Adding a tensioning screw to AR receiver

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While I do not think that this is necessary for a properly machined lower, I have had folks ask me about them so I figured I'd post this.

I set the mill up like this, using the following datum points. The normal "Y" axis zero, the flat area just behind the PG mounting boss for the "Z" zero and .156 to the right of it as the "X" zero.


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Using a 5/16" end mill I machined along the X axis .265 to the left into PG mounting boss. You do not need to mess with either the Y or Z just leave them locked.


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Drilled it out with a #3 bit.


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Tapped it to 1/4"-28


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Added 1/4"-28 X 3/4" set screw. It sits well below flush when fully installed


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That's the hard way to get the grub screw hole in the lower. Just put it into a [quality] jig, drill the hole needed for the tap (1/4-28) and then run the tap from underneath after the hole is made. Less steps, less stress, sofa king easy even a caveman could do it.

Fixxah: It takes up the slop between the upper and lower. Not needed for accuracy, but nice if you don't want the rattle.
 
If I had a mill I'd probably do the same. Otherwise I've seen people use o-rings or those DPMS "accurizing wedges" to fix the problem.
 
And this tension increases wear and resistance on the pins to what end?

Looks like another solution in search of a problem. IMNSHO
 
Nice to see it done well.
Does it really help much.
I get the concept but I often wonder.
Your only aplieing upward pressure on the rear of the upper. How does this affect tension on the front and also the wobble side to side?

Has any one done a accuracy test befor and after?

Its funny old shooter at one of the clubs I shoot cmp at has a mid 80s AR worn and loose looks aweful. Even has a 1/12 barrel ...shoots 52 grain match bullets....and still out shoots most of us.
 
This is what I have on all of mine, And they work great.
yeah, i use them also. bought a bag full of 'em on amazon a few years back, work pretty well. i just fished one out to use on my 7.62x39 ar build, upper and lower tight as a drum.
 
I must be the lucky one. All my friend's ARs are very tight fit. Even with thousands of rounds through a few of them.
 
Very clean set up and nicely done!

I don't bother with any of my fun or SD AR's, but even the slightest play bugs me on a match rifle. After trimming a couple Accuwedges to make them fit, I tried the o-ring trick and never went back. A #7 o-ring on the front trunnion does the job for pennies. The trick to not having to force the front pin is to leave the upper at an angle to the lower when pushing in the front pin. It will slide right in, then 'close' the rear of the upper and push in the rear pin. Perhaps all it does is take away a mental distraction, but when load testing for sub 0.5 MOA accuracy I think they help, LOL.
 
Very clean set up and nicely done!

I don't bother with any of my fun or SD AR's, but even the slightest play bugs me on a match rifle. After trimming a couple Accuwedges to make them fit, I tried the o-ring trick and never went back. A #7 o-ring on the front trunnion does the job for pennies. The trick to not having to force the front pin is to leave the upper at an angle to the lower when pushing in the front pin. It will slide right in, then 'close' the rear of the upper and push in the rear pin. Perhaps all it does is take away a mental distraction, but when load testing for sub 0.5 MOA accuracy I think they help, LOL.

Any positive psychological support is good...
 
That's the hard way to get the grub screw hole in the lower. Just put it into a [quality] jig, drill the hole needed for the tap (1/4-28) and then run the tap from underneath after the hole is made.

So you suggest trying to start either a tap or larger drill bit on a 45º angle instead? Really? I'm still chuckling over your last machining adventures. What was the final tally on broken drill bits and taps? I lost track.

How many folks did you talk into bastardizing their selector stops, before they realized that you had no idea what you were talking about?
I have met a few of the folks involved in that fiasco and every one has confirmed that without malodave involved, that little build session would have turned out to be an even bigger disaster than the winter GB that was associated to it.
 
So you suggest trying to start either a tap or larger drill bit on a 45º angle instead? Really? I'm still chuckling over your last machining adventures. What was the final tally on broken drill bits and taps? I lost track.

How many folks did you talk into bastardizing their selector stops, before they realized that you had no idea what you were talking about?
I have met a few of the folks involved in that fiasco and every one has confirmed that without malodave involved, that little build session would have turned out to be an even bigger disaster than the winter GB that was associated to it.

We've had only a couple of bits explode over the course of 50+ lowers machined. Only tap broken was due to user error and it was a 4-40 tap.

We're not using Dave's mill these days. We have access to a full sized (knee) mill that makes a world of difference. Unlike using a trim router, this does a sweet job...

Keep playing out the jig you're involved with. IMO/IME, it won't beat the ones we have on hand. We don't screw up the top plates since we pull them off before the piece gets locked into the vise.

BTW, keep chuckling. I really DGAF. Also, if you drill down from the top, for the set screw, it's VERY easy to get the tap started. IF you have more than a few living brain cells left that is.
 
We've had only a couple of bits explode over the course of 50+ lowers machined. Only tap broken was due to user error and it was a 4-40 tap.

We're not using Dave's mill these days. We have access to a full sized (knee) mill that makes a world of difference. Unlike using a trim router, this does a sweet job...

Keep playing out the jig you're involved with. IMO/IME, it won't beat the ones we have on hand. We don't screw up the top plates since we pull them off before the piece gets locked into the vise.

BTW, keep chuckling. I really DGAF. Also, if you drill down from the top, for the set screw, it's VERY easy to get the tap started. IF you have more than a few living brain cells left that is.

The drill bits were snapped off in a hole and did not explode, they jammed up and broke due to ignorance and there were several in the first half dozen builds. The fact that you finally figured it out and stopped breaking them after that does not mean that the broken bits were spread out over 50 lowers.

You keep knocking lowers finished of with a router. The majority of folks here do not have a mill at their disposal. These jigs allow the home builder to complete their own lowers with spending a ton of money.

You suggest that they are somehow inferior to a lower done on a mill. This one was done using a Modulus jig, please point out where you feel it falls short.
(bottom of this page) http://www.modulusarms.com/jig-instructions-overview/

Secondly, Take a look at what the folks in the ar15.com BIY forum think about the Modulus jig.
http://www.ar15.com/forums/t_3_4/700712_What_jig__What_method_.html

Lastly, you (inadvertently, I'm sure) forgot to mention the selector issue in your reply. Would you care to expand upon that?
 
I,just bought a mini end mill bit for my dremel tool......watch out.

Come one everyone it's amazing what you can do with some thought....

Although I wish I had at least a bench top mill/lathe combo.... if I ever get my garage built be sure I will !
 
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