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Siemens SINUMERIK Controller
Spindle · Intermediate · 30-120 minutes
Siemens 840D 21670 - Channel block illegal change of tool direction with $AA_TOFF
Alarm 21670 fires when an NC block attempts to modify the tool offset axis assignment (parameter $AA_TOFF[i]) while a directional tool offset is already active. The SINUMERIK 840D NCK prevents mid-block offset axis reassignment to maintain kinematic integrity and prevent undefined tool path geometry.
21670
Siemens 840D 21670 - Channel block illegal change of tool direction with $AA_TOFF
Channel block illegal change of tool direction with $AA_TOFF
Siemens SINUMERIK 840D alarm 21670: Channel block illegal change of tool direction with $AA_TOFF. This alarm indicates a program or channel-level error on the SINUMERIK control. The part program or machine function cannot be executed as commanded.
Root Cause Summary
Alarm 21670 fires when an NC block attempts to modify the tool offset axis assignment (parameter $AA_TOFF[i]) while a directional tool offset is already active. The SINUMERIK 840D NCK prevents mid-block offset axis reassignment to maintain kinematic integrity and prevent undefined tool path geometry.
Safety
- Refer to Siemens documentation
- Ensure qualified technician performs repairs
Causes and Fixes
- 1
Review the NC program block sequence around the alarm occurrence. Identify if G17/G18/G19 command appears after $AA_TOFF activation. Search for pattern: $AA_TOFF[i] assignment followed by plane change in same or next block.
- 2
Deactivate tool offset using $AA_TOFF[i]=0 or TOFF before executing any plane change. Insert explicit offset cancellation block: 'N10 $AA_TOFF[1]=0' before 'N20 G18' to reset offset axis assignment.
Failure to cancel offset before plane change will cause undefined tool compensation geometry.
- 3
Verify MD10200 (AXCONF_GEOMETRY_AXIS_DEF_TAB) parameter settings and confirm geometry axis count matches active plane. Use Siemens HMI Diagnostics > Axis > Geometry to validate current plane state.
- 4
Revalidate partprogram syntax using SINUMERIK Editor with 'Check Program' function (HMI menu: Program > Diagnose > Program Check) to identify all offset-plane conflicts before machine cycle restart.
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