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IndustryArena Forum > WoodWorking Machines > DIY CNC Router Table Machines > are travel stops required? or are limit switches enough
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  1. #1
    Join Date
    Nov 2007
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    197

    are travel stops required? or are limit switches enough

    the x, y and z axis on the router i'm building are all enclosed, and will not allow the machine to over travel.

    however i have been considering using spring loaded stops, to prevent a sudden bind up if the machine over travels.

    what have other people done in these situations?

    here is a pic of where i was at a couple weeks ago.


  2. #2
    Join Date
    May 2005
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    120
    Stepper motors make a lot of noise, but I don't think they will hurt themselves if they run against the stops for a moment.

    If you are using rack and pinion, you could have the rack end just before it hits the stops of the machine. Then the motor can spin freely.

    You can use some prox sensors to tell the mech to stop:

    http://www.syntextech.com.tw/pro6-body.htm


    The mech "knows" where it is in the software, so you might never hit a stop anyway after you do your initial set up.

  3. #3
    Join Date
    Apr 2005
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    3634
    I do cnc machine calibrations (day job).

    Every cnc machine should have at the very least 3 ways to stop.


    1) Software Limits (Machine Control Mach3, etc...)

    2) Hardware Limit Switch

    3) E-Stop


    My 2.0 cents.

    .
    Free DXF - vectorink.com

  4. #4
    Join Date
    Feb 2006
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    794
    Servo's can Fry in less than a hummingbirds heart beat ! Have limit switches set up at each end of travel and use them to set zero and before allowing "auto set to zero" use limits to find extents and settup your Soft limits with something like 3" for the slow zone to start with. Then as long as machine doesn't loose position it will slow down and stop on the limit switch or any distance that you setup from tripping switch. It is a really neat feature so use it. Good luck on your machine build, looks good so far.
    Don
    IH v-3 early model owner

  5. #5
    Join Date
    Nov 2007
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    197
    ok thanks for the help. the machine will definitely have limit switches.

    i may forget the spring loaded stops on some of the axises though (it would still be able to bottom out against a hard stop) - it seems like the gantry can move pretty fast, but right now the screw is not hooked up, and i'm sure that will slow things down alot.

  6. #6
    Join Date
    May 2005
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    120
    Have you guys had any problems with mechanical switches filling with dust and no longer functioning after a year? Even some sealed ones from Cherry seemed to fail (used as homing switches).

    The proximity ones seem to be money well spent. Or am I missing an easy, high quality mechanical switch?

  7. #7
    Join Date
    May 2005
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    1810
    First - you want hard stops. They may never get used but the first time you have a failure that drives your gantry off the rails, you will wish you had tried to put somthing in place. I have seen a gantry driven off the rails on a $1.5M ultrasonic knife cutter during assembly and testing and it's not pretty, nor is it cheap.

    Second - if you are going to use cheap switches as limits, you would be better served operating them through a plunger and putting the $2 switch in a small enclosure to keep it from being contaminated. The industrial switches are expensive but survive rediculous abuse because the case and actuator are well constructed. The contacts inside those Heavy Duty, Industrial limit switches are just as frail and prone to damage as your $2 switch but they are very well protected and sealed.

    Put some hard stops in place.

    Scott
    Consistency is a good thing....unless you're consistently an idiot.

  8. #8
    Join Date
    May 2006
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    1469
    Quote Originally Posted by multiplex View Post

    Are travel stops required? Or are limit switches enough

    the x, y and z axis on the router i'm building are all enclosed, and will not allow the machine to over travel.

    however i have been considering using spring loaded stops, to prevent a sudden bind up if the machine over travels.

    what have other people done in these situations?
    I have sitting here on my desk a CNC machine that has a home switch, no limits and no estop. Works perfectly. Very safe.

    It sprays ink onto paper. It's made by Canon. Made by the millions.

    My point is that every machine is different. So to say a CNC must have this or must have that is just presuming all sorts of things.

    Ask yourself what will happen if through user error, electrical fault or software error it tries to travel beyond it's envelope. Will it damage me? Will it damage the machine?

    There are many small homebuilt type routers that are very happily running without limits.

    Others that have home switches and use soft limits very successfully.

    I have a router about the same size as yours. I have limits and home. But it is more to do with convenience than safety or machine preservation.

    If I hit a hard stop then I go "Bugger!! I lost steps. Need to start again"

    That's it..... No damage...... No safety issues.... Just inconvenient.

    So figure out if your router needs them to stay in one piece and stay safe. If it does then there is your answer.

    If you want convenience then put them on. Or use homes with soft limits. That will work really well.

    In answer to your original question. How about placing some rubber buffers between the relevant components to absorb some of the "BANG" if it hits.

    Greg
    Attached Thumbnails Attached Thumbnails 17_87_img.jpg  

  9. #9
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    Jul 2005
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    Quote Originally Posted by mxtras View Post
    First - you want hard stops. They may never get used but the first time you have a failure that drives your gantry off the rails, you will wish you had tried to put somthing in place......Put some hard stops in place.

    Scott
    Allow me to give a slight correction/modification; you need some soft hard stops. If you stop something over a distance of 0.5 inches with a rubber bumper all the components see a lot less stress than if so stop something over 0.05" with a solid stop.

    If you have steppers I think that is all you need; a stepper does not mind being stalled.

    If you have servoes you need overcurrent protection.
    An open mind is a virtue...so long as all the common sense has not leaked out.

  10. #10
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    Nov 2007
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    yeah just to clarify, i'm not considering not having any mechanical stop - agree thats a very bad idea

    but was thinking about these, which since i bought them i think i will use. Would allow for a more gradual bottoming out, and will de-accelerate the machine in a more appropriate fashion


  11. #11
    Join Date
    Dec 2007
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    1
    Just a suggestion, but if you are worried about a switch getting full of debris and dirt...try using a fully sealed reed switch and have a magnet on your device triger it as it drives by. It would be a very simple solution, and never stop working because of dirty contacts.

  12. #12
    Join Date
    Sep 2006
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    318
    If you have a 3 axis machine do you need 6 limit switches - if so how do you wire them up , my controller board only has one connector for limit switches
    Drakkn Custom Shop http://www.drakkncustomshop.co.uk

  13. #13
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    May 2006
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    One way to do it is to have the switch travel with the axis and have actuators at each end of travel. That way you only need three switches.

    They can all be wired in series as Mach can distinguish between switches when homing. And limits don't matter which one tripped. So long as it stops motion.

    Greg

  14. #14
    Join Date
    Mar 2007
    Posts
    137
    Hi,
    One thing on limit switches: you need proper rated contacts for the amperage of your limit switch circuit. No, I’m not in reference to having heavy enough contacts, but the opposite! Yes, high amperage switches, like the ones on new iron from Taiwan. The problem is, if you are into these "step and direction" controls, your limit switch circuit may be logic level current, and if you have high rated contacts in the switch, you may get false limit switch tripping from vibration of the machine when cutting. Example: if your limit switch is rated at carrying a 20 amp circuit, and you are using logic level current, your switch will be tripping all the time, and your axis may not be anywhere near the switch. If I have a logic level circuit, I use a 1-amp contact. I do service work, and I get calls of “my z axis keeps tripping the switch, and I’m no where near it!” that’s usually the problem, the switch checks out ok with a meter, BUT, I have to replace it with one with lower amperage contacts. A prox switch won't have this problem, although i have had a tool changer that a prox switch was doing the counting, and wouldn't count correctly, I had to put in a schielded wire to the prox, then, like magic, it started counting correctly! moral: don't run unschielded control wire by high power wires. This was on a factory built machine, so i called the manufacture and explained the problem so they could correct it for the future.
    Good Luck,
    Buck,
    Spring Lake cnc llc

  15. #15
    Join Date
    Sep 2007
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    740
    Use NC switches and wire them all in series. If any switch triggers it will break the loop and trigger the limit. The machine won't care which switch triggered.
    This is also a safety thing, since it will show if you break/cut a limit switch wire.

  16. #16
    Join Date
    May 2007
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    55
    I'm going to be wiring my CNC machine up in the next few days. I'm thinking of running each home (x,y,z) to its own input, then the max limit (x,y,z) in series to another input. So a total of 4 inputs(1-Home x, 2-Home y, 3-Home z, 4-Limit x,y,z).

    The reason is I heard that when homing, and sharing multible homes on one input, Mach will only home one axis at a time. It assumes home on whatever one axis it is moving at that time when it hits the switch. I'm thinking if I branch the homes out, it should be able to home all axis at the same time..? Have I overlooked something?

  17. #17
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    Apr 2005
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    1778
    Quote Originally Posted by flyon View Post
    I'm going to be wiring my CNC machine up in the next few days. I'm thinking of running each home (x,y,z) to its own input, then the max limit (x,y,z) in series to another input. So a total of 4 inputs(1-Home x, 2-Home y, 3-Home z, 4-Limit x,y,z).

    The reason is I heard that when homing, and sharing multible homes on one input, Mach will only home one axis at a time. It assumes home on whatever one axis it is moving at that time when it hits the switch. I'm thinking if I branch the homes out, it should be able to home all axis at the same time..? Have I overlooked something?
    Flyon,

    As you know, I am using emc2. It allows me to share home and limit switches. On my router the xneg limit is also xhome, the yneg is also yhome and the zpos is zhome. So when you are homing, you can home multiple axii at one time. The effect is that when homing if a particular axis hits a switch, it is treated as a home switch rather than a limit switch.

    The following is for emc2 but it gives you the picture.

    ### I created 3 loops, one loop for each axis that combines home and limits for that axis
    ###

    # create a signal for the x home and x limit switches combined
    net XLimits parport.0.pin-11-in => axis.0.neg-lim-sw-in axis.0.pos-lim-sw-in axis.0.home-sw-in

    # create a signal for the y home and y limit switches combined
    net YLimits parport.0.pin-12-in => axis.1.neg-lim-sw-in axis.1.pos-lim-sw-in axis.1.home-sw-in

    # create a signal for the z home and z limit switches combined
    net ZLimits parport.0.pin-13-in => axis.2.neg-lim-sw-in axis.2.pos-lim-sw-in axis.2.home-sw-in
    Alan

  18. #18
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    May 2006
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    1469
    Quote Originally Posted by flyon View Post

    The reason is I heard that when homing, and sharing multible homes on one input, Mach will only home one axis at a time.
    It assumes home on whatever one axis it is moving at that time when it hits the switch.
    I'm thinking if I branch the homes out, it should be able to home all axis at the same time..? Have I overlooked something?
    Mach in standard configuration homes one axis at a time.

    If you want to home all axis simultaneous you will have to enter a new script. Easy to do.

    Best practice to let the Z axis home first, then X and Y together if you wish. Less crashes.

    Greg

  19. #19
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    Apr 2005
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    1778
    Quote Originally Posted by Greolt View Post
    Mach in standard configuration homes one axis at a time.

    If you want to home all axis simultaneous you will have to enter a new script. Easy to do.

    Best practice to let the Z axis home first, then X and Y together if you wish. Less crashes.

    Greg
    I agree with Greg. I home z first and then x and y together. If you forget to retract sufficiently and try to home all three at the same time you can break bits, machine into clamps or other little nasties.

    Alan

  20. #20
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    Dec 2005
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    313
    Quote Originally Posted by acondit View Post
    Flyon,
    The effect is that when homing if a particular axis hits a switch, it is treated as a home switch rather than a limit switch.

    The following is for emc2 but it gives you the picture.

    ### I created 3 loops, one loop for each axis that combines home and limits for that axis
    ###

    # create a signal for the x home and x limit switches combined
    net XLimits parport.0.pin-11-in => axis.0.neg-lim-sw-in axis.0.pos-lim-sw-in axis.0.home-sw-in

    # create a signal for the y home and y limit switches combined
    net YLimits parport.0.pin-12-in => axis.1.neg-lim-sw-in axis.1.pos-lim-sw-in axis.1.home-sw-in

    # create a signal for the z home and z limit switches combined
    net ZLimits parport.0.pin-13-in => axis.2.neg-lim-sw-in axis.2.pos-lim-sw-in axis.2.home-sw-in
    This does not work for me.

    I am using it with only a Y axis.
    I have added this to the axis 1 section of stepper_mm.ini
    HOME_IS_SHARED 1
    HOME_SEQUENCE 1

    When homing i get this:

    AXIS error: hit limit in home state 8
    AXIS error: joint 1 on limit switch error.

    Any idea how to solve this ?

    Thanks.

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