Homing Y/A Axis Says Limit Switch Triggered

[Stevenson]

My new table cnc approx. cut area 20”x26” made all in aluminum with 7 limit mechanical switches, Mach3 register, ESS, C11G BOB, ASUS Laptop with Windows 8.1 was working very well and did my first 3 sample cuts until I decided to re arrange wiring for better looks

The attached snip shows at the top left of the screen the Homing of all axis is complete but there are two limit switches on the Y and A axis shown on the right of the screen which are triggered but at least I can back out my axis. Just wonder where to look for to solve this minor problem.

I reroute motors/limits wiring but when I connect the shielded ground to the star point ground and try to Home the Y axis it says “Limit switch is active, fix the wiring and try again”. Try again it says “Limit switch is triggered”. For now I left the ground wiring not connected,

I don’t know if the ESS has anything to do with it but reading on the web some suggest to install a capacitor to the limits wiring but I don’t know what size capacitor and where to connect the capacitor and besides I don’t know if it will help.

Thank you

[cropwell]

If your switches are wired NC then you may have a bad connection or a break in the cable somewhere. Cable internally fractured seem to be the bane of my life. After homing and doing an eight hour print, my 3D printer would not home for the next print as the cable went OC in with the movement. Easily fixed but a PITA all the same. I advise that cables are routed to give the largest possible radius of curvature where they move and the cable ends are clamped where they meet the switch or motor so they don't move relative to the connection.

Hope this helps,

Rob

[Stevenson]

Thank you Rob for your time, I understand it’s a PITA to find this kind of a problem but have no choice except to give it a try again and check the cables for sharp turns. What’s strange is that all limit switches work fine when I press each one individually and homing the X and Z axis I have no problem, it’s only the Y and the slaved A. Eventually I will get it…..

[Neale]

May not be the problem, but I have suffered from this one...

When you home, it is possible that the axis barely moves away from the home/limit switch (and I am assuming that you have a single switch for home and limit). When homing, Mach3 treats the switch as a home switch so that it does not trip the "limit" function at this point. However, as soon as that axis has homed, it reverts to being a limit switch while Mach3 goes off to home the next axis. If the first axis is still just about at the switch trip point, it might actually trip the switch due to vibration, and Mach3 sees this as a "limit" event. The cure is to set the Mach3 parameters to move the first axis a couple of millimetres away from the point at which the switch tripped. I had this problem with proximity switches but I guess it might happen with mechanical switches as well. It's a quick thing to check - just a change of a few Mach3 parameters - so worth a try, perhaps.

[Stevenson]

Good point Neal, thank you sharing your experience, but we must remember that the cnc was working before very well and I have not touch the Mach3 configuration since. It’s true also that I have single switch for Home and Limit. In addition, when Homing if the axis moves even a millimeter I will see/hear it and this doesn’t happen, What happens as soon the Gantry touches the Home switch I hear it clicking the switch and then the Gantry stops moving then the DRO numbers start to move to zeros and when it does I get the message “Limit switch is triggered”.

First I will try to fix it without touching the Mach3 configuration which was good and I did 3 sample cuts with no problem at all

[Neale]

As Rob says, there's a high probability that moving the wiring has broken a core somewhere - I have also suffered my share of broken wires! A real pain in the posterior to track down although at least you generally know which cable is broken (it's the one that's most difficult to get to...)

However, I made my suggestion on the basis that even though it's not very likely, it is so easy to check that it might be worth doing. In my own case, I was getting these limit trips maybe 50% of the time - maybe it would fail once or twice at the beginning of a session and then work perfectly for the rest of the evening. There was no visible movement of the axis causing the trip when it happened. Still, the hysteresis in mechanical switches should mean that it can't happen. And that's what I thought with my proximity switches as well - that wasn't a very sound assumption!

Good luck with it anyway - these kinds of things are a bit of a nuisance but once you've got the thing fixed, you should have a decent machine.

[Stevenson]

You are right Neale but if you have a broken cable would the switch alone still function? I just don’t see that. Anyway if I want to check a cable how do you go about?

Just check the resistance from one end to the other?

If that’s so I can do it and hope for the best.

Just came back from my garage and no matter what I change I still get the same bloody message..

Many thanks

[cropwell]

Just check the resistance from one end to the other?

If you have a buzzer type continuity indicator, clamp the test leads on and wiggle the wires about at the same time muttering the incantation "Sh*t, F**k, B*gger, *rseh*les". Also check all the wiring at the switch many times over. The correct word to say when doing this is B*stard, repeated constantly and vehemently.

Believe me, I have done it many times.

Rob

[Stevenson]

No problem Rob, tomorrow I will do as you say and yes I do have a buzzer type continuity indicator

[Stevenson]

Almost finish checking the cables and will post results soon but I have a question which if I know the answer I may solve my problem

On the attached diagnostic screen where I have the two red arrows pointing what it means the M4++Limits and M4Home? Do they refer to a motor, axis or what?

When I try to Ref Y and the gantry starts to move, when the move stops this M4++Limits & M4Home is also lit but in a few seconds the 2 lights come OFF.