MachIVshooter
Member
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.
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.
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