The cut was fine. The rapid across the top wasn't.
Between operations, the tool rapids up to a “safe” Z, traverses to the next position, and comes back down. That safe Z — the clearance plane — is supposed to be high enough to clear everything the tool passes over: the part, the fixtures, the clamps, any raised feature. When it is, the rapid is invisible. When it isn’t — when the safe Z is set below the top of a clamp, or a tall fixture, or a feature standing above the stock — the tool traverses at that height and drives straight into the obstacle at rapid speed. The cutting was correct. The crash is in the move between cuts, at the worst possible feedrate.
It’s an easy mistake precisely because the safe Z looks fine in the part. Every operation reaches its feature; the finished geometry is right. The clearance plane is only about the space above the work, and that space is exactly what a program listing and a part-focused check don’t show.
How a safe Z goes wrong
- Set below a clamp or fixture. The clearance plane clears the part but not the workholding standing above it. The rapid transit clips the clamp.
- A raised feature the tool must cross. A boss, a previously machined stand-off, or a fixture rib sits above the safe Z, and the traverse to the next operation drives into it.
- Copied from a job with a different setup. A safe Z proven on open, flat stock, pasted into a job with tall fixtures, brings a clearance that no longer clears.
- A modal retract left too low. A clearance height set for one operation carried into another with more standing above it.
- Confusing part-Z clearance with machine-Z retract. A retract to a work-coordinate Z isn’t the same as a safe machine position (G53 Z); assuming one for the other can leave the tool lower than intended.
Every one of these clears the features perfectly. It’s the between-features motion, over the real fixtures, that the programmer didn’t picture.
Why this only shows up with the real fixtures in the scene
The listing can’t show it. A safe Z is a number; whether it clears the setup depends on what’s standing in the setup. Reading the code, the clearance looks like a reasonable height — because it is, for the part. The obstacle isn’t in the text.
A simulation without the real fixtures can’t show it either. This is a genuine collision, so in principle a check can catch it — if the fixtures and clamps are modeled, in the right place. If the verification runs on an empty table or an idealized setup, there’s nothing above the safe Z for the tool to hit, and the crash that will happen on the machine simply doesn’t appear. A CAM simulation driven by its own setup assumptions may not represent the real workholding at all.
So catching it reliably needs two things together: the real program executed the way the control runs the rapids, and the real fixtures in place.
Where Eureka G-Code fits
Eureka G-Code executes the true ISO on a digital twin of your machine, with your real fixtures, clamps, and stock in the scene — so the rapid transits happen over the real setup, at the real clearance height. A safe Z set below a clamp, a traverse across a raised feature, a copied clearance that no longer clears, a modal retract left too low: each shows up as a collision or a near-miss on the twin, before the tool ever transits at that height on the machine. Because it distinguishes work-coordinate moves from machine-coordinate retracts (G53) the way the control does, the “I thought that retract was to a safe machine position” mistake is caught too.
Because it reads the real ISO regardless of origin, a clearance error that entered through a hand edit or a copied block is caught as the control will execute it — over your real workholding, not an empty table.
> Take a job with tall fixtures or raised features — anything standing above the stock between operations — and run the real ISO on a twin of your machine in Eureka G-Code with your fixtures in place. Watching the tool rapid across the real setup, before it does it for real, is how the clamp it was about to clip gets caught at a desk.
FAQ
What is a safe Z / clearance plane?
It’s the height the tool rapids up to between operations before traversing to the next position — set high enough to clear the part, fixtures, clamps, and any raised feature it passes over. Too low, and the rapid transit drives into whatever stands above it.
Why does my tool crash on a rapid when every operation is correct?
Almost always a clearance plane set too low. Each operation reaches its feature correctly, but the safe Z doesn’t clear a clamp or raised feature, so the traverse between operations crashes at rapid speed. The fault is in the between-features move, not the cutting.
Why might my simulation miss it?
Because the crash depends on the fixtures. If the workholding isn’t modeled in the right place, there’s nothing above the safe Z for the tool to hit in the simulation, so the crash that will happen on the machine doesn’t appear. Eureka G-Code runs the real program with your real fixtures in place.
What's the difference between part-Z clearance and machine-Z retract?
A retract to a work-coordinate Z depends on the active work offset; a machine-coordinate retract (G53 Z) goes to a fixed machine position. Confusing the two can leave the tool lower than intended. Eureka G-Code executes each the way the control does.
Does it catch a copied or hand-edited safe Z?
Yes. It reads the actual ISO the control receives, so a clearance error from a copied block or a hand edit is caught over your real setup exactly as the control will run it.
Next step
Eureka G-Code — request a demonstration on a digital twin of your own machine and controller.
Run the rapids over your real fixtures — on a digital twin — and catch the clearance that doesn’t clear before the tool does.