Probing 101 - The Basics

Hey everyone,
I’ve recently noticed a bunch of posts related to probing, so I thought I’d help clear up some things for those that do’t know yet.
By default, GRBL’s probing functionality is no different from any simple switch. A successful probe occurs on the closing of a simple circuit. In most use cases here, that circuit is closed when an endmill (ground) makes contact with a probe block (probe pin - A5).
The actual probing motion is initiated with a simple Gcode command. The most commonly used command is a G38.2
That basically means “start probing in a straight line”. The machine needs more info for the probe, though. A G38.2 probe will stop when contact is made (circuit closed) or when there is an error because it reached the max probing distance. It needs to know the direction and the max distance to travel before an error. It also needs a defined feedrate. Let’s say you have an unusual circumstance and want to probe for your X0 on the right side. You could jog your endmill to the right of your probe and send:

G91 - this sets the motion to relative, or incremental, mode. This means motion commands will be calculated from the current location.
G38.2 X-25 F75 - Assuming mm, this will probe at a feedrate of 75, to the left (negative X) 25mm before failing and throwing an error

On a successful probe, motion will stop, and grbl will store and report the machine position of the probe contact.
That’s it! Probe complete. The useful stuff happens after the probe. Send:

G90 - Always good to remember to do this to get back to absolute mode
G10L20X10.175 - This sets the current position to X10.175 (on the current WCS)

Why 10.175? The probe block (in this imaginary case) is 7mm thick on the X-axis. Add that to half of my 1/4" end mill to get the distance from the center of my endmill to the right edge of my stock. Essentially that zeros on the right edge of the stock.

After that, do whatever you need to…jog somewhere else, probe another axis, start your job, tell your wife you’re too busy probing to put the Christmas decorations back in the crawl space (and then turn off the machine, run inside, and act like she’s crazy for thinking you said something), etc.

I know many of you know this stuff, but like I said, I’ve noticed an uptick in threads related to probing.
I feel that the more you understand that the black box (Carbide Motion) is actually pretty transparent, the more comfortable and confident you’ll be that you’re not going to jack it all up.

Happy probing!


I’d recommend always setting metric mode (G21) before doing this as well as following the probe with a retract at least a few mm off the probe (50mm is pretty convenient). If not in metric mode, and there is a problem (like you missed the probe block, forgot to connect the wire to the endmill, etc) it will jam the endmill into the wasteboard at maximum force and keep doing so for a surprisingly scary amount of time - it will be trying to move down 25 inches instead of 25mm (about an inch).


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