I do not have direct experience with controlling a spindle via an SSR.

Based on the description in http://www.opto22.com/documents/0859...data_sheet.pdf it looks like you may be able to find a suitable SSR that can control a spindle.

If you connect the positive control terminal of such an SSR to +5 volts, and connect the negative control terminal of the SSR to one of the BOB's optoisolator output terminals, it looks like you should be able to switch the SSR on and off via the BOB.

As far as safety is concerned, if you are driving the BOB from Mach3 software, you may want to consider using a "charge pump" detection circuit to ensure that your board can only be activated when Mach3 is "in control" of the LPT port signals. For one example of this, see http://www.micro-machine-shop.com/C4...p%20manual.pdf