threading up to a shoulder, I keep bailing

10K, 16 tpi internal is not the problem, external up to a shoulder is. I pull out early, the thread is short, the nut won't start. I have not cut a relief groove because I'm afraid of the groove becoming the failure. 3/4-16, mild.

Cut the groove. A groove you meant is safer than a tool you yanked. Width of the groove only has to clear the tool. Mark the carriage stop with a clamp so your hand has a place to arrive.

The fear of the groove is how people crash the tool into the shoulder and then write a thread about the half nuts. Ask me how I know. Don't.

Groove cut. Clamp on the bed as a stop, not as a crash pad. Nut starts. I still pull out like the shoulder is hot. That's fine. It fits.