing if I am on a longer run. Usually though I move the tool to X=0, Y=0 and Z=2 before shutting down so I can verify that all is good the next day. When I have used Homing and work offsets it has been when I had a long run of a part and expected to want to do other non-related work during the run. i.e. take a break from the run of 50 widgets to make a specific part and then go back to the widgets. Although I am not sure it would really save time unless I was going to interrupt a long run many times. It only takes me 5 min or so to home X,Y&Z and as a hobby, Time is cheapest part of my investment.
     
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  16. TomS

    TomS Active User H-M Supporter-Premium

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    I tried a few things and still no luck with Y axis homing accuracy. First thing I did was reload Mach3 version 066. When I first started my machine a couple of years ago I couldn't get the mill to home. When I pushed the ref all button the mill would blast past the limits and crash. After reloading Mach all was well. I next took the Y axis out of the series wiring circuit and connected it to pin 13. No luck again. When homing the accuracy is erratic, as much as +/- .020", or it trips the limit switch.

    I'm not convinced it's a switch problem because it does trip when it reaches the machine's limit. I'll troubleshoot it some more and see where it leads me.

    Tom S.
     
  17. JimDawson

    JimDawson Global Moderator Staff Member Director

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    Maybe it's a sticky switch. It would still work, but not be accurate. I would try swapping with the X axis switch and see what happens. Maybe a squirt of contact cleaner would be helpful.:dunno:
     
  18. TomS

    TomS Active User H-M Supporter-Premium

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    I agree, it's the switch. Ran a few more tests and the motor reversed a few times. When it did reverse the dial indicator readings repeated within +/- .001". I also ran two more tests; one test using the ref all button and the other test using the ref Y button. I first homed the axis and set my DI to zero as the basis point then ran ten repetitions. Using the ref all button my DI readings varied between +.001" and -.015" with the switch tripping twice. Using the ref Y button my readings varied between -.001" and -.012" with the switch tripping twice. All of my testing and what others have said leads me to believe it's the switch and not Mach3.

    The switches are rated IP67 but I'll try the contact cleaner and see what happens. Whether I free up the existing switch or buy a new one I guess I won't know if my homing is accurate until I scrap a part.

    Tom S.
     
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