Control a drag knife

Hello! I made a drag knife module with a solenoid. How do I connect this on the hardware level? I’d like to simply use a pen program, but instead of moving the z-table, it controls a solenoid relay. I control the drag knife pressure mechanically. All I need is an on-off signal, and I’m hoping it is inherent in the cut program I send to the laser. Many Thanks!

I’ve been (over)thinking about this one for a couple of months now.

I would attempt to access one of the coolant (M7 or M8) commands.
It’s an on-off toggle on one of the data pins. When a pwm signal is fed to the spindle it can be used to turn air on when the laser is running and off when it stops and traverses.

I think this may make it an ideal candidate to trigger an opto or a power transistor or a relay driver board.

friendly reminder to remember to trap back-EMF from the solenoid field when you disengage the relay. Voice of experience.

Have fun.

I’d think the most simple is to just run the laser on 100% while you want the knife blade down. Run it off the pwm signal…?


For the most part - yes.
I’m not certain if the motion planner would apply power reduction to corners at higher system speeds. Without getting into the firmware in a huge way and commenting out the spindle speed control this would be the fast way to hack it (in my mind). :smiley:

He advises he has a Ruida, so there will be no firmware hacking…


You’re right - I’ve definitely got GRBL on the brain! :nerd_face:

With a Ruida I guess that means there’s an air assist pin right on the controller ready to go. It may even be powerful enough to run a solenoid directly.

My brains been GRBLed a few times… :crazy_face:

Probably easier with the pwm than with the air assist.

Air assist would apply to the layer, enabling the ‘knife’ for the duration of the layers run. You would want more ‘knife’ control that ‘air assist’ would allow.

With the pwm, when the laser fires, the blade is activated…


Thank you everyone for commenting. I am paying back and giving what worked: I used an autofocus 3-wire already at the head. In the back, I moved the LimZ- black (signal) to Wind. No other wires required moving. I used a 24V relay, with the 24V sent into the DC+, ground into DC-, black into ‘IN’. The solenoid is then in series loop with the power supply required: use the NO and COM ports for this, NC is not used. I had to set the relay jumper to low, as that is what my laser supply is using, but this may be high for you (check what the laser power supply is using). Run in pen mode, with blow set to ‘off’. That is all. Do not set ‘enable blowing’ in Vendor settings - this is what messed me up. My view is this is highjacking the z-table move command with the pen setting and just actuating the solenoid instead. It had nothing to do with air assist. I’m using an AP Lazer - wonderful product with an absolutely huge cutting area.

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