Chapter 3

Buffer Tube Hole and Rear Plane

I had originally planned on doing the buffer tube hole on my lathe, which is how I started for the first forging. I had to use my lathe because my mini-mill doesn't have enough Z room to allow the fixture and forging to stand on end in the vise and clear the headstock, let alone allow a tool to be held in the spindle. So I had built a fixture that would index and secure the index blocks and forging to the saddle of my 9x20 lathe and then use the autofeed to feed the reciever into the tool held in the chuck. While it did work pretty nicely, I had to shim under the setup block to obtain a center hole in the rear of the forging. This proved to be a bit cumbersome and time consuming, but it would work.

I didn't take any pics of the setup on the lathe, but I will snap a few for a later chapter when I cut the threads for the buffer tube hole with a tap.

Thankfully while I was up at the CNC Workshop Roland allowed me to use one of his nicely setup Bridgeport vertical mills. This machine definitely had the Z axis room to allow the fixture to be set up on end. The first forging I attempted with a 1-1/8" drill (after piloting to 7/8" with an end mill) for the hole and the surface finish didn't come out very nice and the hole was oblonged and oversized. This was due to the drill wandering off axis as it exited the radiused inside portion of the forging.

Here's a picture of the drilling operation:

Soon after the first one was done and under the instruction of Mark at the CNC Workshop, I switched to using a boring head. This was the first chance I've gotten to use a boring head and let me tell you it was cool! The tolerance you can hold and the surface finish you get with one is unbeatable! Unfortunately I forgot to take pictures of the actual machining process. I was so caught up in making chips (and having numerous machinists more experienced than myself peeking over my shoulder) and answering questions that it totally skipped my mind.

I finished the entire rear plane while I had access to the large machinery they had up there. I figured I'd wouldn't have free access to such a well stocked machine shop in while, so I'd better take advantage of it while I can. The other rear plane operations were pretty easy and straight forward. A 1/4" drill 0.25" deep followed by a 1/2" end mill 0.200" deep makes the stock locking hole while a 3/32" thru to the rear take down pin hole provides the hole for the rear take down hole spring and detent. A 1" 4-flute end mill took care of the facing operation on the back plane and made those cool looking machined swirl marks that I love oh so much! I will mount the fixture on my lathe again using the same setup I was originally going to use to pilot the tap and cut the threads in a later chapter.

Here's a pic of the 4 lowers that I worked on while I was up there:

Behind the first one you can see a part that Rick-O-Matic was showing off after the aluminum casting demo at the CNC Workshop. That a boy, Rick!

And finally the machine I did this on:

My next mill is definitely going to be a fullsize knee mill. Once you use one after using your mini-mill for a year, there's no going back!