I have seen requests from a few guys on the Yahoo group wanting to see pics of how a spindle speed sensor can be fitted to a Bridgeport with a QC spindle nose and decided to post here and link them to iit.
Here are a couple of pics of how I did it. I made the delrin part that holds the slotted wheel to fit into the top of the spindle then bored it and slotted so a bolt can pass through, I hexed the  inside end, at the bottom,  slightly smaller than the size of the nut then as  I tightened the bolt the nut was pulled into the shaft and the delrin  expanded slightly and gripped solidly inside the spindle.

Hood

Hood, I've been following the yahoo posts regarding this feature.  I also added proximity sensor to the top of my QC spindle  on a series 1 BP mill and it works great.
Here is a question thou.  I'm using VFD through Modbus and have pulley #1 set up for 500 - 5200 rpm with varispeed setting sitting at 3300rpm.  This works great.  The issue is when I switch to LO gear spindle pulley #2 (50-499rpm) the index becomes 1/9th of actual index.  Mach just drives the motor at rpm of pulley 2 and uses "ratio" to display new rpm.
How do I set up Mach to drive motor 0 to 100% in LO gear instead of 0 to 10% ?
Thanks

Afraid I dont know about modbus control but I suspect you will have to use your M3 macro to set it accordingly.
Hood