It’s no secret that I have a penchant for oddball guns, be they paintball marker or firearm.  Seeing a design that’s off the beaten path is always enjoyable, be it for mechanical ingenuity or sheer impractical novelty (though usually a peculiar blend of both).  During a recent visit to my friendly local FFL (always good for a rousing discussion and perhaps lightening of my wallet) a friend and I perused a distributor’s sale flyer and immediately spotted an intriguing item – a 12ga shotgun upper for AR-10 lowers.

For those unfamiliar with the AR-10, it is the bigger, older brother to the AR-15 rifle.  In fact, much of what was considered new or novel at the time of the AR-15’s introduction is properly credited to the earlier AR-10 design.  Unfortunately, there is far less standardization on the AR-10 platform than there is on the AR-15.  Eugene Stoner actually updated his AR-10 design decades later to have much greater commonality with AR-15 parts (resulting in the KAC SR-25), while Armalite (not the original Armalite that actually developed the AR-10, just somebody who bought the name and rights) developed the AR-10B using an upper from an SR-25, and somehow DPMS came up with a mashup of these designs, and then…  …yeah, I don’t really understand it all either.  We’re left with saying ‘AR-10′ as a generic terminology for something that looks like an AR-15 but fires a .308 round – as someone once wryly observed, “there’s an XKCD for everything“.

The important part is that the modularity of the AR-15 and AR-10 allows different upper receivers to be mounted to a common lower receiver (which is, as I’ve noted in previous posts, the one part that is itself considered the ‘firearm’ under US law).  While the AR-15 is far more standardized than the various AR-10 incarnations, the magazine well of the AR-15 limits what ammunition can easily be fed through it.  In fact, this limitation has been the underlying factor in the development of various new cartridges such as the .50 Beowulf, 6.8 SPC, .458 SOCOM, etc.  Although a 12ga shotshell by itself will just barely slip through the magwell of an AR-15 lower receiver, designing a practical magazine to feed that ammunition through said magwell is out of the question.  So, the next best thing is to scale up to the larger AR-10 lower receiver.

The ever-popular Magpul .308 magazines will happily accept a standard 12 gauge shotgun shell (extracting said shell is another matter, though).  The rimmed base of the venerable shotshell does not lend itself well to use in a box style magazine (as opposed to the tubular magazine that most traditional shotguns use).  As an aside, this issue isn’t unique to shotshells.  The rimmed base of the famous .44 Magnum round (feel free to insert your favorite Dirty Harry quote here) has limited its use in semiautomatic handguns to only 2 models in history, as far as I am aware – the iconic Desert Eagle, and Emilio Ghisoni’s masterpiece, the Model 6 Unica.

The designers of Kalashnikov derived shotguns (such as the Saiga-12 and Vepr-12) use special magazines with a fairly severe feed angle to improve reliability when stripping and chambering a round.  Unfortunately, that doesn’t translate terribly well to using a straight magwell and magazines designed for rimless ammunition.  The designers of the RAS-12 opted for a pretty radical approach to this problem – they designed their own ammunition.  Which is probably why I managed to snag this very interesting upper for less than a sixth of its original retail price – not much more than 2 years after announcing the product, the company doesn’t seem to be in business anymore.  I may write more on this later, but I’ll limit myself to covering just the ammunition in this post.

The ammunition comes in boxes of 5, with 20 boxes to a case.

The cartridges look very little like a traditional shotshell, and very much like a modern rebated rim pistol cartridge.  In this manner, it is reminiscent of the .50 Beowulf cartridge designed for the AR-15 platform.  The rounded nose and rebated rim makes feeding far simpler than with a standard 12ga shotshell, and allows for easy adaptation of existing magazines.

The most significant feature of the cartridges is obvious – they are not of metal construction, but polymer (US patent 9109850 calls out polycarbonate and nylon as suitable materials, though various online sources specifically note polycarbonate as the hull).  This feature alone is what made me take notice of the system, given that I’ve done a bit of tinkering with 3D printed polymers in gunsmithing applications.  Even if supplies of the original ammunition dry up *cough* Gyrojet *cough* Dardick *cough* EtronX *cough* it should still be possible to recreate the cartridges in a reverse-engineered fashion.  I’m somewhat surprised that the RAS-12 designers didn’t opt to ‘open source’ the design, as SAAMI standardization is precisely what has allowed previously proprietary cartridges to survive in the market if not outright flourish.

I carefully disassembled a cartridge to determine the weights (in grains) for all of the components:

  • Projectile half: 512.4 gr
    • Nosecone: 24.8 gr
    • Nine pellets of 00 buckshot: 481.2 gr
    • Wadding: 6.4 gr
  • Propellant half: 191.0 gr
    • Nitro card: 13.0 gr
    • Gas seal: 17.0 gr
    • Powder: 29.0 gr
    • Hull: 117.2 gr
    • Primer: 14.8 gr

…for a grand total of 703.4 grains for a fully assembled cartridge.  There’s certainly a bit of tolerance to these measurements, but they should serve as a suitable starting point for weights.  Now, to start measuring the hull and nosecone to draw up in CAD…


I recently had another Keiyo Seiki/Homach lathe owner contact me, and I mentioned that I had been meaning to get around to posting a copy of the manual that came with my machine.  It turns out that other owners have been looking for one, so hopefully this helps a few people: Keiyo Seiki Homach KM-1800C lathe manual

When I purchased my lathe, it came with both a 3-jaw and a 4-jaw chuck – the 3-jaw was mounted to the headstock and the 4-jaw was in a box.  However, the first time I wanted to try using the 4-jaw, I discovered that the chuck uses a D-6 camlock mount, not the A-6 mount that the lathe spindle actually has.  Apparently I had been given the incorrect chuck, but it had been long enough that I didn’t have the seller’s information anymore.

My first thought was to see if I could somehow mount the 4-jaw with an adapter plate of some sort – the jaw body itself mounts to a ’spider’ that has the camlock lugs on it, so all I would need to do is to replace the spider with a backplate.  Well, in an ideal world, that would be the case.  The screws that attach the spider to the chuck body are attached from the spider side, not from the front face of the chuck.  Thus, using an adapter plate would be a mechanical impossibility with no way to reach the attaching screws.  So now I have a D-6 4-jaw that needs a new home, and I needed to start from scratch.  Off to Enco!

Enco did in fact have the sort of 4-jaw chuck I was looking for (I selected it based on the size of object it could pass – anything smaller than the headstock ID would be wasteful), so I purchased it during one of their free shipping promotions.  All I needed then was a backplate, which I procured from “Industry Recycles” on Ebay.  I think it was actually an Enco offering but I managed to snag it for about half price.  Score!

Unlike the backplate for the 5C collet chuck, this backplate mounted up just fine.  The trick, then, was mounting the chuck to the backplate rather than mounting the backplate to the lathe.  Mounting hardware was easily purchased from McMaster-Carr, and then it was time to start making holes in things.

Given my experience with the 5C collet chuck mounting, I decided to be a little more precision oriented this time around.  I started by measuring the hole locations on the chuck itself.  Using calipers and screws inserted into the mounting holes, I measured the distance across each pair of screws (both ‘inside’ and ‘outside’ measurements).

I found that the locations of the four mounting bolt centerpoints differed by about 5 thousandths of an inch.  Instead of just basing my cuts on a single measurement, I opted to use the average instead, which came out to be a bolt circle of 5.10425″, or a radius of 2.552″.  Easy work – put the backplate in the vise in an orientation that will clear the existing holes, indicate the center bore, and drill 4 holes at +X, -X, +Y, and -Y locations of 2.552″.

Wait, the Y handwheel stopped….

Noooooooooo!  I ran out of travel little more than an eighth of an inch from the lowest hole location.  I started to resign myself to moving the vise to a new location on the table (I hate having to re-indicate in a vise almost as much as tramming the head), when I realized “duh, just rotate the hole pattern by 45 degrees”.  So each hole would be at X/Y +/- 1.8045″.

After drilling, I ran a M14×2.0 tap through the holes.

The backplate mounts just fine to the lathe spindle….

And the 4-jaw mounts just fine to the backplate!  I started right in on drilling some Delrin for a new product offering.


Filament is the primary consumable on Stratasys machines, but not the only one – the build trays are also considered consumable.  On newer machines that use plastic trays, the official recommendation is to use them for only a single print, but that starts getting expensive fast, so users will wipe them down with acetone or try sandblasting them to freshen up the surface for more use.  The old FDM series machines that I have don’t use plastic trays as a build base, but rather a foam square.  I had already investigated alternate materials with poor results, and John Branlund had started to look into craft store foam instead, as it seemed to be the material to beat.  We eventually concluded that the foam is most likely General Plastics FR-7104 modeling foam based on the sample kits that the company provides.  I haven’t pursued this further, however, as I can’t imagine that shipping on a 4′x8′ sheet of rigid foam could be anything close to ‘low cost’, and I have about half a dozen extra foam bases anyhow.  However, I’ll be turning the FDM 1600 loose to be used by other Makerspace members very soon, and I’d like to give them the ability to simply replace the foam bases as they deem necessary without burning through what I have left.

Some time back, fellow Makerspace member the_digital_dentist was experimenting with various slurries and sprays to improve part adhesion on his very large scratchbuilt 3D printer.  I gave him a Stratasys foam base to try, and it worked like a champ (almost too well, since the ABS support doesn’t break away nearly as easily as HIPS support).  I mentioned my woes in trying to source more of the foam, and he suggested trying the foil-sided polyisocyanurate (PIR) foam that you can find at some home improvement stores, and gave me a few pieces to try.  I cut a 12″ square, peeled the foil from one side, and sprayed the other side with 3M 77 adhesive before pressing it onto my material testing plate.  I put the tray into the machine and let it heat up before testing it.

Unfortunately, I failed to take differential thermal expansion into consideration.  With the chamber heated to 70 C, the foam expanded ever so slightly, buckling upwards due to the foil still being attached on the underside.

I removed the foil from the underside, and lacking anything better, used a bunch of paperclips around the perimeter to secure the foam.

While the PIR foam surface isn’t as smooth as the Stratasys bases, it’s ‘good enough’ for my needs and a test print went fine.

The part and support removed from the PIR foam very easily with just a little tearing of the substrate.  Overall, it’s still not quite as good as the FR-7104 foam (tears more easily and doesn’t grip as well), but for a cheap, readily available material, it’s the best thing I’ve found so far.  The only drawback is that locally available sheets are only 1″ thick and I need 1.25″ of thickness, but shimming up the base from the bottom shouldn’t be much of an issue.


A long-running project I’ve been tinkering with occasionally over the years has been machining an AR-15 lower receiver from scratch (or more precisely, a raw 0% forging).  I’ve made a fair amount of progress thanks to the excellent video done by Frank Roderus (which is a great machining tutorial even if you have no interest in firearms) as well as the Ray-Vin machining tutorial.  However, it’s really slow going, and I’ve made a few small mistakes.  Completing an 80% lower seemed like a good way to feel like I’ve accomplished something and would be a nice CNC project.

Drilling the holes for the takedown pins, selector, hammer and trigger pins, etc. is pretty straightforward after planing the top surface of the forging.  For an 80% lower, the takedown pin holes are already drilled and you can just use a drilling jig to drill out the holes on a drill press.  Call me a purist, but I still prefer just putting the lower in the mill vise as above, indicating from the front takedown pin hole, and drilling the rest of the holes with positions called out with the DRO.  The jig is invaluable for the various milling operations, though.

While the jig works great on the big manual mill, I can’t exactly clamp it into the diminutive vise on the CNC Taig, so I needed to determine a way to secure the fixture to the Taig table.

The 10-32 tapped holes on the A2Z tooling plate I use are spaced at 1.16″ intervals, and it turns out this spacing could be accommodated by the CNC Gunsmithing jig I use.  I drilled a line of holes for 10-32 clearance on the right jig half, and three 1/4″ holes on the left jig half (I used larger holes on one side to allow for slight differences in receiver thicknesses).  The magwell and trigger guard still stick out from the bottom of the jig, so I use a par of 1″ square bars on either side to raise everything up.

Generating G-code was pretty straightforward – I used HSMExpress to generate a path for the upper ’shelf’ and a second path for the rest of the pocket.  Yes, I tend to make Z-steps very shallow, but hogging out material with a 3/8″ endmill with a fairly long length of cut is quite a bit for the Taig to handle, so I try to keep all the cutting lightweight, even if it means really long cycle times.  This was my first attempt to use HSMExpress, and I had no idea how well the toolpaths would work, or if the mill would suddenly take off on its own and carve a giant trough through the piece, turning a $70 part into 62 cents of scrap metal.  If only I had some sort of cheap plastic version for proving out the setup…  Quick, to the 3D printer!

I took a standard AR lower CAD model and filled in the FCG area to simulate an 80% lower.  I made sure to print the part with sparse infill to save plastic and time, but I still needed a way to have a ’solid’ FCG area to provide material for the mill to actually cut.  I cut an opening over the FCG area and mixed up a batch of West System epoxy (I wish every hardware stored carried bulk epoxy supplies – luckily I live near one) loaded with a whole bunch of talcum powder.  I could have used some other filler, but I went with talc specifically to give minimal wear on the cutting tool – many common epoxy fillers can be quite abrasive (to say nothing of the fiberglass, carbon fiber, etc. that generally constitutes the matrix of a composite material layup).  I poured the epoxy/talc mix into the FCG area and let it sit overnight to cure.  In retrospect, I should have tried doing a vapor treatment on the part first, as the epoxy seeped out the sides a bit, but it wasn’t enough to affect the part’s purpose.

After reaming out the takedown pins, I was able to mount the printed 80% lower in the jig bolted to the Taig’s table.  I ran my generated program, and it worked great!  Right up until the very end, when the mill plunged right through the rear of the bolt catch area.  Precisely the sort of thing I wanted to test for, so it was all worthwhile.  I made a few adjustments to the G-code and ran the program through again to make sure that nothing else looked awry.

After that, I switched out the printed plastic lower for my partially completed 0% lower.  This showed me some other issues I hadn’t anticipated, specifically that the endmill liked to bog down in the aluminum.  I hit STOP right away, turned off the spindle power and took a step back to ponder.  The 4-flute endmill was probably not the optimal tool for this pocketing operation, but I had gone with it to increase rigidity due to the long tool length required.  The default toolpaths were also using climb cutting rather than conventional paths – this should result in less power required, but I’ve had bad luck before with climb cutting on the Taig and wanted this pocketing program to ‘just work’ even if sub-optimal.

With those changes made to the program and the purchase of a 2-flute endmill, I felt confident enough to subject a purchased 80% lower to the gauntlet.  One thing that proved very helpful were a pair of 1/4″ ground drill rods from McMaster-Carr that I could slide through the takedown pin holes in the left jig plate all the way through the holes in the lower.  This helped keep everything perfectly aligned while tightening down all the screws, and then I also used the rod to indicate in my X-axis position with an edge finder.  Make sure to remove the rear rod before starting the machining operation, though, as it sits right in the area to be machined.

Gripping a WD-40 sprayer in one hand, and Shop-Vac hose in the other, I exhaled deeply, turned on the spindle, and hit the green Start button in Mach3.  This operation makes a lot of noise, and I worried that something would seize up eventually, but my revised toolpath (I used a slightly different step-down path between Z-levels as well) seemed to do the trick.  When the program finished and the spindle fully retracted, I had no unintended cuts on the part and a nice, bright, shiny FCG pocket.

After washing it off, I installed the FCG components and did a safety check (make sure it will not fire if the safety is engaged, make sure the disconnector is working properly, etc).  Everything functions perfectly!  I’ll do a bit of identification engraving on the lower, and then it’s time to start picking out anodizing colors…


It’s been a busy summer/fall/early winter, but I’ve managed to make a few chips in the shop over the past few months.  First up, dad had a front tractor axle that needed a little work.  The hole drilled through it wasn’t quite at the needed 5 degree angle to allow for proper assembly, so he ground out the sleeve that had been welded in and I gave the boring operation a try.

Clamping the part was a little easier than I thought it would be.  It’s a heavy beast, and is as long as the entire mill table.  There’s a pair of blocks near either end that worked great for aligning it on a a horizontal plane, then I just had to clamp the center section in the vise.  I tipped the head forward by 5 degrees to complete the setup.

I seem to have lost the feeler clamp screw for my trusty Blake Co-Ax indicator, so I used an edge finder to pick up the hole center instead.

I didn’t have a boring bar long enough to plunge all the way through, so I bought one just for this project.  I selected a bar tipped with C2 carbide since the cut would be interrupted due to being a slightly different angle than the original hole.  I started by setting the boring head to a small enough diameter that it would just start cutting at the bottom of the hole (where the offset between the existing hole and my new cutting axis is greatest).  I then adjusted the boring head to make the hole a little larger, set the powerfeed, and bored again.  It was long, slow work, but I finally got to the point where I had better than 50% circumference all the way through.  Finally, I turned a matching sleeve on the lathe that dad could weld into the axle hole for the whole assembly to ride on an axis pin.

A friend is assembling a new upper for his AR, and one of the components is a Sentry 7 adjustable gas block.  Unfortunately, the freefloat handguard he’s using makes adjustment of the metering setscrew almost impossible.  Dremeling out a simple slot would fix things just fine, but would look pretty ugly – milling the slot would look much cleaner.

The problem is, how do you clamp a round object while also properly indicating it so the cut is right over the central axis?  After a bit of pondering, I came up with the above solution.  I first drilled and tapped holes in a 1″ square aluminum bar and clamped that into the mill vise.  I used strips of tape on the vise jaws to protect the anodizing on the handguard, and then used a pair of 1/2″ bolts to secure the handguard – tightening the bolts works to wedge the handguard against the top edges of the jaw faces, perfectly centering and aligning the handguard.

At that point, milling the slot (really just removing the web from between two holes) was a piece of cake.



Perhaps I haven’t mentioned it a great deal on my blog, but I guess I should come out and say it if anyone is wondering: I love airplanes.  I suppose I always have.  Growing up on the shore of Lake Michigan, our house was under a frequently used training flight path for C-130s and A-10s, and the sounds of Hercs or Hogs was enough to send me running outside to see what was passing overhead.  Some of my favorite early memories are of spending time at EAA Oshkosh with my uncles and grandfather on a foggy summer morning.  I distinctly recall seeing sometime in the very early 1980s an F-14 flying with wings swept in formation with a group of WWII warbirds (it may have even been a full Grumman ‘cats’ formation), then pulling out with an immense roar in the most memorable ‘missing man’ formation I will ever be witness to.

As a kid, I actually got kind of annoyed when going to the EAA airshow – I really wanted to see the jet fighters, but my uncles always dragged me first to see the WWII warbirds.  To be honest, I didn’t see their fascination with those old, propeller driven machines, when there were fast, exciting F-15s, F-16s and other such cutting edge combat planes as seen in An Illustrated Survey of the West’s Modern Fighters to look at (I swear, I absolutely devoured that book).  Many years later, I finally came to realize that it wasn’t a fascination at all – it was a deep respect and reverence for the planes and what they had accomplished so many years ago.  Now, when I go to that very same airshow, it’s not the modern jets that I go to look at first – it’s those old warbirds that so enraptured my uncles and grandfather.  The sound of a P-51 equipped with a Merlin V-12 is simply as magical as its engine name would imply.  Seeing the majority of airworthy P-38s in existence lined up together is the very best reunion imaginable.

I still love the jets, of course.  Oddly enough, many of the jets I knew and was fascinated by as a kid are now parked in that same warbird section – you might see an F-4, a MiG-21, or even a Sea Harrier in that area.  Sadly, with heavy restrictions now in place on what can be sold off into the civilian market, it seems quite likely that in another 10 or 20 years, there will still be more WWII vintage planes flying than the fighters I grew up with.  Apparently the government thinks shredding its heroes is for the best, and don’t even get me started on the USAF curtailing practically all airshow participation due to budget cuts (or so they claim – meanwhile, Canada was happy to send a few warplanes and even their phenomenal jet team to the Rockford air show).

Beyond my childlike desire for the sound of afterburning turbofans, I simply love airplanes in general.  I’m one of those weird people that simply have to look skyward when they hear something passing overhead, no matter how pedestrian.  I’m not even very good at being able to tell what type of plane it is.  It’s flying in the air, and that’s all that matters to me.  I’ve tried to scratch the itch – my bookshelves sag with books about airplanes, my hard drive is packed with flight simulators, and I have a pile of RC planes (and parts of RC planes, but I suppose that’s to be expected).  And yet it simply has never been enough.  I need to fly for myself.  I find it absurd that the concept of ‘aviation’ can be encoded into one’s DNA, yet the similar fascination of my uncles and grandfather appear to serve as at least anecdotal evidence that perhaps such desire is more rooted in one’s being than might normally be considered.  To be slightly more blunt, perhaps aviation is indeed in my blood.

I’ve been saving up for several years for flight training.  Flying isn’t cheap – despite the dreams and aspirations of optimistic 1950s post-war America, it’s not something you can pursue on a whim (and expect to succeed), unlike getting a motorcycle license (2 Saturday afternoons of training, a DMV test, bam – you can ride a bike).  I’ve heard that getting a private pilot certificate today actually requires more time/training than getting a commercial rating 50 years ago.  Despite the cost, I knew that this was something I simply had to pursue.  Stephen Force is an utter poet of piloting and has about the best summation that I can possibly imagine – “what do you owe to your 10-year-old self”.

No matter who who are or what are your own passions, I think this is a remarkably succinct distillation of what your true pursuits as an adult deserve to be.  If you had a time machine and visited yourself at 10 years old, what would disappoint them most about you?  Seriously consider that for a moment – looking at not a stranger, but yourself, 10, 20, 30, 40, 50, 60…  however many years younger than you now are.  What would you have to admit to your own self that you hadn’t even attempted, much less accomplished?  In my case, it would be “why didn’t you ever become a pilot?”  I would have to tell my 10 year old self that I had always wanted to, but you know, never quite got around to it, or some other bullshit answer.  And my 10 year old self would have looked at me with the most utter disdain imaginable.  They… you…  had dreams unrealized.  The opportunity was right in your face, yet you never grasped it.

I finally realized that in some way, I had made a promise to myself long ago.  I was going to either become a pilot or make the very best attempt at it that I could.  Your 10 year old self still lives inside of you, after all, subdued though they may be.  And I wasn’t going to let mine down.

Today I hope I did the 10 year old that lives inside of me proud.  I can’t verbalize the experience in any meaningful way, so I hope a simple log entry will suffice.

6 SEP 2014 – First solo – J-3 N42522



A few weeks ago I finally got the GS500 running for the season.  I rode with friends out to Madison and around the Kettle Moraine area as shakeout runs for a trip from Milwaukee to Minneapolis in a few weeks (should be interesting on little 500-650 cc bikes).  My bike, while it worked, needed some attention.

That’s the contents of my gas tank after draining it.  I’m surprised it ran at all – I needed to drain an entire Starbucks cup (thank you, parking lot litterbugs) worth of crud out of the float bowls in order to get home from Madison.  The second issue was the choke – it’s been getting progressively stickier until our 4-person ride to Kettle Moraine, where it seized entirely in the open position.  At stoplights I got to look like a tremendous jerk with my bike idling at 4000 RPM (and a Vance & Hines exhaust to boost the noise even further).  So after the ride I knew it was time to really dive into the bike guts.

After pulling the tank (and discovering the red-brown horror within), I removed the airbox and had a look at the carb itself.  The core issue was plainly obvious – a bent plate and choke plunger end on the left side.  So, just bend the end of the choke plunger back into pla… *snap*

Well, dang.  Fortunately, there’s a Suzuki dealer just down the road, and 2 weeks later I had a replacement plunger in hand.  Once I went to install it, though, something wasn’t right.

Yeah, the new plunger (top) was a bit different.  I don’t have another 2 weeks to get the correct part (if the shop can figure out the right part number), so I chucked the old plunger in the lathe.

A #43 drill and a 4-40 tap provided the threads I was after.  I installed the plunger back into the carb, and attached the plate.  A loctited 4-40 socket head cap screw finished off the assembly.

The carb, airbox, and most hoses are now reattached.  Once I put the gas tank back on and put fresh fuel in I’ll see if everything works.


Continuing from the previous installment

Even after thoroughly cleaning out the problematic support nozzle with appropriately sized tiny drill bits from either end, the nozzle would still clog and jam.  I did indeed have a single spare T12 (0.012″ orifice) nozzle, and installing it exorcised the demons that have plagued the machine for the better part of a year.  *sigh*  It’s always the last thing you suspect.  Frankie notes that when he had nozzle clogs, he used a torch on the nozzles as a ‘take no prisoners’ approach to cleaning out foreign matter.  I think I’ll have to try this method if for no other reason than to exact thermal revenge.

Last week, after verifying that both nozzles were feeding without any jams, I started a test print as a good ’shakeout’ of the system.  Printing Duchamp chess sets are all the rage right now (more on that in an upcoming blog post), so I thought it would be a good inaugural print.

When I stopped in at the Makerspace the next day, I was relieved to see that the print had completed successfully, and I didn’t have a chamber full of ‘3D printer barf’.  The print quality isn’t as good as I’m currently getting with my FDM 1600, but that’s to be expected since I have to fully calibrate the XYZ location of the support nozzle in relation to the model nozzle.  There’s also a fair bit of rippling in the prints that I think may be due to a slightly loose drive cable.  Finally, the layer adhesion is quite poor, but this is standard P400 ABS, not the MG94 (P430 ABS+) from Coex that has me absolutely spoiled (yet another upcoming blog post).


A friend had a Bushmaster rifle that they wanted to add a bipod to – unfortunately, the freefloating handguard has no provisions for mounting a bipod and even more, the handguard is integral to the barrel nut itself.  Still, I’m always up for a challenge.

To make things easier, I decided to leave the handguard/barrel nut in place and put the entire upper on the mill.  This way I could use the upper receiver as my horizontal plane reference.  I stuffed toilet paper down between the barrel and handguard to catch any chips, and used a strip of electrical tape on either side of the handguard to keep it from getting dinged by the vise jaws.

My first thought was to use a test indicator in the spindle against the two flats on either side of the upper receiver’s bottom to get it aligned vertically, then I realized that a much simpler method would be to chuck a large diameter multi-flute endmill and bring it down against a parallel while loosely clamped in the vise.  Then tighten up the vise while keeping a little pressure on the quill to keep things aligned.  Viola, perfect alignment without a test indicator.

Next step was to find the centerline of the handguard.  There wasn’t enough clearance to use a cylindrical edge finder on the jaw faces, so I flipped the edge finder over to use the conical tip.  I just had to make sure to set the stop on the spindle so that I used the same Z height on each side.  Also, this wouldn’t have worked if the top inside edge of the jaw faces was dinged up, since using the conical end of the edge finder in this way gives only point contact rather than the edge contact that the cylindrical end would have provided.

With the Y axis centered, I set my X axis zero as the end of the handguard and used a spotting drill to make a divot for the first hole 1.000″ in.

One of the ball end milled grooves on the handguard would be sitting right under the rail, but the groove is off-center by a little bit, so the spotting drill would be hitting the edge of the groove and not able to make a proper divot.  So for the two center holes, I used a 5/32″ endmill to punch holes all the way through (5/32″ is just a few thou smaller than the #21 drill used for the needed 10-32 threads).

After drilling through all 4 holes, I put a 10-32 tap in the drill chuck and loosened the collet so I could spin the chuck by hand (but still have it perfectly positioned in the X-Y plane).

After using a pick to dig out the toilet paper from inside the handguard, I attached the rail, and everything fit perfectly.  Remove from the vise, blow out any straggling chips with compressed air, and the job is done!  Ideally the screws should be installed with blue loctite (the metal is thin enough that you have to be careful about not torquing the screws too much and stripping the threads), but since my friend wanted to maybe start with having two stub rails installed instead of one long one, I’ll let him deal with the threadlocker.


Support extruder reassembly

After discovering the semi-blockage in the support extruder, I carefully drilled it out with an appropriately sized drill bit and followed up with several pipe cleaners loaded with Mothers Mag & Aluminum Polish.

I then carefully cleaned the extruder tube, and pressed on the last spare inlet buffer I have (made from Vespel, again by John).

I carefully slid the heater coil back over the extruder (and tried to twist it as much as I could in order to keep it as tight as possible on the tube), re-attached the stud for the solenoid paddle, and re-attached the thermocouple probe.

Then I wrapped the woven fiberglass insulation back around the assembly.

The original foil was unusable, so I tried using plain old ‘heavy duty’ aluminum foil from the grocery store.  It wasn’t as thick, so I doubled it up and did my best to wrap it as tightly as I could (using pieces of 3M high-temperature flue tape to hold it all in place).

The next step was to re-insert the pins for the heater and thermocouple back into the circular connector (I had pulled the pins out in order to more easily re-attach the heater and thermocouple to the extruder tube).  Note that the correct insertion/removal tool is the Amphenol M81969/14-01 – it took me several orders from Mouser to finally figure out the right size.

I then used a few zip-ties to neaten up all the wires.

Everything got reassembled back into the housing.

Re-attach the motor drive blocks.

After putting the head back in the printer, I ran a few feet of support filament though the extruder to flush it out.  Things were looking good…  …until I attached the nozzle and tried running more filament through it.  Just as before I started the extruder teardown, it jammed, kinking the filament off to the side between the drive wheels and inlet buffer.  Seeing that the buffer was also now cracked, I said a few choice words and called it quits for the day.

Upon reflection, I figured that the extruder probably wasn’t the issue in the first place (assuming that I had done the rebuild correctly) and that the motor drive block or the nozzle was the culprit.  I removed the motor drive blocks and set the head in the machine so that I could power it up and try feeding the filament through by hand.  I was able to push the ABS filament through pretty easily, but the support filament I could barely budge.  So I removed the support nozzle and put the model nozzle on the support extruder.  Wonder of wonders, I was able to feed through the filament just as easily – the nozzle must be the problem!  I reinstalled the motor drive blocks and sure enough, the support drive was able to feed support filament through the support extruder and model nozzle with no jamming whatsoever.

I’ll try clearing out the support nozzle with a 0.011″ drill bit (even though I know I’ve done so once already), and I believe I have a spare T12 support nozzle floating around in case that doesn’t work.  At least I’ve finally found the core issue and I’ll hopefully have the machine fully functional in only a week or two.  [insert something about 'famous last words']