finishing an 80% AR receiver
Now that I have my DROs mounted (more on that here) http://www.gunco.net/forums/183-tool...mini-mill.html I'm going to try and do this one relying on just the DROs and a blueprint. I did the last one using a drill guide and press. It came out ok, but you can see traces of the where a couple of the holes were drilled.
Let me start with this, I am not a machinist, but have spent a moderate amount time on mills and lathes over the years. Some might say that I know just enough to be dangerous.
This is just meant to be a primer course for the layman thinking about giving one of these a try. I am using a layout that I found somewhere on the net, that others have had success with.
First you need to locate 0,0. everything is done off of this point. The X zero is the centerline of the pivot pin hole and the Y zero is the centerline of the receiver, where they intersect is 0,0. To locate the X axis use an edge finder on the pivot pin. When the edge finder kicks out, 0 your X scale and raise the edge finder up out of the way. Crank your X in an additional .225 and reset to 0. (.100 half dia edge finder + .125 half dia pivot pin = .225) X is done.
To find Y measure the width of the receiver at the FCG pocket area. The AA receiver is .886 so I'm going to use that figure. Set your edge finder on the front of the receiver (facing you, actually drivers side of receiver) when it kicks, set Y axis to 0. Raise the Z up out of the way and dial in an additional .543. reset Y scale to 0 (half the thickness of the receiver .443 + half the dia of the edge finder .100 = .543) To double check, move the edge finder to the back of the receiver (passengers side) it should kick at .543.
The bulk of the material is still removed with drill bits, but with just 7 holes. All of the holes are on the Y zero axis except the hole at 5.608. You need to move the receiver .031 rearward for any operation at that location. I did spot drill the outer holes of the drilling jig just as a visual safety net.
I spot drilled the 7 holes then drilled out the first 5 holes with a 1/4 bit as a pilot hole then out to .500 for the first 4 and .375 on hole 5 with a deck height of 1.245. Remember to add the additional .031 for hole number 5. Do not try to drill out 6 & 7 yet, you will run into the take down pin.