Tooling | Tutorial

Finishing An AR-15 80% Lower Receiver 

- Fire control pocket solid -

(Trigger slot tutorial at bottom of this page)


By: Justin / jwh02017
Rev. Date: 11/30/08 

*** NOTE: This tutorial uses Kevin's newer style 80% lowers ***

Outline:
  I. Introduction
  II. Tools Used
  III. Diagram & Blueprint
  IV. Tutorial
  V. Conclusion

I.
Introduction:
In this tutorial I will demonstrate how easy it is to complete an 80% AR15 lower receiver using the CNC Gunsmithing jig and the milling adapter kit. www.cncguns.com You may have seen some of the other projects I’ve completed using CNC machining centers. Although viewing those projects is neat, it’s usually not possible for the average guy to use CNC equipment to complete his lower. So that is why I want to show how somebody can complete an AR15 80% lower with typical tools usually found in their garage. There are several ways to complete an 80% lower. So what I demonstrate here may not be the way you finish your 80%’er, but it’s what I thought was the easiest at the time with the tools I had access to in my garage. The optional items in the Tools Used list below are items that make the job easier, but they aren’t required.

II.
Tools Used:
  - CNC Gunsmithing Drill Jig and Milling Adapter Kit
  - 80% AR15 Lower Receiver from Kevin (with solid fire control pocket)
  - 12” Craftsman Drill Press - 5 speeds (540-3600 RPM) 
  - Drills & Taps:
      - (C) 1/8 Drill
      - (D) 5/32 Drill
      - (E) 3/8 Drill
      - (F) #40 Drill
      - (G) 1/2 Drill
      - (I) #42 Aircraft Extension Drill
      - (J) #3 Drill
      - (K) 1/4 - 28 Tap
  - 3/16 Allen Hex Wrench
  - Cutting Fluid (WD-40)
  - Calipers

  (Optional Items)
  - Transfer Punch Set
  - 2x4 piece of wood - 7.25” long
  - 2x4 piece of wood - 2.50” tall
  - Columbian Clamp - 15” long
  - Irwin Quick Grip - 8" & 12”
  - Sharpie Marker
  - Dremel Tool w/ Grinding Stone & Sanding Bands

III.
Diagram & Blueprint:

 Click on the pics below to download pdf files of the Hole Size Diagram and AR15 blueprint.

AR15 80% Lower Receiver Tutorial - Hole Size Diagrm Pic.jpg (75294 bytes) AR15 80% Lower Receiver Tutorial - blueprint pic.jpg (115918 bytes) drill holes.jpg (120184 bytes) drill holes2.jpg (81004 bytes)

The diagram will show you which size drill or tap to use in each location.  The blueprint can be used for reference during the project.  Fig.1 and Fig. 2 detail hole locations when using the milling adapter kit.

IV.
Tutorial:

I started with the newer 80% AR15 lower receivers that Kevin is now selling (Email: audibk@yahoo.com)  The new 80%'ers Kevin sells has the fire control pocket solid.  He sells the nicest 80% forged receivers I’ve ever seen. I bead blasted the lower before I started this tutorial to give a better contrast of where cuts will be made. Click on the image below to see what the lowers look like directly from Kevin…

Below is a pic of the drills & taps I used in the original 80% tutorial.  I did not use all of these tools in this new tutorial, but I wanted to use the same pic to show the difference in all the tools.  Depending on how you are planning on finishing your lower, you may or may not need all of these tools.  I used the same drills and tap from the original tutorial so they weren't brand new but close to it.  You don't have to use new drills, you just need to make sure they are good and sharp to avoid any possible problems.  The drill press was operated at the lowest spindle speed of 540 RPM’s throughout this project. I feel this spindle speed is safe for this type of manual work. Any higher RPM and stuff happens a lot quicker and that’s not always a good thing. For reference, an average spindle speed when I run the CNC machines is 10,000 RPM. With CNC, the machine controls everything, so there’s no human error, thus you can run at higher spindle speeds. 

AR15 80% Lower Receiver Tutorial - 1211.jpg (26501 bytes)

The first thing I done was aligned the receiver in the jig. To do this I turned the jig upside-down and set it on the edge of the drill press table. Then I took a flat object and made sure the back of the buffer tower was flush with the back of the jig. Once I was sure both surfaces were flush, I tightened the two 1/4-20 socket head cap screws to around 10 ft-lbs of torque. Below is a reference pic from the original tutorial…

AR15 80% Lower Receiver Tutorial - 1227.jpg (21736 bytes)

A quicker way of aligning the lower in the jig is to run a couple 0.250 diameter pins through the front and rear take down pin holes.  I actually used a couple worn out 1/4 carbide endmills, but you could use a couple drills or anything else you have laying around that is 0.250 diameter.  These 0.250 pins will insure the receiver does not move in the jig while you remove the material from the fire control area. Below is a pic of the lower in the jig ready to go...

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For the rest of this tutorial I will mostly be demonstrating how to use the new milling adapter kit.  During this tutorial I may make references to the original tutorial (fire control pocket milled out tutorial).  

The next thing to do after the receiver is in the jig is to attach the drilling adapter plate.  The difference between the two plates are shown below...

drilling adapter plate - iso pic.jpg (59493 bytes) milling adapter plate - iso pic.jpg (54693 bytes)

To attach the adapter plate to the drill jig, run the 4 bolts through the holes and loosely tighten the bolts.  Make sure the distance from the front of the jig to the adapter plate is 3.410 like shown in this pic...

AR15 Jig 80% Lower Receiver Tutorial_3177.JPG (44340 bytes)

Now make sure the adapter plate is centered over the jig.  Measure both sides to make sure they are the same distance and tighten the bolts to around 10 ft-lbs of torque...

AR15 Jig 80% Lower Receiver Tutorial_3175.JPG (39826 bytes)

Next I got the drill press ready.  Since the the jig assembly will be moved back and forth several times, it's best to clamp some 2x4's to the table for everything to ride on.  I used rough 2x4's, but I would recommend using some wood that has been planed to make sure both pieces of wood is exactly the same thickness.  Even using rough 2x4's I was able to get pretty nice results.  The distance between the boards need to be 1.900 and centered under the drill chuck so you can travel equal distance forward and back.  Here's a pic showing that...

AR15 Jig 80% Lower Receiver Tutorial_3181.JPG (40862 bytes)

The next step is to drill the (37) 1/8 holes.  To make drilling the holes easier, I only used the jig to spot drill the hole locations, then I removed the adapter to finish drilling the holes.  On Kevin's 80% lowers he adds a 0.250 diameter locating hole to help him during his production runs.  Since (4) of the 1/8 diameter holes fall on the edge of this 0.250 hole, I drilled the (4) 1/8 holes at least 0.500" deep.  The 0.250 hole is only around 0.500 deep, so once you drill the 1/8 hole at least that deep, it shouldn't try to walk on you anymore.  I would recommend drilling the full 1.249 depth to avoid any possible problems later on.  Here's a pic showing where the 0.250 locating falls and a pic after the holes were drilled...

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After I had all the 1/8 holes spot drilled, I removed the drilling adapter plate.  Here's what it looks like... 

AR15 Jig 80% Lower Receiver Tutorial_3192.JPG (44753 bytes)

Now I needed to drill the 1/8 diameter holes to 1.249 deep.  To do this I locked the spindle in while the drill was on top of the receiver and then I used my calipers to mark where I needed to stop the drill press.  You can also reference the original tutorial on how I set the stop nut to the correct depth.  Since I didn't care if the receiver had little dimples on the bottom of the fire control pocket, I just used the scale on the drill press.  Here's when I was setting drill depth...

AR15 Jig 80% Lower Receiver Tutorial_3194.JPG (59675 bytes)

The following pics showed how I progressed through drilling the 1/8 holes...

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Next I changed drills to the 3/8 drill.  This drill will remove the meat of the fire control pocket.  I set the depth of the 3/8 drill to 1.249 like I did with the 1/8 drill.  I referenced Fig. 1 for the locations of the 3/8 drill and proceeded with the drilling.  ...

drill holes.jpg (120184 bytes) AR15 Jig 80% Lower Receiver Tutorial_3199.JPG (52132 bytes)

When you start drilling the next 3/8 hole next to the you just finished drilling, make sure you have a firm grip on the jig assembly.  The drill will try to walk on you, but if you go really slow you can force the 3/8 drill to stay centered on the 1/8 drilled hole.  I stopped drilling half way through to take a quick pic...

AR15 Jig 80% Lower Receiver Tutorial_3200.JPG (53715 bytes)

I found it to be a little easier if you skip a row of 3/8 holes and start with a fresh hole.  This way when you go back and drill the remaining holes, there will be equal force on both sides of the drill.  Here's what I'm talking about...

AR15 Jig 80% Lower Receiver Tutorial_3202.JPG (48057 bytes) AR15 Jig 80% Lower Receiver Tutorial_3203.JPG (56355 bytes)

I just barely started drilling this hole and I wanted to stop and take a quick pic.  You can see how there is equal force on both sides of the drill...

AR15 Jig 80% Lower Receiver Tutorial_3204.JPG (65325 bytes)

And here's a pic after that hole was drilled...

AR15 Jig 80% Lower Receiver Tutorial_3205.JPG (58809 bytes)

The next hole can be kinda tricky, but if you go slow and keep a firm grip on the jig assembly you shouldn't have any problems.  If the drill starts to walk off position, just slow down and keep a firm grip on the jig.  With enough patients you can actually force the drill back to the center of the 1/8 hole.  Here's what it should look like...

AR15 Jig 80% Lower Receiver Tutorial_3206.JPG (60048 bytes)

After you progress through all the 3/8 hole locations, the fire control pocket should start taking shape.  Don't worry if the drill walks off position too much, you will be able to clean it up with the endmill.  But the better job you do drilling, the easier the milling job will be.  Here's a pic after the 3/8 holes were drilled and the one 7/16 hole (reference Fig. 1 for hole location)...

AR15 Jig 80% Lower Receiver Tutorial_3210.JPG (56529 bytes)

Now it's time to use the drill press as a mill.  Attach the milling adapter plate as described above.  Put the 3/8 solid carbide endmill in the drill chuck and set the length to 1.900 from the bottom of the chuck to the bottom of the endmill...

AR15 Jig 80% Lower Receiver Tutorial_3211.JPG (40944 bytes) AR15 Jig 80% Lower Receiver Tutorial_3217.JPG (38069 bytes)

With the spindle turned off, plunge the 3/8 endmill into the pocket of the milling adapter.  I actually cranked the table up to the endmill instead of bring the endmill down to the work piece.  I think having the spindle completely inside the drill press makes it a little more rigid.  MAKE SURE there is at least 0.250 length of shank of the endmill touching the side wall of the adapter.  If you fail to do this, the cutting flutes of the endmill could cut into the adapter and ruin the adapter.  Again MAKE SURE there is at least 0.250" of the shank of the endmill touching the wall of the adapter!!!  Like in this pic...

AR15 Jig 80% Lower Receiver Tutorial_3212.JPG (43240 bytes)

Turn the spindle on and slowly move the jig assembly up to the endmill.  You need to conventional mill the pocket.  I don't have to really explain the difference between conventional and climb milling here because you simply won't be able to climb mill the pocket because you won't be able to keep it steady.  Basically you need to go clockwise inside the pocket.  Milling the pocket is a whole lot easier than what you might think.  As long as you take shallow cuts of about 0.025 deep each time.  The first cut may have to go a little deeper to keep the endmill from cutting into the side of the adapter, but other than that, it actually goes really smoothly.  Make sure you use plenty of WD-40 or some other cutting fluid to keep the chips off the endmill so you aren't re-cutting the chips. Here what it looked like after the first step...

AR15 Jig 80% Lower Receiver Tutorial_3213.JPG (49789 bytes)

Keep milling down as deep as you can until the chuck almost touches the top of the adapter plate.  Here's what it should look like... 

AR15 Jig 80% Lower Receiver Tutorial_3215.JPG (64904 bytes) AR15 Jig 80% Lower Receiver Tutorial_3216.JPG (42269 bytes)

Now before I removed the milling adapter plate, I went ahead and drilled a couple of the 1/8 holes in the shelf area of the pocket.  These holes need to be 0.630 deep.  Since the rear shelf is basically complete, a quick way of setting the depth of this hole is to plunge in the hole shown and mark the location on the drill press scale...

AR15 Jig 80% Lower Receiver Tutorial_3219.JPG (45410 bytes)

Then just drill the following two holes... 

AR15 Jig 80% Lower Receiver Tutorial_3220.JPG (47740 bytes)

Next I removed the adapter plate to see how it looked so far.  Not too bad at all considering it was all done on a drill press...

AR15 Jig 80% Lower Receiver Tutorial_3221.JPG (53231 bytes) AR15 Jig 80% Lower Receiver Tutorial_3222.JPG (66245 bytes)

Since the endmill isn't long enough to reach down inside the pocket with the adapter plate attached, you have to finish the bottom of the pocket using what you just milled as the guide.  Make sure you turn off the spindle each time you plunge the endmill into the bottom of the pocket.  Take small cuts and keep measuring the depth until you finally mill a depth of 1.249.  If you go slow and keep a firm grip on the jig assembly you should end up with something like this...

AR15 Jig 80% Lower Receiver Tutorial_3223.JPG (53790 bytes) AR15 Jig 80% Lower Receiver Tutorial_3224.JPG (49324 bytes) AR15 Jig 80% Lower Receiver Tutorial_3225.JPG (49337 bytes)

Now I kinda switched gears and finished up a few of the other features.  I drilled the trigger and hammer pin holes as well as the bolt catch plunger hole.  Visit the original tutorial for details on these holes...

AR15 Jig 80% Lower Receiver Tutorial_3226.JPG (51466 bytes)

(See the trigger slot adapter plate tutorial below if you don't want to use the following method) Now I scribed some lines where the trigger slot should be.  The scribe lines come from the back side of the bolt as show in the pic below.  The locations are as follows:  2.785; 2.969; 3.153...

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Next I measured the width of the receiver, divided that by half and set my calipers to that number and scribed a line from the inside edge of the jig to where the center of the trigger slot should be.  Then I used a transfer punch to center mark the hole locations...

AR15 Jig 80% Lower Receiver Tutorial_3232.JPG (45474 bytes)

Then I drilled the center marked locations with a 1/8 drill. and then the two end holes with a 5/16 drill.  I got in a hurry while drilling these holes so they aren't in the exact location, but it's not a big deal because it's only a slot for the trigger to fit through...

AR15 Jig 80% Lower Receiver Tutorial_3233.JPG (47520 bytes) AR15 Jig 80% Lower Receiver Tutorial_3234.JPG (46403 bytes)

Now I reattached the drilling adapter plate to mill the shelf in the pocket.  Again follow the instructions above for attaching the adapter plate.  I couldn't use the worn out 1/4 endmill in this rear take down pin hole since that is where I'll be milling, so I just ran the 5/32 drill through trigger hole to make sure everything stays lined up.  Here's what it looks like before...

AR15 Jig 80% Lower Receiver Tutorial_3235.JPG (44174 bytes) AR15 Jig 80% Lower Receiver Tutorial_3237.JPG (48215 bytes)

Mill small steps like before to a depth of 0.630 and it looks like this when you are done... 

AR15 Jig 80% Lower Receiver Tutorial_3238.JPG (42099 bytes)

When you remove the drilling adapter plate you will notice two ridges where the endmill couldn't cut.  It's not a big deal.  You can remove these ridges with a dremel tool, or just simply leave them in there because they won't hurt anything.  If you are planning on using a RDIAS then these areas will probably need to be smoothed down... 

AR15 Jig 80% Lower Receiver Tutorial_3239.JPG (54433 bytes)

Next, I setup and drilled the buffer retaining hole.  I didn't have a block that was exactly 0.220 thick so I just stacked up some index cards and placed them under the receiver.  I completely eye balled this hole.  I just made sure it was in the center of the receiver and right up against the buffer tower.  It's not critical at all...

AR15 Jig 80% Lower Receiver Tutorial_3240.JPG (40453 bytes) AR15 Jig 80% Lower Receiver Tutorial_3241.JPG (42313 bytes)

Then I setup and drilled the front take down pin detent hole, pistol grip hole, bolt catch roll pin hole, and selector detent hole.  Visit the original tutorial for details on these holes...

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I used a drimel tool with a sanding stone and a sanding band to finish out the trigger slot and I also smooth down the inside of the fire control pocket even though it wasn't a must.  Here's what it looks like...

AR15 Jig 80% Lower Receiver Tutorial_3246.JPG (46550 bytes) AR15 Jig 80% Lower Receiver Tutorial_3247.JPG (49721 bytes)

Only thing left is to black anodize the lower and she'll be ready to go...

AR15 Jig 80% Lower Receiver Tutorial_3254.JPG (39400 bytes)

I black anodized the lower and it came out looking really nice.  I don't normally toot my own horn, but I'm actually really happy with the way this lower came out considering the fire control pocket was solid before I started.  Below are some close up pics of the fire control pocket...

AR15 Jig 80% Lower Receiver Tutorial_3271.JPG (23891 bytes) AR15 Jig 80% Lower Receiver Tutorial_3270.JPG (22520 bytes)

Click on the pic below to see the final product...

V.
Conclusion:
In my opinion finishing Kevin's new 80% AR15 lower receivers are easier than his original lowers.  Mostly because you don't have to deal with the buffer tube hole.  Using the method above by drilling lots of holes in the fire control pocket to remove most of the material and then finishing it out with a endmill is a whole lot easier than what I thought it'd be.  You just have to make sure you take small cuts and use plenty of cutting fluid.  

I may have skimmed over some of the details in certain areas, but those details should be covered in the original tutorial. (fire control pocket milled out)

All information contained in this tutorial is for educational purposes.

TRIGGER SLOT TUTORIAL

By: Justin / jwh02017
Rev. Date: 09/08/2012

To make life a little easier I have designed and tested a new trigger slot adapter plate.  This new adapter isn't absolutely necessary but it will help eliminate any errors while cutting the trigger slot in the fire control pocket.  If you've completed the tutorial above then you won't need anything else besides a letter L drill. 

The first step is to secure the trigger adapter plate to the drill jig just like you did with the milling adapter plates in the tutorial above (use the same 4 bolts that came with the milling adapter kit).  Make sure you orientate the plate correctly by insuring the 2 holes are closest to the mag well. Using the same method as in the tutorial above, load the letter L drill in the chuck.  It should look like this...

AR15 trigger slot milling adapter plate jig tutorial - 1.jpg (182686 bytes)

Add a little oil to the drill and drill through the receiver.  You should now have 2 holes where the trigger slot should be...

AR15 trigger slot milling adapter plate jig tutorial - 2.jpg (80210 bytes)

Next remove the letter L drill and put the included 1/4 carbide endmill into the chuck.  Set the length from the bottom of the chuck to the bottom of the endmill to 2.110"...

AR15 trigger slot milling adapter plate jig tutorial - 3.jpg (62895 bytes)

Now flip flop the trigger slot adapter plate so the slot in the plate is closest to the mag well.  Again, use the same method as in the tutorial above to located and secure the plate to the drill jig...

AR15 trigger slot milling adapter plate jig tutorial - 4.jpg (76854 bytes)

Lower the 1/4" carbide endmill down into the jig so bottom of the endmill is touching the bottom of the fire control pocket...

AR15 trigger slot milling adapter plate jig tutorial - 5.jpg (71878 bytes)

Again using the same method as in the tutorial above, work the endmill down into the metal in steps no deeper than 0.025".  Guide the endmill around the trigger slot adapter plate in a clockwise motion.  By the time you cut all the way through the receiver the chuck will almost be touching the adapter plate...

AR15 trigger slot milling adapter plate jig tutorial - 6.jpg (73718 bytes)

Lastly, raise the endmill out of the adapter plate and you are finished...

AR15 trigger slot milling adapter plate jig tutorial - 7.jpg (191334 bytes)