The PlanetCNC USB software, like most machine control software, can be used to control a laser engraver/cutter. The typical modification is to use the Z axis down signal to fire the laser, analogous to an endmill cutting into the workpiece from an initial position above it.
For the second question, it is important to note that BYCNC machines are not based on PlanetCNC software, or any other particular machine control system. The machines themselves will accept standard signals from a wide variety of machine control systems, not just PlanetCNC or Mach3. This allows maximum flexibility for the customer to choose their own preferred software for this part of the toolchain.
That said, our machines will accept a laser head either in lieu of or in addition to a spindle. Here is an example of a combination spindle/laser mount for our greenBull machine: https://buildyourcnc.com/item/cnc-machine-laser-spindle-combo-head. We have all the parts you would need to add laser functionality to your machine here: https://buildyourcnc.com/LaserComponents.aspx, if you feel comfortable assembling your own package. Also, feel free to contact our sales department for a quote if you need a custom assembly.
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Can the Planet CNC USB controller be used with a laser engraver/cutter? Can the build your own CNC based on the Planet CNC USB controller accept a Laser head instead of a router?