Slow with proximity switches, stop with limit switches?


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Thread: Slow with proximity switches, stop with limit switches?

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    Default Slow with proximity switches, stop with limit switches?

    I'm going to apologize in advance. I'm pretty sure I'm not the first to ask this question, but I'm having trouble finding the right search term.

    I'm building a homemade mill and plan to use limit switches on all three axes. Running the mill off the end of an axis would really be bad,
    but could running into a limit switch also cause damage? That is, if the mill is moving at full speed and hits a limit switch, could the abrupt
    stop also cause damage? Mach provides slow zones and soft limits. I'm thinking of doing something similar with hardware, using proximity switches
    to detect when the mill is close to an edge and slowing down the rate so that when it hits the physical limit switch, the shock of stopping isn't as severe.

    Is this something that people do, or is it even necessary?

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    Community Moderator Jim Dawson's Avatar
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    Default Re: Slow with proximity switches, stop with limit switches?

    If you are suggesting using a limit switch or a prox as a hard stop, then no, don't do it. If you think you need a resilient stop, then use a rubber bumper. Limit switches and proxes are not designed to be crashed into.

    The limit or prox should be mounted to allow the operating tab to pass by the switch and not damage the switch. This is normally done with a cam in the case of a switch, or a tab in the case of a prox.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Slow with proximity switches, stop with limit switches?

    ...if you look on page 15 you'll see how Two Step Switches work. Is one way of installing Homing Switches
    https://www.eaton.com/ecm/groups/pub...ct_1549250.pdf

    Welcome to the forum,
    DJ



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    Default Re: Slow with proximity switches, stop with limit switches?

    Quote Originally Posted by Jim Dawson View Post
    If you are suggesting using a limit switch or a prox as a hard stop, then no, don't do it. If you think you need a resilient stop, then use a rubber bumper. Limit switches and proxes are not designed to be crashed into.

    The limit or prox should be mounted to allow the operating tab to pass by the switch and not damage the switch. This is normally done with a cam in the case of a switch, or a tab in the case of a prox.
    Thanks for the comment about positioning the switches. I'll make sure to keep that in mind as I position them.

    I'll try to explain my question a little differently:

    I plan to use mechanical limit switches at the ends of the travel of each axis, all wired to a Gecko G540 and configured as axis limits. It's my understanding that the CNC software smoothly ramps up the velocity from zero to the commanded speed with an S-curve, and then back down to zero with a similar curve when it stops. But when an E-stop or a limit switch is triggered, it's an emergency situation, so the G540 cuts power to the motors without doing a smooth stop. The inertia of the moving axes has to be absorbed by the drive screws. So here are my questions:

    1. Is my assumption correct about the system nominally ramping up and down smoothly, but immediately stopping when a limit switch is tripped?
    2. If the answer is "yes," are milling machines typically built to withstand these forces, or is this something that you want to try to avoid?
    3. If the answer is that "you want to avoid" this condition, would it be advantageous to install inductive proximity switches (in addition to the mechanical switches) configured to trigger before the mechanical switch is triggered? When the proximity switch is triggered, the CNC software would be configured to drop the feed rate so that when the mechanical limit switch is triggered, the velocity will be lower and so there will be a lesser chance of damage to the mill.



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    Member CitizenOfDreams's Avatar
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    Default Re: Slow with proximity switches, stop with limit switches?

    All the routers I have seen only had one set of limit switches. If something malfunctions and the machine flies past the limit switch at high speed, it hits the hard stop without destroying itself.



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    Community Moderator Jim Dawson's Avatar
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    Default Re: Slow with proximity switches, stop with limit switches?

    Most machines are designed to have reasonably robust hard stops. In many cases on light weight machines the hard stops are just the ball screw bearing supports. The good news is that your Gecko G540 drive can't run a motor that is capable of doing much damage, just not enough power, nor will any moving part of your machine have enough mass at any normal operating speeds to do any real damage. Also, when over torqued, a stepper motor will magnetically decouple and quit producing torque. The motor will growl at you, but this will not hurt the motor.

    You definitely want to avoid crashing the machine, but this does happen occasionally. This is normally caused by some kind of an oops in the G code or operator error. The most common error is a poorly planned rapid move that crashes the tool into a fixture or something. Rarely does a hardware malfunction cause a runaway type crash, and with stepper motors this is very rare. It is very unusual to crash the machine into the end of travel under any condition.

    You will be amazed at how fast the axis stops once the pulse train to the drive is stopped. On a small machine this is something that is not of much concern. On larger industrial machines with high powered axis drives there are more safety systems built in.


    Jim Dawson
    Sandy, Oregon, USA


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    Member CitizenOfDreams's Avatar
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    Default Re: Slow with proximity switches, stop with limit switches?

    Just a small round rubber bumper serves as a hard stop for the X axis on this SCM router:






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    Community Moderator ger21's Avatar
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    Default Re: Slow with proximity switches, stop with limit switches?

    That is, if the mill is moving at full speed and hits a limit switch, could the abrupt
    stop also cause damage?
    It won't stop instantly, The laws of physics won't allow it.

    It's my understanding that the CNC software smoothly ramps up the velocity from zero to the commanded speed with an S-curve, and then back down to zero with a similar curve when it stops.
    Mach (and most other controls) does not use S-Curve acceleration. It uses a constant linear accel and deceleration.

    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Slow with proximity switches, stop with limit switches?

    Thanks for both your answers. You verified that I was overthinking things. I'll stick to one set of mechanical limit switches per axis.

    "This is normally caused by some kind of an oops in the G code or operator error." -- Yeah, that's what I'm worried about. It's not so much that I expect this to be a frequent occurrence, but that I want the peace of mind that the limit switches are there if I do make mistakes. A friend of mine has the same mill and we were doing a real simple 2D cutout of some carbon fiber sheeting with some auto-generated G code. As a precaution, since it was our first time running the machine, I set the home position with the Z-axis offset a few inches up to do a test run. I don't remember where I made the mistake, but the first thing the mill did is plunge the Z-axis a couple inches towards the bed. It would have crashed the mill. While I plan to be careful to not make such mistakes, I expect that they will happen again.



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    Default Re: Slow with proximity switches, stop with limit switches?

    I think you are overthinking things. Bumping against a hard stop should not do any damage to the machine, and it only happens in rare circumstances (usually as a result of a tool crash or a hardware malfunction). Crashing the tool into the part or into the bed is much more destructive, but limit switches cannot prevent that.



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Slow with proximity switches, stop with limit switches?

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Slow with proximity switches, stop with limit switches?