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Posted: 12/4/2015 11:08:34 AM EDT
| I'm planning on starting my first 80% lower today on lunch or Monday. I'll be using a mill and do not have a jig. I found a couple drawings on the web I'll be using. None of them have +/- on them. Any tips. Any spots to be more precise than others? I'm pretty mechanically sound but haven't done much on the mills. Any advise/tips would be great. |
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The important tolerances are with the width of the .500 pocket. Too tight and the rear lug doesn't want to go in. Too loose and your upper will flop around a little bit. The depths aren't really critical as long as they clear all the trigger and safety components. More is better in the fire control group area, just not too deep. The safety, trigger and hammer pin diameters are kind of important or they will slop around. There dimensions from one another is fairly critical but a few mils here or there and it will function fine.
Good luck! |
| By the way, I looked over the blueprint like a million times but for the life of me cannot find the position of the trigger cutout on the bottom of the fire control pocket at all. It lists the cutout size but NOT where they should be relative to say the takedown pin holes or whatever... it's just floating around in the air. I had to "guess" them and enlarge them later when it wouldn't fit the trigger. |
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If memory serves, standard tolerance for dimensions listed to two decimal places is +/- 0.020". Three decimal places is +/- 0.005", and 4 decimal places is +/- 0.0005". Always "unless otherwise noted"; you may have a 3 decimal figure that specifies +0.006"/-0.002" or somesuch.
Off the top of my head, I don't remember which tolerances are different than standard on an AR lower, but if you can hold 0.003" or better, you'll be fine. |
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Quoted:
If memory serves, standard tolerance for dimensions listed to two decimal places is +/- 0.020". Three decimal places is +/- 0.005", and 4 decimal places is +/- 0.0005". Always "unless otherwise noted"; you may have a 3 decimal figure that specifies +0.006"/-0.002" or somesuch. Off the top of my head, I don't remember which tolerances are different than standard on an AR lower, but if you can hold 0.003" or better, you'll be fine. No, two decimal places (x.xx) are +/- .010, unless otherwise specified. Three decimal places (x.xxx) are +/- .005, unless otherwise specified. Four decimal places (x.xxxx) are normally always specified. |
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