CNC Mill Limit Switches

[electrosteam]

I am 'slowly' working on my CNC mill and I need now to consider limit switches.

I think the way to go is an oil-proof inductive switch with a sealed cable connection.
I think a NC (normally closed) switch with a target on the moving item operating the switch to open the contact at the limit position would be sufficient.

I have read elsewhere that if all the switches are put in series, the CNC controller software is 'smart enough' to realise which switch is activating by virtue of the axis/direction currently taking place when the switch activates.
Putting inductive switches in series is probably not acceptable, so interposing relays will probably be necessary.

Not sure yet just why the CNC controller needs to know which limit switch activated, surely any switch operating should stop the program and the recovery actions are sure to be done by manual intervention (jogging).

Any suggestions on a suitable economical supplier of inductive switches (in Sydney) ?

Or contrary suggestions ?

John.

[Stustoys]

I have read elsewhere that if all the switches are put in series, the CNC controller software is 'smart enough' to realise which switch is activating by virtue of the axis/direction currently taking place when the switch activates.

Not so sure about this. If the controller was so smart it wouldn't be running into the limit switch in the first place. :) As for guessing which axis, what if it is moving two?


Not sure yet just why the CNC controller needs to know which limit switch activated, surely any switch operating should stop the program and the recovery actions are sure to be done by manual intervention (jogging).

It could be handy to know, but like you say no biggy. What would be a pain is if the controller is guessing and guesses wrong. It's telling you X and its Z.

Inductive switchs are nice but whats wrong with a boring old NC micro switch?
Stuart

[Dave J]

Hi John,
I found a supplier in either Taiwan or China that sold really nice protected switches. It was on ebay but I don't have them in my watched items anymore.
Have a look around ebay world wide and you will find them,I know they were cheap and had different configurations.
Dave
PS
Just found them for you it was Taiwan, have a look around his shop or email him if it's not in there.
LIMIT SWITCHES TZ-5104 tend - eBay, Switches, Electrical, Industrial. (end time 01-Sep-10 21:00:34 AEST) (http://cgi.ebay.com.au/LIMIT-SWITCHES-TZ-5104-tend-/280552171389?pt=AU_B_I_Electrical_Test_Equipment)

[RayG]

Hi John,

I am using inductive prox switches, for limits and homing, the "normal way" of wiring is to just inhibit the direction the axis is travelling when the limit is hit, rather than complete shut down, that way you can still back off the limits. If they were wired in series (like an e-stop cct) then you would have to manually wind it off the limits to restart.

The servo amps I'm using have seperate inputs for X-fwd-inhibit X-rev-inhibit, and so on. When the servo hits the X-high limit stop, which is wired to the fwd-inhibit, the drive stops instantly, and the software generates a following error "position error" and the program halts, but because x-reverse is still enabled, you can back off the limit switches under program control and restart (assuming you fixed whatever caused the overtravel in the first place.. )

Regards
Ray

[Stustoys]

Unless you want to fit two sets of over travel limit switches, I would go with shut down. If it has over traveled something is fubar, relying on the drive to stop the machine could be a problem if the drive is the fault.
Having said that I've never tried to push a 4tpi ballscrew backwards off a limit.
Stuart

[RayG]

Unless you want to fit two sets of over travel limit switches, I would go with shut down. If it has over traveled something is fubar, relying on the drive to stop the machine could be a problem if the drive is the fault.
Having said that I've never tried to push a 4tpi ballscrew backwards off a limit.
Stuart

Hi Stuart,

Agreed, two sets of switches are needed. mechanical limit switches for overtravel "shut down" wired into the e-stop circuit to kill the mains power. is the best way to provide protection for the machine and operator for that matter.

The proximity switches I am using are for homing and "normal" over travel events, like a programming error. They won't help in the event of a hardware failure like a drive failure.

Nothing beats good old mechanical limit switches for reliability. But proximity switches for homing etc. are easier to set up and can be reliable so long as swarf is kept away.

I got mine from this guy http://stores.ebay.com.au/Easycnc

Regards
Ray

[eskimo]

Unless you want to fit two sets of over travel limit switches, I would go with shut down. If it has over traveled something is fubar, relying on the drive to stop the machine could be a problem if the drive is the fault.
Having said that I've never tried to push a 4tpi ballscrew backwards off a limit.
Stuart

I'm with Stuart also

safety devices are exactly that..the but stops with me so to speak....
when every thing else goes wrong its your last "fail safe" mode before untold damage happens....

[electrosteam]

Thanks for all the comments - illuminating as always.

Stuart,
Ballscrews are amazingly easy to push.
I have seen it commented that ballscrews should not be used on a manual machine because they have such little friction.
In the CNC machine, the energised stepper motor holds the position.

Ray,
Thanks for the link to EasyCNC, I will check them out later.

The manual intervention I was considering was a jog station.
A button could be held to bypass the limit switch and E-stop series contacts while the machine was re-positioned.
I think that if the machine gets to a limit, something had gone wrong and better to stop everything.

But, I will consider your comments about drive failure - food for thought.
I have been involved in several industrial incidents over the years where components, some safety critical, have failed putting personnel or equipment at risk.
Perhaps a contact from the limit switches and E-stop in the stepper motor power supply would be wise.

John.

[markjaffa]

The supplier Dave J provided has good gear - I purchased all the push button and rotary switches I needed to re-wire my lathe 3-phase with VFD from them, and the switches I need to convert my mill to VFD when I get around to it. Good quality and smart designs.