Simon,
My Boxford had a GEC Gemini DC controller that failed, so I replaced it with one from BEEL:-
http://www.beel.ca/smcmanual.pdfYour Lenze one should be very good quality and design. Have you found a manual for it? I don't think it will be an inverter, as they are used for AC drives and I think that yours is DC, have a look at the motor plate. The transformer looking device is a choke (an inductor), wired in series with the armature supply to slow the rise time of the current.
Assuming DC, it is not a simple mater to reverse it at speed. Just throwing a switch to reverse the voltage on the brushes will cause the back emf from the motor to be added to the applied voltage and will fail the motor and/or driver. Lots of amps will be gererated, which would cause arcing of the brushes and on any changeover relay. So, what is usually done is that at a reverse command, a relay drops out cutting the drive current to the motor and shorting the motor terminals with a low value, high wattage, braking resiistor. This brings the motor to a stop. Either a time delay or a motor voltage sensor is used to make sure it is stopped. It is then safe to start the motor in reverse with another relay. My 125 has the two relays mechanically interlocked to avoid forward and reverse being energised at the same time.
Your Lenze controller may be more inteligent and have all this built in so that it is only a matter of applying a control voltage to a forward or reverse terminal. It should also have terminals for speed control. This usually 0-10 volts DC with a centre tap so as to connect a 10K ohm variable resistor (pot). You can then use this voltage to power the speed control on the C11 card and the output from the card to the centre tap on the motor controller. If it is an AC drive, the inverter will have all this as standard and will be programmable by inputting various parameters via a keyboard.
Unless you must have rigid tapping or use a fixed die for threading, just use a pot to manually control the speed to begin with. If you want variable speed control from Mach, you need to replace the timing disc on the back end of the spindle with only one wide slot - see the Mach turning manual. You might be able to use the Boxford slotted opto switch.
Have fun!
Ian