I started making the backplate for the 3" independent 4-jaw chuck for the dividing head (1 3/4-8 TPI). I bored it first :
I used a small threading tool (too small for the 1 3/4-8 thread) to try and get the thread close. I switched over to a larger boring bar to hog out as much as I could without deforming the thread. Then (because I wanted the thread perfect) I used a 1 3/4-8 tap to finish the threads :
One has to check the thread to make sure it fits the intended target, so, yeah, I picked up the entire dividing head and threaded it in to make sure :
Next was getting the thread sizes. It's a three-bolt-pattern. I measured using actual pins here, but took the picture without the pins as the reminder of what I did when doing the calculations :
With a segment spacing, of 2.25", I could start calculating the diameter of the pattern. I spent about a half hour on it before my brain kicked in, and I grabbed my Machinists Handbook.
There is a table of segment sizes based on a 1" diameter. For the three segment pattern, the segment size is 0.866025" . I divided the segment size (2.25") by the table size (0.866025") and got 2.598". Divided by 2, and it's a 1.299" radius. Now, I can put the back plate backwards on the dividing head, install it to my mill centered on the spindle, and then move the table 1.299" in any direction, and then I can mill the pockets and drill the holes for the bolts, yank the plate and install the chuck.
