This is an educational explanation of a common lathe crash, not operating instructions for your specific machine. Always follow your machine’s manual and shop procedures, and verify depth and offsets before boring.
Boring is internal work: the bar reaches down inside a hole where you cannot see the tip. On a lathe the Z axis runs along the part, so the depth of a bore is a Z move. That is exactly why this crash is so common. One Z number too far, and the bar drives into the back wall of a blind bore or the far end of the part, as the lathe and boring overview describes.
Why the back wall gets hit
Every cause comes down to the same thing: the tool ended up deeper in Z than the part allows.
| Cause | What happens | Fix |
|---|---|---|
| Depth value too deep | Programmed Z passes the back wall | Leave clearance from the bottom |
| Wrong Z zero / work shift | The whole job is shifted deeper | Verify Z zero before cutting |
| Absolute vs incremental mix | A Z move stacks from the wrong reference | Confirm G90 is active |
| Wrong tool offset | The control mislocates the tip | Re-check the tool geometry offset |
| Bad retract order | Bar drags or stays at depth | Come off the wall, then exit |
Depth is the dangerous number
The most common version is simply a Z that goes too far: a blind bore is, say, 30 mm deep, but the program feeds to a Z that reaches 32 mm. Because you cannot see it, nothing warns you until the bar bottoms out. Always leave a deliberate clearance between the programmed depth and the real back wall, and treat the depth value as the number to double-check. A wrong Z zero does the same thing to every move at once, which is the lathe version of the offset problems behind a G00 crash. The positioning behavior of these Z moves is documented in the LinuxCNC reference.
Absolute vs incremental, again
An absolute and incremental mix-up is brutal in a bore. Leave G91 active when the program expected G90, and a Z move adds to where the tool already is instead of going to a fixed depth, so it sinks further than intended. This is the same trap explained in G90 vs G91 crash prevention, but with no clearance to spare because the tool is already inside the hole.
Retract off the wall first
Getting out matters as much as getting in:
| Step | What you do |
|---|---|
| 1 | At depth, move X slightly toward the center to leave the bore wall |
| 2 | Rapid Z out of the hole |
| 3 | Then reposition for the next pass |
Retracting Z while the tool is still pressed on the wall drags the insert and can gouge the bore. Coming off in X first breaks contact cleanly. The standard code references show these as ordinary positioning moves, but the order is what keeps the bar safe.
Prove it before you cut
Because boring is blind, verification matters more than usual. Single block the first pass, keep a hand on feed hold, and confirm the depth, the Z zero, and the mode first. That is the same discipline as safely testing a program without crashing, and it pairs with the lathe spindle caution in G50 max spindle speed. The code literacy to catch a wrong depth quickly is what a practice routine on the G-code practice hub builds.
Bottom line
A boring bar hits the back of the part when Z travels too far: too deep a depth, a wrong Z zero, an absolute mix-up, a wrong offset, or a bad retract. Leave clearance from the back wall, verify Z zero and offsets, confirm G90, retract off the wall in X before exiting, and single block the first pass.
Sources
- Wikipedia: Lathe (boring, internal work)
- LinuxCNC G-code reference (positioning, Z moves)
- CNCCookbook: G-code and M-code cheat sheet
Frequently asked questions
Why did my boring bar crash into the back of the part?
Because it traveled too far in Z. Boring is blind, so any error that sends Z deeper than intended drives the bar into the back wall: a depth that is too deep, a wrong Z zero, an absolute versus incremental mix-up, or a bad retract.
What order should you retract a boring bar?
Come off the bore wall before exiting: move X slightly toward the center to break contact, then rapid Z out. Retracting Z while still on the wall can drag and gouge.
How do you stop boring crashes when learning?
Leave clearance from the back wall, verify the Z zero and tool offsets, confirm absolute mode, and single block the first pass with a hand on feed hold. Treat the depth as the most dangerous number.
What is the best way to learn lathe G-code safely?
Drill the codes with active recall so positioning codes and modes are automatic. A free app like G-Code Sprint quizzes the everyday codes and repeats whichever ones you miss.
G-Code Sprint is a study and practice tool only. Always follow your instructor, employer, machine manual, and shop safety procedures.