I'm pretty new to the forums but not new to Shopsmith. I found the forum relatively recently when I found and Googled a table flatness issue with a used 520 that I had bought. Since then I've done a lot of reading of the entries, but only recently registered so that I could join the discussion. I found this thread while looking for advice on a "wobbly bandsaw." What I found was that manufacturing variations may create cases where screwing the studs in, then turning them out an equal amount, does not assure success. This is written regarding a 2004 Model 520.
Variation in the Headrest is the first and most obvious contributor to my problem.
Note that the lock/clamp hole on the right (front) of the Headrest is visibly deeper than the one on the left. It is difficult to measure accurately, or I don't have the right tools, but the two holes differ by about 0.07 inches. The SPTclamp, as it is called here (aka Accessory Mounting Lock) shown above the headrest is adjusted with the front side out farther to offset for the casting difference. The studs are 16TPI, and the square head gives us 1/4 turn (1/64 inch) resolution. To offset for this casting feature I need to back the front stud out between 1 and 1-1/4 turns more than the rear stud.
The other contributor in this case was that the specific SPTclamp, or Accessory Mounting Lock, had its own bias such that with both studs screwed in all the way, the left (rear) stud protruded farther from the handle:
The offending clamp is on the top. A clamp that I consider normal is on the bottom and they are sitting on jigs for emphasis. To offset for this I needed to back out the right stud between 1/2 and 3/4 turn before making the adjustment to offset for the casting.
Using the approach I've used in the past, and as described in this thread, does not work on this machine. It results in only one lock engaging. I calculated I needed to back the front stud out 1-3/4 turns more than the back stud, but that was too much to install it. So I used 1-1/4 turn and this was well within the tolerance of this casting, the handle wasn't centered but had gaps on both sides, indicating both locks engaged.