[KipKirol]

We have a CNC machine that has a brake on the Z axis. The brake is activated when there is no power to it. Right now we have a relay so that when the main power is turned on to the drivers and controller the brake gets power and releases it's grip.

I had an issue a few weeks ago where the Z axis went offline - it was an over torque or something like that. Anyway it has only happened once so far and I would like it to not happen again.

When the z servo went offline it allowed the tool to fall into our plenum, partially destroying it.

I am thinking of adding a limit switch inline with the brake power supply, this way if the z axis goes offline and the head starts to fall the limit switch will cut power to the brake, causing the brake to stop the ballscrew motion.

My question is - does this seem like a good plan? It seems pretty simple, am I missing something?

Thank you!

[drdos]

The only issues is where do you place the switch? If you have a part on the machine and the Z axis fails, then you risk hitting the part way before the switch. What controller is on this? If it were a Fanuc then you can edit the ladder to use the SA bit so when the axis faults out you engage the brake.

[Jim Dawson]

If I understand correctly, I think there is a small flaw in your control logic. You would have to have some way of overriding the limit switch in normal operation. If you just put the limit switch in series with the brake, then the brake would apply anytime the limit was triggered.

You really need to use the servo enabled/OK output from the servo drive, and use that to energize a relay that will allow the brake release.

[KipKirol]

There are a lot places I can mount this switch. I basically need a reference and sensor (switch) mounted to a fixed point and to my z carriage. The easiest way I think to start is with an adjustable plunger and roller switch mounted somewhere on the Z carriage and using the plenum as the reference. I would add a bypass switch also. I want the brake to apply any time the limit switch is hit because in normal operation it would never be hit. This machine is very simple, it only has a Y and Z axis and only makes one type of part.

I think I am going to use the servo enable circuit to trigger a relay on our e stop circuit.

One of my main concerns is wondering whether I can damage the servo by the brake suddenly stopping it - my assumption is that the overload or overtorque protection would save the servo in this hypothetical situation.

The controller is Mach3

If I understand correctly, I think there is a small flaw in your control logic. You would have to have some way of overriding the limit switch in normal operation. If you just put the limit switch in series with the brake, then the brake would apply anytime the limit was triggered.

You really need to use the servo enabled/OK output from the servo drive, and use that to energize a relay that will allow the brake release.

The only issues is where do you place the switch? If you have a part on the machine and the Z axis fails, then you risk hitting the part way before the switch. What controller is on this? If it were a Fanuc then you can edit the ladder to use the SA bit so when the axis faults out you engage the brake.

[Jim Dawson]

The servo drive will probably save itself if the motor is suddenly stopped by the brake. If the brake hooks up hard enough there could be mechanical damage on a sudden stop, depends on how fast things are turning.

[KipKirol]

OK, if the z goes offline it should not fall very fast. There are 2 air springs acting as a counterbalance to the rather heavy z carriage.

The servo drive will probably save itself if the motor is suddenly stopped by the brake. If the brake hooks up hard enough there could be mechanical damage on a sudden stop, depends on how fast things are turning.