[joeavaerage]

Hi,
I've been using Mach for years, first Mach3 and a parallel port and for the last 8.5 years Mach4 and an Ethernet SmoothStepper.
I've never had a contactor to cut the AC power supply to any components.

I rely instead that when I hit the <Estop> button or a Limit or servo fault or whatever that Mach will stop. In thirteen years CNCing it never failed to stop.
Do you actually need a contactor to cut the power?

You might be required by Electrical or Industrial regulation to do so.....if you were selling this machine......but for your own machine there is no legal requirement to do so....and who would enforce
it anyway?

It occurs to me that if you have a contactor to depower the machine it suggests that you do not have confidence that your CNC software solution and/or CNC motion control system will not reliably stop
when commanded eg <Estop>. That suggests that you should get a better CNC software and/or motion control.

Craig

[catahoula]

Yeah that seems reasonable to me. The PrintNC diagrams as well as the Centroid Acorn manual both recommend the contactor circuit, and I understand the idea that the E-stop is not a normal use function but a last resort so having redundancy built in may be appropriate. But the VFD is already going to be controlled by a relay/contactor so it does seem a bit overkill.

[phomann]

Yeah that seems reasonable to me. The PrintNC diagrams as well as the Centroid Acorn manual both recommend the contactor circuit, and I understand the idea that the E-stop is not a normal use function but a last resort so having redundancy built in may be appropriate. But the VFD is already going to be controlled by a relay/contactor so it does seem a bit overkill.

Hi,
I would suggest that you follow the lead of the CNC manufacturers and have your Estop remove AC power from the drives and spindle. Leave the motion controller powered off Its 24vdc or other dc power.

Removing AC power from the drives and VFD is safe and easy to do. There really isn’t a reason not to do it.

As to your relay controlling the VFD, many breakout boards/ controllers use FETs to turn the relay On/Off. when a FET fails, the majority of time it fails in the on state, meaning that the VFD will not turn off if you press a software Estop.

This issue is also the failure mode for solid state relays. It’s why mechanical relays are used in contactors.

If you’re CNC system can’t harm anyone then maybe you don’t even need an Estop, but once you VFDs and spindles then it should be controlled by a proper EStop circuit.

Cheers

Peter


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[phomann]

many breakout boards/ controllers use FETs to turn the relay On/Off. when a FET fails, the majority of time it fails in the on state, meaning that the VFD will not turn off if you press a software Estop.

For reference.


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[joeavaerage]

Hi,
well kool, and this has actually happened to you? Then you have evidence that a depower type Estop is worthwhile.

I, on the other hand have never had such a situation. For 11 of the 13.5 years I've been CNCing I was using two of your own (MB2) breakout boards, and not once did they ever let me down
with respect to any Estop event. If I recall the outputs of your breakout boards are CMOS IC's....and yet never a problem. I did manage to 'blow' a couple of inputs over the years, my own fault
on each occasion, but I never had Mach fail to signal a stop event for which the breakout board ignored.

I've built my own breakout board for my new mill, and its been in service now for 2.5 years, and neither has it ever failed to stop the axes or stop the spindle when so commanded.
The ESS signals the breakout board, a CMOS buffer IC and that in turn switches a 2N2007 MOSFET to turn ON the spindle relay. It not impossible that something could fail that would
preclude the relay from turning off, or the relay contacts could get 'sticky' and remain ON despite the armature of the relay going to OFF state but its not done so yet.

Craig.

[metalmayhem]

For 11 of the 13.5 years I've been CNCing .

in those 13 years you've been machining what machines and controls have you used that do not cut power when estop is hit .

[joeavaerage]

Hi,
I used first Mach3 and a parallel port and then for the last 8.5 years Mach4 and an Ethernet SmoothStepper. Over that period at no time whatever
have I pressed <Estop> , or had a Limit induced Estop, or servo alarm induced Estop where the machine failed to stop.

About 13 years ago there was a bad earthquake in Christchurch where I live, over 200 people died. I can say that the chance of a fatal earthquake is higher
than the chance of my motion control not stopping when commanded to do so. Statisticians are likely to jump up and down....but consider this there has been a fatal
earthquake in the same time where I have never had my CNC motion control fail to stop when so commanded.

Al is a long time user of Galil motion controllers. For many years they have been the standard against which other motion controllers are measured....and thus I'm very interested
to know in all Al's long experience whether he's ever had an occasion that a Galil motion control failed to stop when so commanded. My guess is that he has not for otherwise
I feel sure he would say so. If that is indeed the case then is a depowering strategy necessary....aside from an outdated rule?

Craig