You say that you put two limit switches in series and it "does not work right". Does it work with just one limit switch connected? Does it work with no limit switches connected? I've been looking at a couple of examples recently where the BOB or motion controller has an analogue output for spindle speed, and it looks as if these often have a separate ground connection that should not be connected to the same ground that the limit switch inputs use on the BOB. I agree with Rob, though - it would be a lot easier to understand if you could provide a diagram and some details of the devices you are using. It's too easy to guess and make the wrong assumptions otherwise. If you are talking about spindle speed control here (not power to the spindle motor) it would be surprising if the problem was a lack of power because the limit switches and speed control inputs take quite low amounts of power. Interference is possible but shortage of power - probably not the problem.