Finally stopped the spin...

[dubby]

I've tried lots and lots of different methods to tap these little 1/2" thick, 1.25" rounds to 5/16-18. Lots. Some methods have worked, some have failed. Every one of them has had some sort of drawback.

Using a vise to hold them, I ended up with either severely scarred up edges from the teeth on the jaws, or I'd end up with an out of round thread where I actually tightened the vise down so much it would oval-shape the center hole.

I've tried clamping them from the top, using v-blocks, tapping in my lathe, tapping in my drill press, and even holding the tap in the vise and trying to spin the work on by other means. Lots of time the actual setup to tap the hole took longer than the process. Either way, it's just been frustrating.

Finally, I believe I've hit upon a method that works. It's fast. It's simple. It has minimal scarring on the workpiece. The drawback is that it's not adjustable for any other sizes than the two that I built the rig for. I put this little chunk of aluminum in the CNC mill and programmed it to cut two pockets, one 1.5" and one 1.25". I had it go ahead and clear out a 1/2" hole in the center all the way through the piece so the tap and chips could clear. I then changed tools and ran a line to help me find the center of the holes on the edges. I pulled it off the mill and drilled a couple holes for 1/4-20 threads into the upper pockets.

Again, not real complicated but it proved to be extremely effective. Here's the block in my hand tapper ready to work. I need to find or make a speed handle to manipulate the screw but had a water leak to fix and had to stop for the day...