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Use halscope to do a plot of encoder.0.velocity pin during a pierce, might be interference, although as you are getting 55 and 75V readings it also seems not right, and this might be due to scaling and/or due to interference.
What divider ratio is Mesa THCAD set to? Usually 1/32.
If this is a new machine, disable auto volts, disable void lock and velocity lock, well disable everything automated until you get the machine working correctly, only then enable the automation.

What type of control do the drives/VFD take?
What encoders/resolvers are on the motors?
If they do turn out to be analog +-10V ones, a Mesa 7i97T should do just fine, and if you do need more I/O you can add a 7i84 with 32 inputs and 16 outputs.
Here are some topic to give you an idea of what is involved:
forum.linuxcnc.org/30-cnc-machines/31792...-sbz-130-01-retrofit
forum.linuxcnc.org/30-cnc-machines/33529-hurco-bmc-20p-retrofit
forum.linuxcnc.org/show-your-stuff/34986...nd-proma-thc?start=0
And detailed info on wiring and tuning
forum.linuxcnc.org/10-advanced-configura...to-example-mesa-7i77

Those motors do not seem to have encoders at all, so will not do for CNC control as you found out and i am late to point out.
You can try adding encoders as the lid is easy to open, but i fear it might be way to much work to attach them due to short shaft and limited space.
Might be possible using those magnetic encoders as they are simply glued to the shaft, so only drilling a hole for it and for the electronic on the lid, but i never used them so i have no idea how good and reliable they are.