Chapter 1

Alignment and Take Down Holes

I had seen all of the various ways that others had finished out thier 0% lower forgings, but due to the size of the equipment I was working with and being hard-headed and wanting to do things my way, I came up with my own series of operations to machine the lowers. I can't take all the credit for all of the machining processes though - some of them were inspired by others.

First thing I had to figure out is where to start! The raw forgings give you a lot of extra stock (about 0.100") around some of the edges to clean up and you can't properly dimension off of those edges, plus they're slightly tapered from the forging process. Since I plan on making a few of these forgings into receivers for different projects I had to figure out a way to properly index each forging in the set up blocks that I purchased from Justin at www.cncgunsmithing.com. So using a trick from the .pdf book on machining the lowers available from www.ray-vin.com I located off of the 1/2" round boss on the left side of the forging. This allowed me to find the two take down pin holes and drill them out. Luckily the top plane of the forging was square enough that I could parallel that against my vise and make the 2 take down pin holes parallel to the rest of the forging. I also had to make a custom right clamp in order for the center finder to clear the 1/2" boss and the 2 take down pin holes. I used a 1/2" end mill arbor and a 1/2" transfer punch to find the center of the 1/2" round boss. I also had to drill two 1/4" holes in the left clamp to allow two 1/4" steel dowel pins to slide through it and into the take down holes in the forging.

This would now allow me to pop each one of the forgings into the blocks and index them with 1/4" steel dowel pins. This would make fixturing, setup and indicating much easier for all of the rest of the operations. Since basically everything on the lower reciever is based (not only on the drawings, but in the finished product as well) off of the two take down pins, even if they are off a little bit, everything will be machined based off of them, and therefore everything will fit all up.

Here is a picture of the custom clamp and the first drilling operation:

Here's a pic of how the 1/4" dowel pins index each forging:

At first I was frightened when looking at the drawings and seeing how close the tolerances had to be on everything. But after purchasing a lower reciever locally and putting a lower parts kit in it along with test fitting a few magazines, I soon saw that the tolerances didn't have to be that precise and the design gives room for a large margin of error, relatively speaking. The design is still quite over-engineered and very machine-time intensive as opposed to other simplier designs, like the Kalashnikov style rifles where everything is +/- 0.030 and that still leaves room for slop.