Mori Yasnac loses zero position after threading cycle

I’m using a G96 threading cycle on my SL-3. It’s in a subroutine that gets called several times. It seems that after the first run my threading depth, x-axis, may change and my threads are cut too deep. It’s inconsistent, may change after the first part or after the tenth part. Has anybody seen this before?

More likely to be a mechanical issue. Check the belts and couplings for the motor and encoder on X.

Since it is an SL series machine it is going to be an older control. Those were a bit fussy about encoder signals. I have seen random mis-positioning on older Yasnacs due to imbalanced encoder pulse width where the on time and the off time of a channel were not well balanced. If you have an oscilloscope you can check that.

Has it ever mis-positioned outside of a threading cycle?

Thank you for your responses. As usual, you guys have brought up an issue that is likely to be the problem. Thank you! Although not what I wanted to hear. The drive belts are old and probably need to be replaced. But, let me add to the explanation of the current job. I'm using a G92 canned cycle. The G code I originally posted was incorrect. This Yasnac control seems to have 3 different canned cycles for threading. Is that normal?

Another thing, and to answer a question, this is the first time I've had trouble with positioning. I wondered if it might be because right after the threading cycle I'm using the same tool to chamfer the edge using a regular feed.

Finally, if the belts are the problem, wouldn't slowing down the spindle speed help? I haven't tried that, but I'm thinking a slower spindle could eliminate the problem for this job and also confirm that this is really the problem.

Here's my code:

O0011
N104 (*** SUBROUTINE *** SUBROUTINE ***)
N106 M18 (REVERSE TOOL CHANGE)
N108 G00 T0909(LOAD TOOL - DRILL)
N110 M17 (FORWARD TOOL CHANGE)
N112 G00 X0.000 Z5.700(MOVE TO DRILL POSITION)
N114 S0600 M03 (SPINDLE ON)
N116 M08(COOLANT ON)
N118 G01 Z4.200 F0.01 (DRILL)
N120 G00 Z5.700 (PULL OUT)
N122 M05 (SPINDLE OFF)
N124 M09(COLLANT OFF)
N126 G00X-5.0Z10.0(MOVE TO TOOL CHANGE POSITION)
N128 G00 T1111(LOAD TOOL - INSIDE THREADER)
N130 G00 X-0.590 Z1.600(MOVE TO START POSITION)
N132 S500 M03 (SPINDLE ON)
N134 M08 (COOLANT ON)
N136 G92 X-0.702 Z0.100 F0.0984 (THREAD CUT CYCLE)
N138 X-0.732
N140 X-0.742
N142 X-0.752
N143 X-0.757
N144 X-0.762
N145 X-0.767
N146 X-0.772
N148 X-0.777
N150 X-0.782
N152 X-0.787
N154 X-0.792
N156 X-0.797
N158 X-0.798
N166 G00 X-0.815 Z1.300 (LOCATE FOR CHAMFER)
N168 G01 Z1.220 F0.006 (CHAMFER)
N170 G00 Z1.550 (PULL OUT)
N172 M05 (SPINDLE OFF)
N174 M09 (COOLANT OFF)
N176 G00 X-5.0 Z8.00(MOVE TO TOOL CHANGE POSITION)
N178 G00 T0404(LOAD TOOL - TURNING/FACING)
N180 M08 (COOLANT ON)
N182 G00 X-1.110 Z-0.795(MOVE TO POSITION)
N184 M03 (SPINDLE ON)
N186 G01 X-1.255 Z-0.860 F0.008(CUT CHAMFER)
N188 M05 (SPINDLE OFF)
N190 M09 (COOLANT OFF)
N192 G00 X-5.0 Z4.00(MOVE OUT OF THE WAY)
N194 M00 (*** TURN PART OVER ***)
N196 M08 (COOLANT ON)
N198 G00 X-1.110 Z-0.795(MOVE TO POSITION)
N200 M03 (SPINDLE ON)
N202 G01 X-1.255 Z-0.860 F0.008(CUT CHAMFER)
N204 M05 (SPINDLE OFF)
N206 M09 (COOLANT OFF)
N208 G00 X-5.0 Z6.00(MOVE TO TOOL CHANGE POSITION)
N209 M18 (REVERSE TOOL CHANGE)
N210 G00 T1111(LOAD TOOL - INSIDE THREADER)
N211 M17 (FORWARD TOOL CHANGE)
N212 M08(COOLANT ON)
N214 G00 X-0.810 Z1.300 (LOCATE FOR CHAMFER)
N216 M03 (SPINDLE ON)
N218 G01 Z1.220 F0.006 (CUT CHAMFER)
N220 G00 Z1.550 (PULL OUT)
N222 M05 (SPINDLE OFF)
N224 M09 (COOLANT OFF)
N226 G00 X-5.0 Z12.00(MOVE TO TOOL CHANGE POSITION)
N228 M00 (*** NEXT PART ***)
N230 M99 (END SUBROUTINE)

Yes it is common to have multiple threading cycles available. Just depends on which options the machine builder or initial buyer selected.

Your mis-position problem might be caused by old or worn timing belt on the axis. I would expect that to show up in all types of cutting, not just threading.

Using the threading tool to cut a chamfer should not cause a positioning problem.