Hi Arch, I haven't been able to check into the group much anymore (too busy turning), but it's always a pleasure to see your active involvement.
I have a 3520 and prefer the spring loaded spindle lock over the type that stays in place by itself. I find it significantly faster and safer because it is quicker to engage and release, and it releases automatically so I don't accidentally power up with it engaged. I started up my drill press one day with the key still in the chuck. I haven't been wearing my face shield at the drill press but now I am rethinking that.
Most of the time I don't need to hold in the spring loaded spindle lock on my lathes that have that feature because once it is engaged, simply maintaining rotational pressure on the chuck or workpiece will hold the lock in place. On the RARE occasion when I really want it to stay engaged by itself, I use the nail through the flanges method described in another post by "robo hippy" . I guess if I felt I needed it to stay in place often I would come up with something quicker - ought to be easy to figure out something spring loaded or using a strong magnet. I can envision a cam on the end of a lever (like on a cam operated bar clamp) attached to the flanges; that might be the best solution.
I prefer tommy bar chucks and have made bars that hang from the chucks and wedge against the bed so it frees up my left hand to work the other tommy bar or hold the spindle lock. The hanging bar is shaped like a "7" - a short nub goes into the top hole, it wraps around the chuck, and the long handle hangs down all the way to the bed so it can wedge against or between the ways if I want that hand free (nearly always). This kind of bar, by the way, will never make a hole go oval.
best wishes,
-mike paulson, fort collins, co