CNC Touch Probes for Desktop Routers: Wiring, Tool Length Offset, and Work Zero Setup

Our own guide to zeroing without a touch probe covers the manual paper-slip method that works fine for a single bit and a single job. A touch probe earns its keep the moment you start swapping bits mid-job or running repeat production, since it turns "carefully lower the bit until a piece of paper just barely drags" into a repeatable, automated measurement that takes the same two seconds every time. This guide covers wiring a probe into GRBL, using it to set work zero and tool length offsets, and building a simple one yourself if you don't want to buy a dedicated probe.

What a Touch Probe Actually Does

Electrically, a touch probe is nothing more than a switch: the spinning bit (or a probe plate touching the bit) and the machine's frame or a clip form two sides of a circuit that closes the instant they make contact. GRBL's G38.2 command drives an axis toward a target at a controlled feed rate and stops the instant that circuit closes, recording the exact machine position at the moment of contact. That single primitive is what every "probe work zero" and "probe tool length" feature in gSender, UGS, CNCjs, and LightBurn's GRBL module is built on top of — the software just automates running G38.2, reading the result, and applying it as an offset.

Probe Types

TypeHow It WorksTypical CostNotes Conductive plate + alligator clipBit touches a metal plate wired back to the controller's probe input$10-25Cheapest, works with any conductive bit; the DIY option below Spring-loaded touch probe (BLTouch-style repurposed)Plunger contact closes a switch on deflection$15-30More common on 3D printers, adaptable to CNC with the right firmware config Dedicated 3-axis CNC touch probe (tool setter puck)Precision-ground plate with a known, repeatable trigger height$30-80Best repeatability, worth it if you run multi-bit jobs often Toolsetter (fixed-mount, for tool length only)Bit is jogged down onto a fixed switch to measure length relative to a reference$25-60Mounted permanently on the table edge; fastest for repeat tool changes

Wiring It Into GRBL

GRBL boards expose a dedicated probe input pin (often labeled PRB or shared with an "A5"/analog pin depending on the board) that's separate from the limit switch inputs, and it needs to be normally-open: circuit open when the bit isn't touching, closed on contact. Wire one lead of the probe plate to that probe input and the other to the machine's ground/frame — if your setup uses an alligator clip directly on the bit or collet instead of a plate, clip it there and let the plate (or workpiece surface, for certain jobs) complete the circuit through the frame. Check your controller's $6 setting (probe pin invert) if your probe reads triggered when it shouldn't be or vice versa; this is the single most common first-setup mistake and takes thirty seconds to flip.

Before trusting it on a real job, jog the bit down by hand toward the plate in a GRBL sender's console and watch the probe status indicator (most senders show a live probe pin state) to confirm it flips exactly on contact and not before, from static or a loose wire, or after, from a dirty or oxidized contact surface.

Setting Work Zero with a Probe

Most senders (gSender, UGS, CNCjs) have a dedicated probe dialog: you jog the bit near the corner or point you want as zero, enter the probe plate's known thickness, and hit probe. The software runs G38.2 downward until contact, then automatically sets Z-zero at the workpiece surface accounting for the plate thickness you entered — no mental math, no paper-slip feel test, and it's exactly repeatable every time you re-zero the same setup. For X/Y zero on a corner, the same probing sequence typically runs in each horizontal direction using the plate against a squared edge or corner block.

Tool Length Offset for Multi-Bit Jobs

The real payoff shows up on jobs that need more than one bit — a roughing pass with a larger bit followed by a V-bit detail pass, for example. Set a reference tool length once at the start of the job by probing the first bit's tip position, then every time you swap bits, re-probe at the same physical location and the sender calculates the difference (the tool length offset) and applies it automatically, so the new bit's Z-zero lines up with the job's original zero without you touching a single setting by hand. This is exactly what a dedicated toolsetter is optimized for: a fixed, always-in-the-same-spot probe point specifically for this workflow, versus moving a plate around the table for basic work-zero probing.

Building a Simple DIY Plate Probe

A basic plate probe is genuinely a weekend-afternoon build: a flat piece of aluminum or steel of known, consistent thickness (measure it with calipers and write the number down somewhere you won't lose it), a wire soldered or bolted to one corner, an alligator clip on the other end of that wire for connecting to the bit or collet, and a second wire running from the controller's probe input to the plate. Keep the contact surfaces clean — a plate covered in aluminum oxide or wood dust residue causes inconsistent trigger points, which shows up as slightly-off Z depths that seem to wander for no obvious reason.

Safety Notes

Never probe with the spindle or router spinning — the whole method relies on slow, controlled contact at rest, and probing into a spinning bit is both pointless (you'd damage the plate) and a way to fling metal shavings or the plate itself. Set a sane probe travel distance limit in your G-code or sender settings so a probe that fails to trigger (bad wiring, dirty contact) doesn't drive the bit straight through the plate and into the spoilboard or table. Disconnect the probe's ground/frame lead before doing any work near the machine's electronics with the probe circuit still live, since an unexpected momentary short can trigger a probe cycle mid-adjustment.

A ten-dollar plate and a wire is genuinely one of the highest value-per-dollar upgrades available for a GRBL router, and it pays for itself the first time you run a two-bit job without babysitting the Z-height change by feel. Once it's wired in, the workflow gets out of the way entirely — probe, cut, swap bit, probe again, keep going.

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