Ready Steady Eddy

[Ger21]

Ok Both motors drive upto first switch then both back off then Slaved motor drives upto it's own switch while Master stays still then backs off.

Sorry my friend, but that is not correct. I didn't think so when I read it, so I just went out to the garage to check. My machine has a lot of flex, so I can easily rack the gantry one way or the other. I have dampers on my steppers, so with the power off, I can move one side by turning the dampers.

First I moved the master side about 3mm away from home, then powered up the machine, and homed the X axis. The slave axis hits the switch, and backs off, while the master keeps moving to it's switch, then it backs off.

Next, I moved the slave side 3mm farther from home. This time, the master hits the switch, and backs off, while the slave keeps moving.

Each side of the gantry (master and slave) move independently to their own switches. For all practical purposes, they are not slaved during homing.

1. Do both X home switches have to set so they both activate together exactly when the gantry is at the physical 'square' position ?

Yes, but technically, no. The Home position is not set when the switch activates, but when it deactivates as it's backing off. So technically, the switches need to be set so that they both deactivate when the gantry is square

2. If so, we are now relying on the repeatability of the switches for machine accuracy.

Yes, but as Jazz said, even cheap switches are quite accurate. The DIY Hall switches I use are supposed to ba accurate to about .01mm or better, and only cost about $3 each to make them. The most important factor is accurately positioning the switches.

3. Imagine a cutting job has just been completed so that the gantry has been up and down loads of times and it's now out of square. When it comes back to the home position it will hit one home switch first, so then what happens ?

The side that hits the switch will back off and stop, while the other side continues to it's switch, squaring the gantry.

Depending on the type of switch, you may or may not actually see the machine back off the switch. On my machine, it backs off such a small amount that it appears to just stop.