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AR Lower--from wood!
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  orions_hammer

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Posted: September 21 2008 at 5:52pm | IP Logged Quote orions_hammer

I finally started getting excited about the AR platform after seeing the new Troy Arms AR57 upper, which takes P90 magazines on top; and the UltraMag 50, a 50BMG upper with the magazine to the side.  The basic appeal to me of the AR platform is that the pain-to-ship legal "gun" part, the lower, is almost trivial to fabricate; while the tough-to-fabricate barrel, trunnion, bolt, etc can be purchased separately in a variety of calibers.  Plus, I need more evil black rifles!  So I ordered a couple of raw lower forgings from DS Arms, hoping to do the finish work on my mill via the famous Ray-Vin method.

But DS Arms is backordered on lower blanks at the moment, probably because everybody else is worried about the Democrats too (didja know VP candidate Joe Biden wrote the 1994 "assault weapon" ban?).  I'm sure they'll eventually be back in stock, but dangit, I want to get this project going!  I considered doing a billet receiver, but it's like $60 for a big enough chunk of aluminum, and it'd be a shame to turn that mostly into chips just because I couldn't wait to get crackin' on my lower.

Also, as you'll see, my machinist skills do need some... er... practice.

So I decided to mill a billet lower out of a block of wood.  This will help me practice my layout, clamping, and machining on a cheap and easy-to-cut substance, and if it can survive actual shooting without exploding into a pile of toothpicks, that's an interesting bonus!

I started with the biggerhammer.net AR15 receiver plans, which I scaled properly, printed, and glued on.  Because my buffer tube tooling hasn't arrived yet, I'm doing an integral shoulder stock and pistol grip, in the style of the Cavalry Arms nylon lower.  Here's the rough profile, glued and sketched on a random but relatively knot-free 2x8.  I'm leaving the bottom of the magazine well flat, to make it easier to clamp down.


OK, with the rough cut out, I'm ready to mill the top deck flat.  Here's where I learned a couple of things about machining--first, your eyeball can tell level to maybe a tenth of a degree, but a dial indicator can tell level to a thousandth of an inch, so use a dial!  Second, you can only mill along until you hit your clamps.  Third, if you re-clamp, your work will move.  Lessons learned!


Next I flipped the receiver, used the dial indicator to make sure the top deck was level, and milled off the bottom of the magazine well level, for clamping.  I also milled out the big hole where your trigger finger goes.


Now it's on to the 5/32" fire control group holes, 3/8" safety selector hole, and 1/4" upper mounting holes.  My paper template's measurements looked spot-on for all the holes except the upper mounting hole, which was about 0.040" off--I think I stretched the paper as I glued it on or something.  Be sure to double-check your measurements, especially for the upper mounting holes, because otherwise your upper won't fit!


Here was the first major malfunction I disovered--either I mounted my mill vice non-level, or else it worked itself crooked over a few months use.  So I used the dial indicator to true up the vice, by clamping in a piece of (known-flat) cold-rolled stock, and running the table forward and back and left and right, shimming up the vice, until the dial indicator doesn't move.  (Probably every real machinist on the planet can do this in their sleep; I say this as FYI for the clueless newbies like me!)


Now that my vice is trued up, I clamped my billet vertically (that level magwell bottom is sure handy!) and drilled the mag well corner holes (standard Ray method).  I deviated from the canonical Ray-vin method a bit for the trigger hole, by milling it out with my long 1/4" mill before clearing the FCG pocket--this lets me just follow my paper template rather than needing to measure in the location at the bottom of the FCG pocket.


Now back to standard Ray-vin technique, I used my long 3/8" mill to drill the magazine's forward 3/8" hole, then my 3/8" mill to clear the safety selector area, and finally took little bites with my 1/2" drill to get the magazine well mostly open.  Then I could finish the magazine well with my big 1/2" end mill, and I did the FCG pocket at the same time.


A little bit of finish filing on the magwell, and voila!  We're closing in on function-test!


I think I'm going to leave the sides of the receiver thick rather than milling them down to milspec dimensions; this extra-thick "bull receiver" should partially compensate for wood's pathetic tensile strength.  The buttstock-to-pistolgrip connection actually feels pretty darn solid, but in the thin areas around the magazine well some wood has already chipped out.  I should have used a backer board while drilling and milling some of those holes to avoid splitting off the last few layers, but that's life.

Any predictions for what will happen when I slap on an upper and fire off the first round, using a long string to pull the trigger?  I'm thinking it'll be fine, but I'm going to be ready for anything from cracks to splintered flying fragments of flaming receiver!
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  mikey_mick

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Posted: September 21 2008 at 7:58pm | IP Logged Quote mikey_mick

You are going to make a video of the tests right?  If it breaks I want to see it happen in video...
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  dcorb

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Posted: September 21 2008 at 8:37pm | IP Logged Quote dcorb

My guess if something breaks it will be the take down pin holes for the upper. 
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  orions_hammer

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Posted: September 21 2008 at 9:54pm | IP Logged Quote orions_hammer

Mikey, I do hope to get slow-motion video of the explosion; er... first test firing!

Dcorb, I agree the takedown pins are the most marginal part to fabricate in wood, though I'm a little worried about the hammer pin too.  There's supposed to be less than 1/8" of material around the front takedown pins, which might be fine for aluminum but probably wouldn't even hold together during fabrication in wood.  For now I'm cheating by leaving extra wood in all directions around the upper mounting holes, and I'll just trim it down to clear whatever upper I finally choose, probably an ordinary 5.56 to start with.  I definitely won't start with 50bmg!

Here are some pix of the front upper mounting pin hole.  I started with a 1/2" end mill, but can't fit the lower + stock vertically in my mill, so I squared off the corners using a dremel:


The wood's about twice as wide and thick as a milspec receiver, and it's fully supported underneath.  We'll see how she holds up!  I think it'll be marginally OK when both pins are in, but easy to break while hinged open.  As insurance, I glommed on some fiberglass epoxy, the honey-consistency Bondo-brand stuff; but in retrospect a thinner epoxy might have soaked into the wood a little better.


You can also see my magazine release slot, and recoil buffer hole above.  I also milled down the area around the safety selector switch to milspec dimensions, so hopefully the safety selector switch will fit too.  Now I'm just waiting for my lower parts kit to arrive before I can do a function test!
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  tr6guns

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Posted: September 22 2008 at 7:45am | IP Logged Quote tr6guns

I wouldn't load more than one round and NO WITNESS when i pulled the string, On recoil the hammer pin will probally elongate the hole in the wood thus missing the disconnector and Slam Fire.. OOPS full auto 
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  hawcer

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Posted: September 22 2008 at 8:38am | IP Logged Quote hawcer

You coud always insert steel sleeves in oversized holes in all your pin locations.Thin your epoxy with denatured alcohol,that may allowit to soak into the wood more readily.Or use a high strength CA.

Since DSA forgings are on back-order,I picked some up from Tactical Innovations for $21.99 each.

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  chapaev

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Posted: September 22 2008 at 12:23pm | IP Logged Quote chapaev

hawcer wrote:

You coud always insert steel sleeves in oversized holes in all your pin locations.Thin your epoxy with denatured alcohol,that may allowit to soak into the wood more readily.Or use a high strength CA.

Since DSA forgings are on back-order,I picked some up from Tactical Innovations for $21.99 each.

+1 on that

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  orions_hammer

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Posted: September 22 2008 at 5:48pm | IP Logged Quote orions_hammer

Thanks for the Tactical Innovations tip, Hawcer--they're a little cheaper too.  Hopefully all of us ordering lower blanks won't run them out of stock though!  On the phone DSA said they get lower forgings in batches of about 1000, so it takes a while to make more when they run out...

I sanded and filed down the first layer of epoxy.  The epoxy-coated wood is a lot harder and less hairy to work than raw wood, which is nice.  But except at the end grain, the epoxy really didn't penetrate into the wood much.  So I added a second coat, this time thinned with about 40% acetone by volume (acetone's the recommended solvent for this epoxy).  This thinner coat soaked in really nicely.  It's drying now, so we'll see how this works.

Other than that I'm just waiting on shipping for my lower parts kit.  It's tough for me to stop working on a fun project due to shipping delays, so I built a little test magazine latch from a chunk of #10-32 threaded rot, a nut, and some 1/4" bar stock.  It seems to both fit in my lower's slot and hold/release the magazine.  Hopefully the real milspec mag latch will fit and work too!


I'm not super worried about the wood lower accidentally going full-auto; for the first firing of a new gun I usually chamber one cartridge by hand, no magazine involved.  And if the pin holes mush out badly, or I have some other catastrophic failure, I'm totally ready to just write off the "wooden lower" experiment as a bad idea and switch to milspec aluminum--I welcome failure, since it's usually a better teacher than success!
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  bikergunnut

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Posted: September 22 2008 at 10:54pm | IP Logged Quote bikergunnut

Whether or not there is any failure, as quickly as you turned this out so far, I for one would like to see a complete AR from wood . Maybe a couple kinds of wood to set off the different parts .

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  orions_hammer

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Posted: September 23 2008 at 1:23am | IP Logged Quote orions_hammer

Biker, I'm imagining a rifled wooden 223 barrel--remember the Mythbusters tree cannon?  (video / synopsis)  Wood's low tensile rupture modulus means you'd be limited to *maybe* 1Kpsi, but you could do it!

Wood definitely works fast and easy.  I've literally got like two (long) evenings in this project, and I'm ready (and itching) to drop the parts in it and test fire!
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  Inabadhood

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Posted: September 23 2008 at 10:26am | IP Logged Quote Inabadhood

LoL! 

You need to post a video of the first test firing! 

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  nacht

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Posted: September 23 2008 at 4:39pm | IP Logged Quote nacht

 I hope it works. 

 

Of course, you could use the wooden receiver as a way to make a mold, then fill the mold with bondo.  That should be stronger than wood anyway.

Perhaps you should consider using "Iron Wood".  Now, that just might work.

 

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  orions_hammer

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Posted: September 25 2008 at 4:02pm | IP Logged Quote orions_hammer

I need to find some black locust wood for future projects of this sort.  It's heavy, but it sounds like it's the strongest wood going, dimensionally stable, and rot-resistant.

I finally settled on a Model 1 Sales 16" flat-top float-tube 5.56mm upper.  Still no sign of my lower parts kit, though.  C'mon, shipping!
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  c322348

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Posted: September 26 2008 at 12:27am | IP Logged Quote c322348

nacht wrote:
Of course, you could use the wooden receiver as a way to make a mold, then fill the mold with bondo.  That should be stronger than wood anyway.

No it wouldn't! Bondo is worthless for strength, it is too brittle. If you ever dropped it, it would shatter.

You could make this far stronger using thin plywood like they use for model airplanes and laminating it. Then you wouldn't have all the splits and slivers and you might stand a chance where the pins go through to keep it from splitting.

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  orions_hammer

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Posted: September 26 2008 at 10:45pm | IP Logged Quote orions_hammer

I've gotten strangely inconsistent results from Bondo.  It seems like sometimes it'll cure into a tough and resilient solid,  but other times the same can will give wimpy fracture-prone results.  Bondo's MSDS indicates it's a mix of epoxy resin and talcum powder, so it might just be I'm not mixing the goopy stuff in the can well enough (mostly epoxy -> strong; mostly talc -> weak).  Or maybe it depends on the cure temperature or amount of hardener or something. 

Anybody have any experience with cast urethane?  Urethane's tensile strength rating is 7,800psi, way higher than wood, but still way lower than the 40-70ksi rating of 7075 aluminum.

Anyway, my lower parts kit finally shipped today, so sometime next week I'll finally get to see if an AR FCG fits and works in my wooden lower!
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  bikergunnut

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Posted: September 26 2008 at 11:33pm | IP Logged Quote bikergunnut

There is an alluminum epoxy used in porting heads, as I recall it's fairly expensive but something like that might be the ticket. JB weld and the like are prone to shrinkage with heat and age. Bondo types work best with something to hold it together like a mesh as a base (like rebar and wire in concrete).
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  orions_hammer

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Posted: October 01 2008 at 3:28am | IP Logged Quote orions_hammer

While I was waiting for my lower parts and upper to arrive, I built a little remote test rig with 2x8 sides, a 2x8 bolt-down clamp piece, and 2x4 feet.  Here it's holding a Mauser while I tweak the scope:


I also got my Tactical Innovations aluminum lower forging order.  Forged lower blanks are sure pretty, and match my wood lower geometry pretty darn well!


And finally, waiting for me today was my lower parts kit!  All it took to get the lower parts to install properly was the awesome arfcom guide and a little bit of minor lower fitting, where I'd underestimated the clearance that parts would need.  I tried to stay oversize on everything, since it's way easier to mill away excess than to glom on more wood, but it looks like you can safely run pretty far oversize on the FCG pocket--I had to open up everything (just a bit) for my FCG parts to fit!

It looks like I'm going to have to stick with my self-fabricated magazine latch, because a real magazine latch doesn't have enough clearance to screw into the magazine latch button.  I could make clearance on the receiver, but that would mean losing a lot of thickness right next to the magazine, and I'm not willing to do that.

The only major malfunction was on the selector switch, where my end mill managed to work itself down, leaving a huge thru hole instead of a flat area.  Luckily, the hole is under the unused full-auto selector position!


But finally, with the fire control group fully assembled, the moment of truth--pulling the trigger...


Success! 

The trigger pull is fairly heavy, but it's smooth, short, and breaks cleanly.  The safety selector works.  The disconnector also works reliably, even after I smacked a piece of scrapwood to bits by dropping the hammer on it about fifty times.  The FCG holes don't look "egged" at all, although I noticed my hammer pin turns along with the hammer.  But it turns out wood *is* strong enough to keep my 5/32" FCG pins in place reliably, for at least a few magazines!

I ended up just saving the stock FCG pins, which are a lot shorter than my extra-wide receiver, along with the crazy variety of tiny plungers, pins, etc.  For now, I want to keep the minimum number of parts at risk when I pull the trigger on my upper receiver, which should arrive any day now.  I can't wait to drop it on here!


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  justashooter

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Posted: October 02 2008 at 12:22am | IP Logged Quote justashooter

no reason why this wouldn't work if you use a better wood, and machine the upper receiver to stock mating surface tight. adding the steel tube inserts around axis pins would also help with durability (common trick in older wood stocked subguns).

i would trash this pine abortion and make a better receiver/stock out of black walnut, which machines more cleanly and is stronger, as well as finishing nicely. would also consider adding a steel or aluminum machined bearing surface between the wood and the rear of the upper.

limit this to 5.56.

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  orions_hammer

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Posted: October 02 2008 at 2:55am | IP Logged Quote orions_hammer

Dude!  I love that name! 

And I finally got the upper receiver for "the pine abortion"  but I need to mill out the lower's slots a tiny bit for it to fit.  And right now I need to go to sleep.

Pix coming soon!
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  jeff47

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Posted: October 02 2008 at 1:12pm | IP Logged Quote jeff47

I think the Pine abortion is just a proof of concept.   If it works in pine it should work for just about anything and he can go crazy.

I for one am impatiently awaiting a range report.  This is too cool!  
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