Why does my CNC forget its position or lose Z zero?
The machine cuts at the wrong depth, starts in the wrong place, or seems to forget its origin between jobs or after a stop. Underneath almost every version of this is a confusion between two coordinate systems - where the machine is, and where your part is - or a lost reference after an alarm.
Quick answer. The machine tracks two zeros: machine coordinates (its own home reference) and work coordinates (your part's zero). Most lost-origin problems are either setting work zero incorrectly, or an alarm wiping the machine's known position so it needs re-homing. Home first, then set work zero, and use a Z-probe for repeatable depth.
Work down the list, most likely first
These are ordered by how often they are the answer, not by severity. Check each in turn and stop at the first one that confirms.
- 1
Machine vs work coordinate confusion
Confirm it's this
The job runs in the wrong place or depth even though 'zero looks set'. You have set one coordinate system but the job references the other.The fix
Understand the split: machine coordinates come from homing and never move; work coordinates (G54 and friends) are where your stock is. Set work zero to your material's corner and top, and confirm the job is referencing work, not machine, zero. - 2
Z zero set inconsistently
Confirm it's this
Depth varies job to job or after a tool change. You are eyeballing Z or touching off by hand differently each time.The fix
Use a Z-probe or touch plate to set Z the same way every time, and re-zero Z after any tool change. Consistent Z reference is the whole game for depth accuracy. - 3
The machine forgot position after an alarm or power event
Confirm it's this
Position is lost after a hard-limit hit, an e-stop, a reset, or a power cycle. Open-loop machines have no memory of where they were.The fix
Re-home the machine to re-establish machine coordinates, then re-set or reload your work zero. After any alarm, homing is the first step, not just unlocking - see the GRBL guide. - 4
Lost steps masquerading as a lost origin
Confirm it's this
Position drifts progressively during a job rather than jumping after an event. The zero was fine; the machine slipped.The fix
This is a lost-steps problem, not a zeroing one - work through acceleration, binding and noise on that page.
Stopping it coming back
- Home at the start of every session so machine coordinates are always valid.
- Set and, where your sender allows, save work zero; re-zero Z on every tool change.
- Use a touch plate or probe for repeatable Z rather than touching off by eye.
Frequently asked
- What's the difference between machine coordinates and work coordinates?
- Machine coordinates are the machine's own reference, established by homing against its limit switches - they never move. Work coordinates are where your material sits, which you set for each job. The job is cut in work coordinates; homing gives the machine a stable machine-coordinate frame so soft limits and repeat positioning work.
- Why does my CNC lose its Z zero?
- Usually because Z was set by hand slightly differently each time, or because a tool change moved the reference and Z was not re-zeroed. Use a Z-probe or touch plate for a repeatable Z, and re-zero after every tool change. If position drifts during a cut rather than between setups, that is lost steps, not a zeroing problem.
Keep going
Diagnostic patterns are distilled from manufacturer documentation and established tooling references; where the community converges on a real-world tell, we state it. Exact feeds and speeds always come from the calculator, never invented here.