Machines do not skip lines out of moodiness, so a skipped tool change is really a misunderstanding with a short list of authors: a switch, a convention, a restart, or the magazine itself. The audit is fast because every author leaves a signature in the program text or the control’s panel, and reading for them beats rerunning on hope by exactly one crashed tool.
Why this symptom deserves an immediate stop
Before the doors, the stakes: a skipped tool change is uniquely corrosive because everything after it still runs. The program continues confidently with the wrong iron in the spindle, every length and diameter offset from that point pairing with a tool that is not there, and the failure surfaces wherever the mismatch first matters, an air-cut if you are lucky, a plunge if you are not. So the operational rule precedes the diagnosis: the moment a change you expected did not visibly happen, stop the cycle, because the cheap mystery is about to compound into an expensive one.
The five doors
| The door | The mechanism | The signature |
|---|---|---|
| Block delete | The change line starts with a slash and the switch is on | A / at the line’s start; the panel switch lit |
| T without M06 | The line stages a tool but never executes the change | T word present, M06 absent |
| Staging conventions misread | T calls ahead; M06 changes; the reader confused the two | T words mid-operation, changes elsewhere |
| Restart past the change | Mid-program entry skipped the block entirely | The restart point sits below the change |
| Magazine logic | The called tool was already in the spindle | No visible skip at all: the call resolved silently |
Doors one and two: the mechanical explanations
Block delete is the honest switch with the dishonest reputation: lines beginning with a slash run only when the switch is off, the block-skip slash feature documented per control and used for optional operations, and a tool-change block parked behind a slash by some previous process decision vanishes whenever the switch flips. Programs age, slashes outlive their reasons, and the audit step is a text search for the character.
The T-without-M06 door is the dialect’s own design: on most machining centers the T word stages, instructing the magazine to ready a pocket, while M06 executes the exchange, the split that lets the next tool rotate into position during cutting. A line carrying T5 with no M06 changes nothing, correctly, and a program that lost its M06 in an edit drills tool 4’s holes with tool 3, which connects this page to its air-cutting sibling: the wrong tool under the right offsets produces Z errors in formation.
Doors three through five: the misunderstandings
Staging conventions vary enough by builder that reading someone else’s program means learning their choreography first: some shops put the T call lines before the change, some stage the next tool immediately after the previous change, and the tool-change anatomy on your specific machine defines which T means now and which means next. The restart door is the procedural one, a mid-program entry below the change block inherits the previous tool silently, which is precisely why tool changes make the gold-standard restart points. And the magazine door is the non-event: calling for the tool already in the spindle resolves invisibly on most machines, no skip occurred, and the surprise belongs to the operator’s expectations rather than the run.
The post-skip audit, before anything reruns
A skipped change contaminates everything downstream that assumed it happened: H and D offset numbers pair with tools, so each call after the skip is now wrong-length, wrong-diameter arithmetic waiting for contact. The recovery is mechanical: identify what is physically in the spindle, re-enter at the skipped change itself rather than after it, prove the first motion in single block, and let the program-versus-reality discipline carry the rest. The reading skill underneath, T-and-M06 choreography, slash conventions, offset pairings, recognized at a glance, is the standard recall material the free 60-second rounds on the G-code practice page keep sharp, and tool-change blocks are among its highest-value applications: small, dense, and expensive to misread.
Sources
Frequently asked questions
Why did my CNC program skip a tool change?
Five doors: block delete active on a slashed line, a T word without M06, staging conventions misread, a restart that entered past the change, or the magazine resolving a call for the tool already in the spindle. Read the change block and its neighbors against your control’s conventions.
What is the difference between a T word and M06?
On most machining centers, T stages the tool in the magazine while M06 executes the physical exchange, a split that lets the next tool ready itself during cutting. Builders vary on the choreography; the machine’s documentation defines yours.
How does block delete cause skipped tool changes?
Lines starting with a slash run only when the block-delete switch is off. A change block parked behind a slash for some past purpose vanishes when the switch flips. Search the program for slashes.
What should I check before rerunning after a skipped change?
What is physically in the spindle versus what the program believes, because every downstream offset call is now suspect. Re-enter at the skipped change itself and prove the first motion in single block.