The concept
Tool lengths are measured (on a presetter or on the machine) and stored in the controller's offset table. When compensation is active, the controller shifts Z by the stored length so programmed Z values refer to the tool tip, not the spindle gauge line.
How controllers activate it
| Controller | Activation | Cancel |
|---|---|---|
| FANUC / Haas / Mitsubishi | G43 H5 Z50. | G49 (or with retract) |
| SINUMERIK | T5 M6 then D1 | D0 |
| Heidenhain | TOOL CALL 5 Z (length from tool table) | Next tool call |
| Okuma OSP | G56 H5 Z50. | G49 |
Safe activation
T5 M06
G00 G90 G54 X20. Y10. S8000 M03
G43 H05 Z50. M08 (Z move in the same block as G43)
G00 Z5.
G01 Z-2. F400.Activating G43 with a Z move to a safe height is standard practice: the first Z move after compensation is the one where a wrong length is most dangerous.
H-number mapping
- Most shops use H = tool number (T5 → H5), which the post enforces.
- Some use separate length offset numbers from the CAM tool library.
- Sister tools and tool life management may use different offset numbers.
Common mistakes
- Wrong H value after a tool renumbering.
- Compensation not reactivated after a
G28or tool change. - G49 cancelling at the wrong moment, causing a sudden Z move.
- CAM tool length not matching the real tool, making simulation unreliable.
Tool length errors are among the most common causes of crashes. The post should make them nearly impossible.
Safe tool length handling
See our CNC post processor development service and G43, G43.4 and TCP explained.