CNC setup
CNC Touch Probes and Zeroing: Stop Eyeballing Your Origin
Updated July 7, 2026
Every ruined carve traces back to one of three zeros. X and Y decide where the cut lands; Z decides whether a V-carve is crisp or bloated and whether a through-cut goes through. A $20 touch plate sets all three faster and more repeatably than any human with a sheet of paper — this is the highest value-per-dollar upgrade in hobby CNC.
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The three ways to zero, honestly compared
| Method | Z accuracy | Time | Cost | The catch |
|---|---|---|---|---|
| Paper drag | ±0.05–0.1 mm on a good day | 1–2 min per axis | $0 | Depends on feel; worse when tired, rushed, or new |
| Z touch plate | ±0.01–0.02 mm, repeatable | 15 seconds | $10–20 | Z only — X/Y still by eye |
| XYZ corner plate | ±0.01–0.02 mm all axes | ~60 seconds | $25–50 | Needs a corner; bit diameter must be entered correctly |
How probing works on a GRBL machine
Every GRBL board (which means every Genmitsu, FoxAlien, and most hobby machines) has probe pins. The plate sits on your stock, a magnet clips to the bit, and the `G38.2` probe command drives the bit down slowly until electrical contact — then the controller does the math: contact height minus the plate's known thickness equals stock surface. Senders like UGS, Candle, and gSender have one-button probe macros, so in practice you click 'Probe Z' and wait four seconds.
The XYZ corner version repeats the trick against two machined edges to find X and Y. Enter the bit diameter correctly — the controller offsets by the bit radius, and a 3.175 mm bit probed as 6.35 mm puts every cut 1.6 mm off.
Where each zero should live
Z on the stock top for carving and engraving — depth errors then land harmlessly at the bottom of through-cuts. Z on the spoilboard when the through-cut face matters more than engraving depth, or when stock thickness varies (rough lumber). X/Y on the stock corner for one-offs; X/Y on a fixed fixture corner for repeat jobs — probe once, and every future blank clamped against the fence or fixture pins inherits the origin.
That last pattern is the production unlock: a batch of 20 coasters with a fixture and corner probe means one setup, not twenty.
Probing hardware worth owning
XYZ corner touch plate
The full three-axis zero in one probe cycle. Machined aluminum with a known edge thickness — check that your sender has a matching macro (most do).
See options on Amazon →Simple Z touch plate
If your workflow zeroes X/Y off fixtures anyway, a $12 puck does the only zero that changes every job.
See options on Amazon →Digital calipers
Verify the plate's actual thickness (engraved values are sometimes off by 0.05 mm+) and measure stock. The single most-used tool at any CNC.
See options on Amazon →Feeler gauge set
The trust-but-verify tool: after probing, a 0.05 mm feeler under the bit at Z=0.05 should just drag. Also sets dust shoe and limit-switch clearances.
See options on Amazon →Probe-day setup checklist
- Measure your plate's real thickness with calipers and put that number in the sender's probe settings — once.
- Test the circuit before the first probe: touch the plate to the bit by hand and confirm the sender shows the probe pin triggered. An open circuit means the bit dives through the plate.
- Set probe feed slow (25–50 mm/min for the final touch). Fast probing overshoots and lies.
- Clip the magnet to bare bit or collet metal — coated bits can insulate the contact.
- After probing, jog to X0 Y0 and sanity-check it's where you think the corner is before pressing run.
When zeroing is not your problem
If the origin drifts during a job rather than between jobs, probing won't save you: that's lost steps from a snagged dust hose, a slipping pulley, or feeds beyond what the machine can hold. And a consistent depth gradient across the bed is spoilboard flatness, not zero error. Probe accuracy only shows up when the rest of the machine is behaving.
Frequently asked questions
Do touch probes work with every hobby CNC?
Nearly — any GRBL-based controller (Genmitsu, FoxAlien, most kits) exposes probe pins, and Marlin and LinuxCNC machines support probing too. Check for a two-pin 'probe' or 'A5' header on the board; the plate is just a switch that closes when bit meets plate.
How much accuracy do I really need for wood?
For profiles and pockets, ±0.1 mm is invisible in wood. V-carving is the exception: letter width errors are about 1.7× the Z error with a 60° bit, so 0.1 mm of Z slop visibly fattens fine text. Carve signs and the probe pays for itself immediately.
Why is my probed Z consistently too deep or too shallow?
The plate thickness in your sender doesn't match reality. Measure the plate with calipers, including any label or anodizing, and update the setting. A constant offset is always a constant somewhere in the config.
Can I probe the top of every workpiece even if it's cupped?
You can probe only one point, so pick it near where the detail matters most, or flatten the stock first. For seriously cupped boards, some senders support probing a grid and applying a height map — but at that point, surfacing the blank is faster and better.
What's the difference between machine zero (homing) and work zero?
Homing (limit switches) tells the machine where its own frame is; work zero tells it where your stock is. Probes set work zero. You can run without homing switches, but homing plus a fixed fixture means you can recover a crashed or power-cycled job mid-cut — worth setting up together.