How to Finish an 80 Lower Without a Jig

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First, I realize gunsmithing is the more appropriate venue for this post, but I feel that rifle country is where it really belongs.
Now I imagine most members and non-members alike are buying a jig to machine any 80% lower they buy, be it an AR15 or .308 AR pattern. And that's fine. But there are those who own or have access to a vertical mill, and you absolutely do not need a jig to finish an 80% lower on a mill if you know how to read dials or DROs. However, the math for putting your cutter in the right place is tedious, and it's easy to make a mistake, such as forgetting to account for cutter radius. I may do a step-by-step tutorial with pictures later, but for now will supply the 80% lower finishing plans I just finished drafting, along with a few machining tips.

This will take the guess work out of it, eliminate the possibility of making a mathematical error trying to do the calculations on the fly.

As you can see, I laid this blueprint out with the zero/zero point at the outside edge of the receiver's driver side and the front edge of the BHO pocket. I made these plans around a receiver that is .900" thick at the FCG; if your receiver is thicker or thinner, you need to adjust accordingly. for example, if your piece mics .908" thick, then you need to set zero/zero .004" in from the edge (or add .004 to all of the Y-axis numbers).

I did modify the spec for the rear of the main pocket where the selector lives. I have it at .600" deep from the rear of the FCG pocket, while the actual spec is .491. The reason for this is that many people prefer to drill the selector hole after milling the pocket; if you only cut to the .491" depth, the drill may deflect as it passes through, causing your selector bore to be misaligned. If you want your lower at the exact spec, then reduce the X-axis number by .109" and bore the selector hole before you mill the pocket.

The numbers are calculated for a 7/16" (.438" cutter), which is the proper size to give the specified .218" radii in the pocket. One would have to recalculate everything to run any other cutter size. The trigger cut out is done with a 5/16" (.312") cutter.

On a quality full size vertical mill, one should be able to do the main pocket in 3 steps. I do my roughing with a 4 flute at 1,000 RPM with a .620" DOC on pass 1, 1.240" DOC on pass 2. I also stay ~0.010" out from the final dimensions on pass 1 & 2. On the third and final pass, I make the 1.249" DOC and bring the pocket out to it's finished dimensions. I run the 3rd pass at 2,500 RPM and climb mill for a nice finish.

The rear step can be done in only two stages, since it is only .630" deep.

Drilling the pin and selector holes ain't rocket surgery, but I will say that I prefer to do those with a slightly undersize drill, and then poke them to finished diameter with an end mill @ 2,500 RPM. Much cleaner finish.

Using this template, you should be able to knock these out in under an hour. The one on the left I did yesterday; took 42 minutes. The one on the right was a couple weeks back, and took about 10 minutes longer. I expect the next one to be under 40 with this cheat sheet.

Last edited: Sep 16, 2014
Thanks for posting this. I've toyed with the idea of machining from an 80% lower, but I have a mill and using a jig would take the fun out of it! It still doesn't make economic sense to do this but most of my home machining projects aren't financially justifiable anyway.
Thanks again!
I just now remembered to do this. Unfortunately, I can no longer edit the original post, so.....
IMPORTANT!

When editing these schematics, I decided to omit the measurement for the rear takedown pin, as they are already drilled on every 80% lower I've seen. However, when I was playing with lines in the magnifying glass view, I clipped the wrong line, giving an incorrect measurement for the selector hole.

I did not realize this until a few days later, when I was helping a friend get set up to do his.

The very top measurement that reads 6.375" should be 5.582".

I also made an error when setting the destination for the rear takedown pocket. The red X-axis number reads 3.669X. It should read 3.569X

I apologize if these two errors caused anyone trouble. With these two revised figures, everything is good to go.
Thos of us with Bridgeports say
*thank you*

Willie

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Very nice, thank you. I have access to a very nice Acer CNC mill and often thought about milling my own from an 80% lower. My idea was to chuck and index a completed commercially-produced lower, index it from the takedown pin locations, and "teach" the machine the dimensions without even referring to a blueprint. Then I'd index the 80% and let the mill go to town. It assumes, of course, that the 80% has with the takedown holes already drilled.
Think that would work?
Don't know. I have virtually no experience with CNC. Is that kind of capability within this machine's programming? Usually CNC are read-only and require a CAM file. It's a very simple file for finishing an 80%, though.
Do bear in mind that unless the machine has a 4th axis, you will have to manually index the receiver to go from pocket cutting to hole boring.

Also, I meant to add this earlier, but I learned the hard way that not all lowers have the selector detent pin hole drilled in exactly the right place, and going off of specs for that can cause your selector lever to end up slightly off of 0* and 90*. Won't affect function, but annoying to look at. So, when boring the selector hole, center your drill on the existing detent pin hole. Using 5.582" from front pin hole has resulted in the detent hole being as much as 0.008" rear of center or 0.011" forward of center.