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Thread: finishing an 80% AR receiver (condensed)

  1. #1
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    Default finishing an 80% AR receiver (condensed)

    Now that I have my DROs mounted (more on that here) Sieg X2 mini mill 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.
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    Pic 1) Time to break it down and drill pin holes so I can remove alignment pin from take down hole to be able to work on the shelf area. After pin holes were drilled, I remounted the jig, squared it up and re-established 0,0

    Pic 2) Shelf area roughed out. Deck height of shelf is the only thing finished to final dimension, everything else only roughed at this point.
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    I finished it up today. I have recorded the DRO settings based on end mill size. Please feel free to contact me if you would like any of that info.
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    I used a center drill to mark the selector locations then filled them in using model paint. I don't know how well they will hold up to solvents, we'll see.

    I got a chance to put an upper on it and take it for a test drive. (actually 3 different uppers) It runs like a swiss watch.
    I also finished up the second receiver.
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    I finally received the rest of the parts that were on backorder so that I could assemble the upper.
    18" fluted SS barrel with mid length gas system.
    Matrix upper.
    ICE BCG.
    15" FF tube.
    Low profile gas block
    Brownell's shorty MD
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    Quote Originally Posted by Gunco View Post
    Who made the 15" tube? and why the fluted barrel. I like this look
    Fluted barrels are lighter, more ridged and run cooler than non-fluted barrels

    The fore ends are made by MI https://www.midwestindustriesinc.com...ategory_ID=149


    I ended up using the 15" tube on the 2nd rifle. (16" carbine)
    Attachment 37001

    Attachment 37002

    The first one (W/18" fluted SS barrel) is now sporting a 12" tube.

    Attachment 37004

    Attachment 37003
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