Hey /k/, I want to build my own AR15 from scratch (always bought

Cont'

I guess what I'm asking is, how exact are the tolerances on the holes? As in, what's the play for distances? 0.001 mm? less/more? The machine can handle tolerances down to that but can differences in distances be handled by the finished weapon?

Yes, I've done it. Drill the holes undersize and ream them to the correct size first before milling the pocket.

>>28363763

Why not drill them to correct size in the first go? The Bridgeport can definitely handle the material. Not disagree just curious.

File: original.jpg (56KB, 866x561px) Image search: [Google]

56KB, 866x561px

>>28363724
if you can measure you can do it without a dedicated jig
you may need sometype of jig to hold the work piece in the correct orientation but that doesnt mean an 80% jig

>>28363793

Well I figured (and please, correct me if this line of thought is wrong) is zero the machine to a known point, say the center of one of the main, pre-drilled, pin holes (or even an edge of one) and then use that known location to move the machine head to the location I need to drill each hole. My concern is if I'm off by say a 0.01 mm, will that effect the ability of the weapon to function? I don't know much about the internal tolerances of each part in an AR15 and a google-fu search doesn't seem to answer it.

>>28363829

cont',

Also, do you have that one (image) in metric? Engineers tend to prefer that to imperial :D

>>28363829
it wont matter i have a practice lower i fucked up where the safety selector is 2mm low the hammer pin is crooked and i was still able to get it functional
you can be off by like a full mm and still make it work if you file other parts to compemsate

>>28363884

So the tolerances aren't engine tight? My experience thus far has been building micro-turbines, micro-pumps, and assorted parts. Their tolerances are very unforgiving but I could make it work without too much issue since it was my own designs, this being someone else's I'm just concerned if I don't get it down to the 0.001mm the whole gun won't work.

>>28363787
Drill bits tend to wander through material and find the path of least resistance. Always drill undersize and then ream to size.

>>28363853
This engineer doesn't.

>>28363934

Even on a bridgeport? I've never had that issue on it with my micro-turbines (when milling metal at least). I should mention it's the big, 3 axis version that's about $28k

>>28363946

You must be older. They don't even teach standard in school anymore.

>>28363954
Even on a Bridgeport. Peck drill, use proper speeds and feeds, and then ream it once you're through. Plus, the holes will not be oversize like they often are if you just drill them.

>>28363829
You'll need to make a couple fixtures to hold the forging.

The milled pocket's dimensions don't especially matter so long as the components fit correctly and you don't go all the way through the side. What matters most are the drilled holes.

See

http://www.lecreg.com/P4x4/ARblank.pdf

For fixture designs.

>>28363970
Maybe not for an engineering degree, but I'm finishing up my AS in Machine Tool and we ONLY work in Imperial.

Doesn't really matter on the production side of things anyway. Metric being "more accurate" than Imperial is only true when you're talking from a metrologist's POV.

>>28363925
everything critically important is in the upper or already cut for you on an 80%
the only thing you are doing is making enough clearance for the the fcg to move freely

it doesnt really matter if the correct part of the hammer hits the firing pin (preferably yeah you want it to be perfect but you know) just that the hammer hits the firing pin

you do want the trigger and hammer holes to be in the correct locations but with milspec parts theres plenty of extra space

>>28363984

Okay, sounds like a plan. Other question, since you guys sound like you know what youre talking about, what's the apparent preference between the 6061 and 7075 80% lowers? I'm a Chemical Engineer by trade so I appreciate the metallurgic properties, but how well do these translate into firearms? The 7075 should be better because it's harder right? Or does it matter at all (and thus save a few bucks getting the 6061)?

>>28364018
Really doesn't matter. The lower doesn't experience anything in the way of real stress. The only thing a 7075 lower will be better at is maybe not scratching as much.

>>28364018
The nature of the forging is going to make it the better choice from a strength of materials standpoint. Best to avoid the pre-anodized ones, too.

>>28364050

Why avoid the pre-anodized?

>>28364050
Forging vs machined billet also won't matter to you in the long run. Inconsequential differences.

>>28364063
Because chances are you'll scratch
it all up beforehand and need to have it re-anodized anyway.

>>28364063
Harder on the tooling and tends to chip. Once you're done, send it out for anodizing, or do it yourself with one of the guides posted around the web.

>>28364083

We have the machine here at our machine shop so no need. Was just curious.

>>28364077
Great, so inconsequential gains in strength at a cost reduction. Go with the forging.

>>28364113
Can I send you some lowers to get anodized? I hate the home method.

The only other question I have is less 80% based and more just general firearms; I was (as everyone rightfully assumed) going for a 5.56 but I'm also considering a 7.62. Anything special for them (other than the obvious of hole placement)? Is it a good idea long term? I err towards the 7.62 only cause ammunition is more universal to find (in my experience at least). I've never had a 7.62 AR and have only shot them out of either M1A's and the 240 bravo when I was in the Army.

>>28364113
if youve already got the stuff needed to anodize dont buy the preanodized cause then you will have to sandblast it to remove all the anodizing before anodizing it

if this is just a fuckaround beater that you dont really care about you could just spraypaint the aluminum and be done with it

>>28364131

It still costs machine time to do it at our machine shop, just convenient since it's here.

>>28364156

Na I want it to be pretty nice. I'm half tempted to work up an autocad doc. to make my own logo.

Out of curiosity, if I put my *own* serial number on it, is there any legal implication? Basically number it 00001.

>>28364161
I'll pay.

>>28364147
if you are talking 7.62x39 buy an ak
the ar pattern mags can be problematic and the bolt heads tend to break
if you are talking 7.62x51/308 just keep in mind that there are several different ar10 pattern guns and not all of the parts are compatible. you'd have to figure out what spec you want to use and plan it out

>>28364189
Firearms you make only need a serial number if you are going to sell it, in which case the number has to be a certain depth and height, which I don't know of the top of my head.

>>28363970
University of Idaho here, we sure as fuck do

All your bar stock and practically all manual machines are only in Standard units, and most machining and fabrication shops do not have inspection capabilities in metric.

Rule of thumb is if you dimension something in metric, expect 3x the cost

>>28364198

I would be the Nato (.308). And please explain? The uppers and parts aren't standardized like the 5.56 (.223)?

>>28364218

All our machines here at University of Buffalo are in Metric. In fact you have to specially ask for the standard drill bits and heads. We have a ridiculous amount of exchange students (highest in the Country I think) and our school is hugely about universality of Engineering so we use metric to exclusion of all others. Hell as a Chem Eng we don't even use Bar often (which is industry standard) in favor of Pa.

>>28364224
they arent standardized like the ar 15
the original ar10 is well the ar10 and the modern one is more of an ar 15 scaled up to take 308 its not really based off the ar 10
off the top of my head theres a minimum of 3 different types
the original ar 10 with f/a and everything
the semiauto armalite ar 10
dpms ar 10
they take different mags too youve got some that take ar10 mags while others were setup to take g3 or fal mags

if you do decide to go 308 make sure all your parts are compatible preferably form the same brand and make sure to mill the 80% to the right specs

>>28364361

Sounds Like I'm best off sticking to the AR15 size since I haven't even really customized one of those before, much less played with an AR10. All good help though, ty all who contributed.