Z-axis safety - prevent fires on power failure

I am looking for ideas on how to prevent my heavy spindle from plunging into the workpiece if the power to the Z-stepper is cut while it is spinning. I do have a brake on the VFD that stops it in 1 second when you turn off the VFD, but not when power is unexpectedly cut. So for 6 or 7 seconds it burns it way into whatever I am working on as it falls down.

My first thought was a spring to balance the force of gravity. But I know that the spindle’s weight is probably the only thing preventing any Z backlash, so in that sense, it is a valuable thing.

My latest seed of an idea is to create a brake on the Z screw that stops the coupler from spinning as soon as the power to the stepper goes away for any reason. How to implement that? I was imagining some sort of ratchet cog fixed to the coupler, and a solenoid with a spring that engages a catch if it loses power. (powering up compresses the solenoid’s spring and releases the brake)

Or do you have a better idea?

I’m, uncertain of the scenario here. First any e-stop system should shut down both the router and the control board, so that should not be an issue. So I’m guessing that it is a brief power interrupt to the control board. For my system, I use an IOT relay driven by a fan pin from the control board to control the power to the router. If the board loes power, then the IOT relay is deenergized.

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The problem is that as soon as the Z stepper is not powered (for any reason), the Z axis falls down as far as it can go. The spindle has a lot of momentum at 24,000 RPM, enough to keep it spinning until a fire is started as it falls into the wood.

  1. Never run your machine unattended.

  2. Anything that unpowers the steppers is probably going to ruin the work. Probably near impossible to resume unless you have very careful measurements of your absolute start coordinates, and even then there will always be repeatability error.

  3. Any brake system can also fail, producing lots of heat and potentially starting fires.

  4. Never ever run your machine unattended.

  5. If the unpowered rotational inertia of your spindle can start a fire in the wood, then any brake system canceling that must also dissipate the same heat. This means either a large heatsink, or burnt fingers changing bits. Probably though the inertia is not so much as you expect, and you might get a little blackening at most. The reason that these things start fires is the unintended application of power into friction.

  6. Did I mention to never run the machine unattended yet?

The few times mine has dropped it didn’t even burn the wood. Just drilled down through it. One of the times it was my 1" surfacing bit. Now if the spindle stays running, yea, that’s bad. But we’re not leaving the room when it’s cutting anyway.

You say “unexpectedly” in your initial posting, but I’m wondering whether this mostly happens at the end of the job. If so, what about an “end gcode” section that moves the machine to a parking location that “catches” the axis as it falls before the bit contacts the spoil board, or maybe one that already has a clearance hole for the bit (and collet, I assume) to pass through?

Could you tap the “enable” pin of the Z axis and use that as a trigger for the VFD brake?

Perhaps, rather than a brake on the spindle, a brake on the Z axis threaded rod would be better, to prevent the spindle from falling, rather than stop it from spinning.

This isn’t as safe, because the spindle can keep spinning, but it helps the common use case where the job ends.

I have the same problem but it is benign as my small router never digs more than a few mm before stopping. If I was to fix it, I would do what Dan said and put a solenoid brake on the z screw; wired such that it releases when powered, and a spring latches it when power is removed (similar to truck air brakes… you could also use compressed air).

Sorry if I was not clear because I mentioned two brakes. I WAS talking about a brake for the Z stepper coupler, not the spindle. I don’t mind if the spindle slowly spins to a stop if it is in air or a cut it has just made. It is the plunging straight down that is the problem, hence the need for a Z-brake.

Yes, that is a good rule in theory. During a CNC workshop, there is a lot of chaos and distraction, and an enhanced concern for safety due to the presence of newbies.

“In theory, theory and practice are the same. In practice, they are sometimes different.”

You can replace the screw with one that has a lower pitch. A 8mm 2mm pitch with one start would probably not slide down on its own. The trade off is that you will lose Z speed by 4x. One possible speed up is to increase the supply voltage, which normally isn’t needed, but could boost your top speed torque.

IMHO, any mechanical system (or even electrical) that needs to work in that situation is adding more possible errors than fixing things.

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I really like your idea, Jeffe. I had planned to up the voltage to 24V as soon as I had a good reason to. (So far, 12V is surprisingly effective!)

Another way to install a shallower pitch screw and not suffer Z slowdown is to use a controller capable of much higher pulse rate. I have that on my electronics bench, also waiting for the excuse to get it installed on the MPCNC: grblHAL board that uses the Teensy 4.1 microcontroller, designed by Phil Barrett.

My LR2 doesnt drop when i turn the machine off, and that has 2mm pitch screw for that reason… I did turn my max Z feedrate down from 15mm/s to 10 and have no issues with Z skipping (though i dont have many hours on my machine, so do your own testing before blindly copying plz)

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The problem isn’t cpu speed. We used to use a thread that had 4500 steps/mm, that was stressing the cpu. The problem is that when stepper rpm increases, so does the back emf. At some point, the drivers have the 12V on all the time and you’re still not getting your set current. Above that speed, you will lose torque at higher speeds and you need that torque to lift that heavy motor.

We have the default speed for both Z at 10mm/s, which is conservative for a 4 start screw, but several people have had trouble at 10mm/s with a 1 start. Maybe you are lucky Ned, but I recommend drooping to 3-4mm/s and raise it if you want from there on a 1 start with 12V.

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Thanks for that helpful explanation, Jeffe. I think the grblHAL board can go all the way to 48V if desired.

Correction: the TB6600 stepper drivers I got to go with it can go up to 40V. They were less than 6 euro each at Aliexpress. I don’t think the grblHAL Teensy breakout board cares about the voltages.

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I was looking through marlinbuilder today and it set the speeds to 15 i thought, but maybe I missread.
I also run on 24v actually, should have mentioned that as well. I also have active cooling on my TMC2130s so probably not a good example in this case…
Though highly recommended 2mm pitch screws if you want want your z to drop

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You’re right!

The 24V makes your example make more sense. I don’t have 1 start leadscrews (I have 8mm 2mm pitch, 4 start) and 24V on my LRv1. But I have told more than a dozen people who bought the wrong screw to drop the speed to 4mm/s and that solves the problem for them. I also have told a ton of people that 24V doesn’t help in most cases, but it does help in this one, I believe.

So far, it happened when the USB cable momentarily jiggled loose from the computer sending a job. It also happened when I halted a job by hitting Emergency Stop on the Repetier Host screen. Both times I was right there watching, and could not shut down the spindle motor fast enough to prevent the dreaded drill-down. So it is not about running the machine unattended. At the end of all my jobs, Z raises 10mm above the workpiece (the set Safe Z value in VcarvePro) and that works fine.

Have you considered a relay to control the spindle? At least then the relay would cut power in those circumstances.