The left hand thread is started at the headstock end and fed toward tailstock. Bore the stock to minor diameter, Cut a relief groove at the bottom of the bore. The bore dia. should be a tad larger than the dia of major dia. of thread. example( 3/4" -5 acme) 3/4" would be the major. Relief groove should be wide enough to accommodate threading tool. Tip width of Acme threading tool is different for each Acme pitch. 29 deg. included with flat at tip.( Look up in machinery handbook) Set compound to 14.5 Deg. Run tool by hand to bottom of bore and set stop. This is so you can return to relief groove blind. Then it just like right hand threading but backwards. If you are cutting a big acme thread you want to rough out with different tool. Take your time and go slow. You will take very small depth of cuts and scratch the metal out. It will take many passes to cut the tread. Good luck.