Inside finish on an 80%

[Stape]

Just finished out an 80% AR lower. I used the Modulus jig and a Rigid R24012 finish router. It turned out pretty good, my only complaint is the interior well/pocket for the fire control group. Looks like I need to do a final pass to smooth it all out but at the same time, I don't want to go messing with it to the point of just removing material to make it looks better, I mean, whos gonna pop open my AR and inspect the floor of that cut.

Its about the thickness of a piece of paper or two, running right down the center. I guess I focused on the edges more and thought the middle was cleared out. Off the top of my head I cant think of how this would affect anything.

Of course Id like it to be smooth as glass with a mirror finish but it was my first time using this setup. So am I missing anything? Let it be, or bolt the jig back up and make a final cleanup pass?

 

[Boris]

you need to get control over your OCD.

the only thing that matters is that you can fit trigger group in. That's it. If you want to obsess over finish, 80% lowers are cheap and practice makes it perfect.

When I cast lowers out of the plastic, I can say that the biggest stress on the lower comes from you handling the rifle.

 

[ReluctantDecoy]

Are you talking about the FCG floor where the trigger sticks through is about the thickness of a couple pieces of paper? I could be wrong, but I think the depth should be 1.25. Unsure of what that leaves for floor thickness, but I'd imagine it would be more than a couple pieces of paper. The trigger springs rest on the FCG floor, so the thickness does factor into performance and reliability a bit. Mill too deep into it and you're removing spring resistance.

 

[84ta406]

You can sand and scotchbrite if you really want but unless you're using a mill I imagine you'll always have a bit of a rough finish.

 

[Stape]

Thanks fellas. Decoy, in reference to my comment about the thickness of two pieces of paper I was speaking of a portion on the FCG floor being proud by that much. I kinda knew the answer but wanted to run it by someone first. It doesn't bug me too much, and in all honesty I think I'd stand a better chance at F'ng it up trying to fix such minutia.

 

[CatSnoutSoup]

Aluminium doesn't care if it looks pretty, but it is rather picky about its dimensions...

 

[allen-1]

Stape,

It shouldn't matter - unless you're using a "drop in" trigger - one that's already assembled and contained within a housing. In that case it *may* matter because the pinholes *may* not line up. If you're using a "standard" trigger, (separate pieces and springs), you'll be fine.

Other than your OCD of course... We can't fix that.

I had to remove extra material from a couple of AR's that I put American Trigger Gold triggers in.

 

[PEW]

Aluminium doesn't care if it looks pretty, but it is rather picky about its dimensions...

OP,
According to that drawing, the maximum allowable flatness variation across the floor of that pocket would be .020

A piece of standard printer paper is about .004; you are estimating a step of .004-.008. Making a couple of assumptions about coplanarity and location, you are well within tolerance. It may be worth your time to confirm that no point on that surface is less than 1.239 inches from datum C.

 

[Stape]

Good to go. Ive got a few LPK's laying around, I'll assemble it tonight and make sure.

 

[akma]

You could bead blast to even out the finish. In production all these things go through some kind of vibrator to clean up and smooth out everything.

 

[Stape]

Update: LPK went in fine.

 

[HarryPottar]

Update: LPK went in fine.

oh if I could count the times I said that to the wife

 

[Wrench75]

The floor doesn't have to be smooth. It just has to be deep enough that when the trigger moves, the front of the trigger, the part with the sear face, doesn't hit the floor before releasing the hammer. With a standard lpk the only part of the sides that matter is the area immediately around the pin holes. I'm talking about a pencil eraser sized area around the pins. If they are rough and the trigger fits snugly, then you'll feel that grittiness/drag when you pull the trigger. If your trigger goes in without touching both sides simultaneously then you probably won't feel the difference either way.

 

[92G]

if floor of pocket is not cut deep enough or has leftover material, can have issues with selector switch correctly fitting over the trigger mechanism. the floor of the pocket is quite important that it's cut at correct depth and evenly. the sides of the pocket are overall less critical. the above posts all are hitting on similar issue. if this was your first cut, don't worry about the finish. as you do more it will improve. slowing down on cutting depth and taking sweet time improves the results. these are some of the most rewarding firearm projects out there i'm always excited to hear some other nerd is playing with them.

 

[Stape]

if floor of pocket is not cut deep enough or has leftover material, can have issues with selector switch correctly fitting over the trigger mechanism. the floor of the pocket is quite important that it's cut at correct depth and evenly. the sides of the pocket are overall less critical. the above posts all are hitting on similar issue. if this was your first cut, don't worry about the finish. as you do more it will improve. slowing down on cutting depth and taking sweet time improves the results. these are some of the most rewarding firearm projects out there i'm always excited to hear some other nerd is playing with them.

While putting in my LPK, I noticed that the portion along the center of the floor wasn't raised as I had thought, in fact is was deeper than the rest of the floor. I might have pressed down more firmly when making that particular pass in that area. Everything went in fine, but the trigger pin gave me some headaches. Additionally, my trigger pin and hammer pins are a little loose for my liking. I can push them with my thumbnail without too much effort. So, whenever I put this lower to use I might get some anti-walk pins, or some anti-walk pins in addition to a drop in trigger unit.

I did all of my safety and function tests a dozen times and its all up to snuff.

 

[92G]

While putting in my LPK, I noticed that the portion along the center of the floor wasn't raised as I had thought, in fact is was deeper than the rest of the floor. I might have pressed down more firmly when making that particular pass in that area. Everything went in fine, but the trigger pin gave me some headaches. Additionally, my trigger pin and hammer pins are a little loose for my liking. I can push them with my thumbnail without too much effort. So, whenever I put this lower to use I might get some anti-walk pins, or some anti-walk pins in addition to a drop in trigger unit.

I did all of my safety and function tests a dozen times and its all up to snuff.

anti walk pins are great especially for an 80 build

 

[greencobra]

the only thing that matters is that you can fit trigger group in.

right, i like to tell the story of a friend of mine, an 80 yo guy, who hogged an 80% out at his kitchen table with a hand drill. pretty its not but it stuffed, pin holes line up and best of all, it fires.

 

[CatSnoutSoup]

right, i like to tell the story of a friend of mine, an 80 yo guy, who hogged an 80% out at his kitchen table with a hand drill. pretty its not but it stuffed, pin holes line up and best of all, it fires.

And to this day he is probably still finding aluminium flakes in his Cheerios.

 

[akma]

While putting in my LPK, I noticed that the portion along the center of the floor wasn't raised as I had thought, in fact is was deeper than the rest of the floor. I might have pressed down more firmly when making that particular pass in that area. Everything went in fine, but the trigger pin gave me some headaches. Additionally, my trigger pin and hammer pins are a little loose for my liking. I can push them with my thumbnail without too much effort. So, whenever I put this lower to use I might get some anti-walk pins, or some anti-walk pins in addition to a drop in trigger unit.

I did all of my safety and function tests a dozen times and its all up to snuff.

Always under size the hammer and trigger pins for a snug fit. Raw aluminum is soft an can wear over time especially on the hammer pin because of the heavy spring pressure, so the anti walk pins are recommended. Ebay has some that lock everything in place. Black Steel Complete Anti Walk Rotation Pins Kit Set .154" | eBay
They are like .152 ish but work great.

 

[92G]

there are few events on earth that can make
me lose my marbles like watching a hammer pin rotating inside a lower. cringe! once those anti-walk pins get installed the panic resolves.

 

[amm5061]

anti walk pins are great especially for an 80 build

I exclusively use them now. It does prompt some questions, but I stopped caring a long time ago.

 

[84ta406]

I use the KNS anti walk pins in all my builds now. The Facebook and YouTube experts say they're evil but I'm going to keep using them

 

[amm5061]

I use the KNS anti walk pins in all my builds now. The Facebook and YouTube experts say they're evil but I'm going to keep using them

Huh? Why would they be evil?

 

[Golddiggie]

When finishing 80% lowers I always drill then team the holes to size. That means drilling slightly under and reaming to size. Never had an issue with pins moving too freely. I have an AR10 and AR9 80% lower on deck once I get my DROs mounted (on the mill I bought in July).

The first 80% lower I finished was using a drill press for the side holes. Holes were not true round so it's not in the usable set. I used it to test home anodizing before I set up for spraying Cerakote. At some point I'll destroy that lower and toss it's remains.

 

[akma]

TIG the holes and redrill?

 

[pastera]

TIG the holes and redrill?

I would just oversize drill, ream and press in bushings

For a ghetto build, a little green loctite works wonders...