I've been busy with handling various life issues lately, but I did manage to slip in some time yesterday to do some machining work on one of my AR lower receiver forgings. I got all the necessary holes drilled, including the buffer tube hole. Originally, I set it up on the mill and got the buffer tube hole lined up and drilled out to 5/8" before taking it to the lathe. A large block of aluminum allowed me to chuck the lower up and get the buffer hole centered before boring the hole out to 1.125" in preparation to cut 1.1875x16 threads. I went ahead and threaded the receiver in the lathe, and it turned out nicely.
Having a lathe around to do this with makes a tough job much easier, but I thought I could make the job even easier and if nothing else, I could have some fun building something.
I priced the taps that are normally used to thread these, and even though I'm planning to build several lowers, I don't want to spend that much money. I finally just ordered a one foot length of 4140 prehard 1.25" solid round stock. I then turned one end to 1.125" and the other end to 1.188". I cut progressive steps in the stock, with each step 1/2" in length and .01" in greater diameter. I left the smaller end blank and then started cutting 16 tpi threads down the length of the stock, stopping when full thread depth had been reached.
I set it up in the mill vice and cut four flutes down the length of the whole thing then cut down 1/8" across four faces so I could use a 1" box wrench on the newly machined tap.
Did it work?
The pics of the second receiver I'm machining speak for themselves. It worked better than I thought it would, and I'm only out about 10 bucks.
There are times that having a mill and lathe come in real handy, and this was one of those times.