acme thread for large wood screw

I made one of those, I used a 55 degree thread, 3.5 threads per inch. turn the tap out of high speed steel, mill in three flutes. The tap was so large I easily relieved it with a file, after blueing it. After hardening, it was sharpened with a fine stone on a die grinder. I cut very sharp threads in clear maple for the nuts useing beeswax as a lubricant. The screws were made with a router mounted in a metal lathe, I ground some 55 degree cutters, I calculated thread depth, and cut the thread one pass in slowest backgeared speed. Dennis