Dear All,

I am considering adding encoders and home switches to my Sherline CNC mill.

I see 2 benefits:
1) Being *sure* of not loosing steps (estop, ...)
2) Using (low precision) home switches and the index of the encoder, I get
*excellent* repeatability when homing.

The mill is equipped with regular sherline steppers (135 oz-in, 1.8 degrees)
and a P/N 8760 driver (800 step/rev.) connected to a parallel port. The
steppers have a double ended 1/4" shaft. They are driving a 1mm master
screw. Everything is connected to EMC2 running on a dedicated PC. All is
actually working fine, with rapids of 450 mm/mn. I have a 4th axis on a
turntable with the same stepper.

I expect to use US digital E2 quadrature encoders with 400 CPR and index,
giving a maximal theorical resolution of 1/400 mm, more than needed :) for a
cost of about ~200$.

I expect to use the E2 T model, which will be sticked on the rear of the
stepper, in place of the handwheel, using the US digital centering tool.

I expect to connect the outputs of the encoders to a second parallel port on
a PCI card.

My questions:

1) Is it stupid :) ?

2) Has anyone already done that?

3) Is a parallel post fast enough for reading the 4 axis encoder pulses at
full speed?

4) Are there other encoders suitable? Is 400 CPR the correct value?

5) I suspect that HAL+classicladder is powerful enough to handle the
*complex* homing (eg. Zero till the home switch trips (low precision
microswitch), then forward to the encoder index pulse, for all joints). Am I
right? If not, is it possible to program the homing sequence in C ?

6) Any tips in setting up HAL? PID tuning? Best mode for the parallel port?
Any existing example?

Thanks a lot,


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