Doc, plywood is a darn good idea. I haven't been able to get epoxy to soak into wood very effectively, though--but the vacuum-bag trick might help! I've also tried MDF, which is a really fine-grained fiberboard--it mills nicely, and it's about twice as strong as pine, but it still splits apart too easily. I also tried coating both wood and MDF in polyurethane, which soaks in beautifully, but sadly polyurethane doesn't add much strength.
(Modem folks beware: many images coming below!)
But I really do like how a fairly soft plastic like HDPE machines on a mill, so...
Spiral bits for a scrollsaw sure are awesome--you can cut in any direction!
Step two was to "weld" the above slices of cutting board into one solid block. I used a hot air gun, which just barely had enough heat output to keep the whole surface liquid. Hot HDPE gets really sticky, so once both sides of a weld are hot, be sure it's lined up before you squish the surfaces together!
I'm not super happy with this welding technique--I got the edges basically fused properly, but as I discovered large sections inside there aren't fully fused, and HDPE warps badly due to its high thermal expansion coefficient. It'd be better to start with one solid 1.5" thick block of plastic, or to heat the sections in an oven broiler and then fuse in a nice level aligned jig. Ah well, at least it's one piece now!
I pasted my template on, ready to mill the top deck flat. "High helix" aluminum end mills work very nicely on plastic, scraping off weird blue chips at a ridiculously high feed rate!
Ready to drill the buffer tube hole. Little did I know, I'm actually following the wrong line in my template while drilling this hole, so it's going in the wrong place! (D'oh! I've cleaned up my template substantially after this screwup...)
I usually start with a 3/8" pointer to find the center of the hole, then lock down the mill table so nothing moves in the horizontal plane. I then open up the hole enough to fit a boring bar with a 1/2" end mill, switch to the boring head, and work outwards to 1.130". Then I can drop in the huge buffer tube tap, and tap out the threads. The only tricky part here is that my (crappy!) weld failed, visible as a thin white line above, and the laminations came apart a bit--hence the clamps during tapping.
HDPE sure works beautifully. There's a tiny bit of hair on the corners, but it's definitely a precise substance to mill!
On to the trigger finger hole:
I ended up setting up a shop vac to deal with the hairy blue plastic shards going everywhere--they're soft, but lighter and fly a lot farther than metal chip, and they seem to electrostatically cling to everything in sight (think shopping bag fragments!).
Now that the main exterior profiling is complete, ready for the interior templates. It turns out that Elmer's glue doesn't stick at all to HDPE, but
Plumber's GOOP actually sticks paper to HDPE pretty well, but can eventually be
peeled off cleanly. I *love* using templates for machining, because it means I don't have to measure anything; I just set the vertical depth stop based on the side profile, and then mill out a pocket based on the top outline:
I usually do a FCG hole starting with a long 1/4" mill to get the trigger hole. I always make this hole too short and too narrow to start with, so go outside the lines a bit! Then I use a 3/8" mill (above) to clean out the narrow area around the safety switch, and finally a 1/2" mill to do the rear upper mounting pocket and main FCG pocket.
Then it's on to the magazine well. I began with the usual ray-vin
1/8" holes in the corners, but rather than slowly step up a series of drills down the centerline, this time I just plunged out the magwell outside with a 3/8" end mill. It's literally like five minutes work in plastic, and the narrower end mill leaves a little island of uncut material that falls out of the middle of the magwell.
The narrower endmill lets you get a lot closer to the corners, so it only took a tiny amount of hand filing to fit the magazine proper. Rather than carefully remove all the chips between test fittings, I just wrapped the magazine in a plastic bag--this protects the magazine, and adds a little clearance for drop-free function!
Here's where I realized my big mistake--the buffer tube hole should be just exactly lined up on the bolt, not 1/8" lower than the bolt!
So, I just cut the back end off and re-welded it!
Sadly, it's a lot trickier to place the hole to within 10 thousandths when you're squishing hot sticky HDPE, so I had to cut off and re-weld again:
I thought I was pretty darn close, so I just drilled the rest of the holes. The trigger, sear, and hammer all fit fine on the first try, just like with the Pine ABORTION!
And again, there wasn't clearance for a factory magazine latch, so I recycled the longer ghetto magazine latch from the ABORTION:
HDPE's strong enough to drill and tap well, so I even did the pistol grip screw, and buffer tube retainer hole, the milspec way:
Ta dah! Tactical tupperware at its finest! And unlike the wood model, nothing broke off during assembly either!
Only one tiny problem: the bolt doesn't cycle, because the welded-and-re-welded buffer tube hole isn't lined up with the upper mounting pins. Solution? Weld on a blank plate, and mill and tap it fresh and concentric:
During finishing I popped another one of the crappy lamination welds on the magwell, so I re-welded it and put in some 1/8" brass rivets just to hold it together better. Note to self: build next billet receiver from... one billet, not a set of laminations.
My magwell was a bit tight, and thanks to some post-weld warping the bolt no longer ran over the magazine lips properly, hanging up as shown below. Filing out the magazine well just a tad on the proper side, to let the magazine jiggle out of the way of the bolt, cured this problem.
I even put in the buffer spring retainer hole, which works nicely:
I fired off a few rounds yesterday, and dang it, the bolt wedged itself up way back in the buffer tube again, like a misaligned buffer tube! It's a bit stiff when inside, but it really siezes up in the (0 deg F) cold. I think the problem is that plastic contracts a lot more than metal, so I need to add a bit of expansion room around the upper mounting area for it to function properly. Also, in a warm inside room, the pistol grip feels a tad mushy--HDPE is pretty flexible (low elastic modulus), despite its toughness (high tensile modulus).
But overall, I'm really happy with this version! HDPE mills like a dream--they actually make practice "milling wax" from some variant of HDPE. It seems to be plenty tough enough for actual use, unlike wood--except along my crappy welds, nothing has broken off yet.
I was considering making version 2 in UHMW, but I managed to find
some little pieces locally, and I don't like how they mill--UHMW is way
more stringy and chattery to mill, and it actually feels floppier than
HDPE (low elastic modulus). I ended up ordering Grainger item
which is 1 square foot of 1.5" thick ABS plastic. It's $70, but that
should be enough for at least five more receivers! I can now crank out a receiver in about two evenings (assuming I don't screw up!), most of which is time spent lining up and clamping the work.
Here's the final firing version, together with its "parent" cutting board and "brother" giant bag o' chips.Grizzly
buffer tap and end mills: $100.
Building an AR-15 from a cutting board: priceless!