CNC router controllers

GRBL, Mach3, or something with nobody behind it

GRBL is open, free, well documented and enough for most desktop work. Mach3 is older, Windows-only and paired with more capable hardware. A board running unlabelled firmware with a QR code to a dead forum is the one to avoid, whatever it costs.

Open loop or closed loop

Open-loop steppers lose steps silently: the part is scrap and nothing told you. Closed-loop steppers and servos know their position and fault instead. On a machine cutting metal unattended, that difference is the whole argument.

Drive current has to match the motor

A driver rated below the motor runs hot and derates; rated far above it costs money for nothing. Ask for the motor’s rated phase current and match the driver to it, rather than buying the biggest number on the listing.

The VFD is part of the controller, not the spindle

Motor poles, rated frequency, current limit and acceleration ramp all have to be set for your specific spindle. A correctly matched pair configured wrongly behaves exactly like a faulty spindle, and that is where most "dead on arrival" reports come from.

Mains supply and safety circuit

Most desktop controllers ship with a Chinese plug and an e-stop wired only to the board rather than to the spindle contactor. For a workplace rather than a garage, that gap is what an inspector looks at first.

Machine guarding — US OSHA 1910 Subpart O ↗

Ask what it does on power loss

A controller that forgets its position on a brownout turns a two-hour job into scrap. Whether it homes on restart, and whether it has limit switches wired at all, is worth confirming before purchase rather than after.