AR 80% lower finishing - Order of operations?

[ScrapMetal]

I am planning on making a couple of ARs for myself from 80% finished castings. I just finished making some vise jaws for my Kurt using Ray Brandes' drawings - http://www.lecreg.com/P4x4/ARblank.pdf

I"m sure that I'm over thinking this but now that I'm ready to drill some holes and mill out the receiver I'm looking for the "most accurate" or the "least error prone" way to do it. The way I see it I have three options for how to proceed.

One, fixture the receiver on it's side, locate the holes, drill all the way through and out the opposite side of the casting, then mill out the hammer and trigger cavity.
With this method, I know that drills can drift somewhat and the sides of the casting might not be perfectly parallel anyway. Holes on each side might not "match up" to where they should be.

Two, mill out the hammer and trigger cavity, fixture the receiver on it's side, locate the holes, and drill all the way through the casting out the other side.
This method has the same "problems" as the first but you are drilling through less material (less chance of deflection?) but when the drill hits the other side there is no pilot hole or punch mark and therefore a chance for the drill bit to "walk"?

Three, mill out the hammer and trigger cavity, fixture the receiver on it's side, locate the holes, drill out only that one side, flip the receiver over, re-indicate the holes, and drill the second side out.
The final method drills through less material, as "#2", but alleviates the chance of the drill "walking" as a spot drill can be used on both sides. Where this one is prone to error is simply in the indicating of the holes on both sides and how repeatable those measurements are.

All of the methods look to me to have both "pros" and "cons" but what I'm trying to find out is which "order of operations" would be the least prone to error and why.

Hopefully my explanation is adequate as to what I'm doing/looking for, so now I'd like to hear your thoughts.

Thanks,

-Ron

 

[Eddyde]

As long as your drill is sharp and your work is square, you shouldn't have any problem with drift.
You could also make a drill jig that would fit over the sides of the receiver and drill it from both sides, might be worth the effort if you're doing multiples.

 

[Karl_T]

You may want to join a forum named weapons guild. there's got to be 20 build threads doing this very thing. One fella there is selling a real nice fixture to make this job doable on a drill press.
http://www.weaponsguild.com

Here's a nice 0% thread:

http://www.mdshooters.com/showthread.php?t=4197

 

[Andre]

#2, and use a long, thin center drill to reach through the first hole and spot the hole below it.

 

[ScrapMetal]

As long as your drill is sharp and your work is square, you shouldn't have any problem with drift.
You could also make a drill jig that would fit over the sides of the receiver and drill it from both sides, might be worth the effort if you're doing multiples.

I don't think I want to get in to using a jig for this as jigs can have some issues of their own. If I was doing this on a drill press it might be the ticket but on a mill with DRO I can see some problems with it.

I should expand on this point a bit... I borrowed a jig setup from a buddy of mine to play around with and possibly use it as a model for some vise jaws I was planning to make. What I found was that using the jig setup was more trouble than it was worth. It got in the way of indexing off the casting itself forcing you to index off the jig and it's holes. The jig was made from 3 or four pieces that screwed together and not necessarily square, parallel, or perpendicular to where it should be plus it wasn't really designed to be clamped in a vise (go figure). It also made the depth of cut a bit of a guess and obscured where I was trying to cut.

That being said, I can see where I could make a one-piece jig for just drilling the couple of holes that need to be drilled but I'm still not sure that it would be more accurate than using the mill/DRO/indicators method when all is said and done. I'm not too sure that I would be building a better mouse trap than the guy who designed the original jig.

Thanks for the suggestion. I'll still keep it as a "plan B".

You may want to join a forum named weapons guild. there's got to be 20 build threads doing this very thing. One fella there is selling a real nice fixture to make this job doable on a drill press.
http://www.weaponsguild.com

Here's a nice 0% thread:

http://www.mdshooters.com/showthread.php?t=4197

Just signed up on that site. I'll take a look at it when I get approved. Thanks.

#2, and use a long, thin center drill to reach through the first hole and spot the hole below it.

Another thing I hadn't thought of. This is exactly why I posted/asked questions on here. That does take care of at least one issue with that method.

I'll post some pics tomorrow of the setup I made so far.

Thanks guys,

-Ron

 

[Andre]

Good luck on the build! I've used that method many times and has worked like a charm. If you do not have a long center drill a split point drill bit with a extremely thin web will work also. Hand grinders with cutoff wheels work wonders for thinning drill webs.

 

[ScrapMetal]

I will be checking my supply of spot drills tomorrow when I'm in the shop and see if I have anything long enough. I've got some longer center drill bits but I don't trust them for anything requiring accuracy.

Thanks,

-Ron

 

[ScrapMetal]

Didn't have time to do much in the shop today as I took my wife to the shooting range to put many little holes in a target or two. She was chomping at the bit to try out the new grip I installed on her Ruger LCP. A good time was had by all. :biggrin:

I did take the time to take some pictures (not the best quality but what the heck...)

I started with a set of 6" x 3" x 1.5" aluminum soft jaws from Monster Jaws, fired up the mill and the chips were a flyin'.





Here's a couple of shots with the lower receiver sitting in them...





I expect that they will work out nicely.

-Ron

 

[Andre]

Nice work on the jaws! Looks like your set up for mass production of lowers by the looks of things :biggrin:

Do you do any work on 10/22's?

 

[brav65]

Nice, before I bought a mill I purchased a set of jigs for AR15 and AR10. They work great, but were $$$. Now that I have the mill I could have saved the money. I am still setting up my new mill, and have not started on a lower yet, but I am getting close.