The first version of the Benchcrafted Tail Vise used a hex nut bolted to steel angle. While this was functionally good, the design was later refined to allow more vise capacity and easier installation.
Thus was born the massive 1" thick nut that we currently use. This nut starts out as a solid chunk of free-machining steel. Each block is sawn to rough length, then the milling begins. Each end of the nut is milled to precise length. Although this serves no functional purpose in the finished vise, it allows accurate positioning for the other operations that the nut must go through. Plus, it makes for a nicely finished part when you take it out of the box. The pilot hole for the acme tap is drilled and then a large chamfer is cut around each side of the pilot hole. Finally, the two holes in the bottom edge are drilled and tapped for the cap screws that hold the sliding plate to the nut.
Then the blanks are tapped with a 1 1/4" x 4tpi left-hand acme tap. And that's where the interesting part happens. Acme tapping removes huge amounts of material, so acme taps are usually two-stage. The first section of the tap is for roughing in the profile, and the teeth reach to near full depth, but they are narrower than the finished thread. A short "no cut" section is followed by the finishing portion of the tap which takes the threads to final dimension. If you've ever tapped steel before you know that it takes some force to turn the tap (even with a small 1/4-20 tap) and there is always the risk of breakage. Taps are easy to break. So when tapping by hand you reverse the tap every turn or so to break the chips.This is impossible when tapping acme thread. The thread needs to be cut in one shot. So to help clear chips the tap is flooded with cutting fluid. If a chip gets jammed, the tap can break. And a couple weeks ago, that happened. We had a tap break even at the slow speeds that we tap at, and the tool launched itself across the shop. Acme taps are huge, and when they break they make a nice sharp end that wants to lodge in soft flesh. Thank God, no one was injured. This part is probably the most "touchy" part of our vises to make. It's always a tense day when Tail Vise nuts gets tapped.
Here's a video of the process.