I’ve been building control panels for nearly 50 years, so I am well aware of noise issues and very seldom have a problem. Proper grounding, shielded cables, control wires on opposite side of cabinet from power wires, flyback diodes, etc.  Recently I was building a solenoid pump driver. I needed one flyback diode on the coil and with the demise of Radio Shack could not get one quickly. So I took the one on the spindle relay on my Mach 3 mill.

A few weeks went by and I set up job and noticed a large thump noise as I turned the spindle on and off. Then I saw that all 4 axis were jumping a full step every time the spindle turned off!  I’m thinking “Nah, couldn’t be that missing diode.”  The replacement had come in and sure enough that solved the problem!

I’d bet half the troubles we see here are due to simple stuff like this.