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3D Printing Guns & Parts
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  orions_hammer

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Posted: October 08 2012 at 3:04am | IP Logged Quote orions_hammer

So, there are these 3D printers around.  They're cheap, starting at a couple hundred bucks.  They take a roll of ABS plastic filament, heat it up, and a moving head extrudes arbitrary parts from it, layer by layer.

They can make anything you can draw in CAD, which is exciting, but for the cheap home models of today, it's made of plastic.  Plastic ain't bad!

This summer a guy named "haveblue" did an interesting project where he used his Stratasys to 3D print an AR lower (citing my HDPE work here!), and it worked reliably.  Granted, media haters gonna' hate, but it's clearly perfectly legal, and frankly it's currently way easier to buy a gun at Walmart than to program, tune, debug, tweak, and assemble one from 3D printed parts.

There's even a 'wiki weapon' group called Defense Distributed who are trying to design a 100% printable gun.  Stratsys won't let 'em have a uPrint, despite the tiny build volume and limitation to only work with ABS plastic.  A downloadable preassembled (or easy-to-assemble) gun would definitely raise grave moral issues, and cause the gun confiscators and controllers to freak out, but we're still quite a ways from getting there. 

As you know, I've been trying to figure out how to build a gun that compiles with my state's Firearms Freedom Act for years now, but I've been having trouble building safe, reliable fire control parts.  I've gotten usable parts via pressurized urethane casting, but it's a pretty long complex unreliable process (fighting bubbles, keeping the mold from sticking).

Anybody doing 3D printing out there?  I've got a definite positive experience to share in the next post...
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  orions_hammer

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Posted: October 08 2012 at 3:48am | IP Logged Quote orions_hammer

So I wanted to try 3D printing AK fire control parts.  The process starts in CAD.  I've currently tried FreeCAD (good from my Inkscape .svg outlines, nice CSG, but you need to be very careful to keep track of the XYZ coordinates + origin in your head), Blender (awesome for freeform triangle manipulation via "sculpt" tool, but not precise), and Autodesk Inventor Pro (decent for embossing and part cleanup).  Frankly, I hate them all--it'd literally be faster for me to machine a part from steel than built it the first time in CAD.  But I'm learning!

Here's a screenshot of some 3D FreeCAD AK parts, which started as 2D photos, traced in Inkscape, and extruded in FreeCAD.  I need to use Salome or Inventor as an intermediate export to get to a printable STL file, but I can get it printing!


Luckily my buddy had a 3D printer, a compact rickety build-it-yourself contraption made with laser cut wooden sides.  Finally, a 3D fabricator that makes my tiny Grizzly mini-mill look big and solid by comparison.  But after 'borrowing' it for a week, I bought it from him!

Above is about halfway through the printing process.  The extrusion head (buried in a ball of Kapton tape) is over the disconnector, making the spring hole.  The hammer axis hole is in the foreground.  It's very interesting to watch your parts grow up layer by layer!

Here's how the finished parts look directly from the 3d printer.  This version is printed onto a sheet of acrylic, which sticks better than the pink platform above.  The hairy stuff is where the extruder didn't quite shut off ("ooze") while moving between parts.


Here are some finished, cleaned up parts--about five minutes with a Nooga deburring tool takes care of the hair.  The shape looks like AK parts (except where I beefed it up), but the white color looks really odd!  I've got some black plastic filament on order, so I can make them in black.


They feel pretty weak and springy in the hand, but ABS plastic springs back into shape rather than breaking.

It actually took a roundtrip back to CAD to tweak the sizes a little so the finished parts fit each other and the AK receiver without a lot of filing, but as of rev 2 they fit!


With some trepidation, I armored up, and went out to my test area.  But when I pulled the trigger, the hammer dropped, but the round did not fire.  The primer was lightly dimpled, but clearly not struck hard enough.

The problem: this version was printed at 25% "infill", meaning the inside of the hammer is 75% air.  I don't know if the problem is mostly the light weight (this hammer only weighs 3.5 *grams*) or the fact that the face of the hammer is only 50mils of plastic followed by air, so it deforms instead of smacking the firing pin.

So I reprinted at 75% "infill" (only 25% air), and the thing fires!  The only problem is 75% infill makes a very tight little crosshatch pattern that shakes the machine badly.  This caused the extruder to shake a screw loose and stop extruding for a few layers until I noticed it, so both trigger and hammer quickly separated at the resulting weak spot.  3D printed parts seem to be weakest between the layers.

This trigger and corresponding hammer were repaired with goopy black ABS plastic cement (in the plumbing aisle), and seem to work reliably, but they're ugly.

Printing at 100% infill avoids the crosshatch shaking, prints reliably, and seems to produce a clean and reliable part.  Here's some slowmo semiauto video:

    http://youtu.be/aWrp-afq4hw  (link should work now)

Advantages of a 3D printed FCG:
    - 5x LIGHTER: The plastic FCG weighs under 15 grams, versus the normal metal version's 80 grams.
    - 2x FASTER: The plastic hammer should have about 1/5th the rotational inertia of the metal hammer, which makes for faster 'lock time' between pulling the trigger and firing the round.  I'm having a hard time seeing the black metal hammer at 1000fps slowmo to compare, but in the video I've got, a metal hammer moves at least half as fast as the plastic hammer.  Faster lock time is one reason bolt actions are famously more accurate than semi-autos.
    - 10x CHEAPER: It only costs about $0.60 in plastic filament per FCG, which makes them cheap enough to experiment with, versus $5 for an imported set or about $30 for a US version.
    - The parts are not only made in the USA for 922(r), they're made on your own printer!
    - It takes the machine about 2 hours to print a complete FCG set.  I can go to sleep, and wake up with new parts!
    - Plastic parts won't ever rust.
    - Each swipe of a file takes off a lot of material, which lets you fit the trigger group to the rifle quickly.  I've learned more about fitting AK trigger groups in the last week than I have in the last decade!
    - You can customize/redesign the trigger group directly in CAD, to make a bullpup, fit existing parts, or a higher or shorter hammer reach, etc.  I'm planning on making a version with tiny adjuster screws to let you adjust the trigger pull.

Cautions:
    - CAD is an annoying timesink.  I thought it was just the open source stuff, but the commercial programs I've tried are also really finnicky.  It's way faster to use a file to round off a corner than to get the dang thing to actually fillet correctly onscreen, and programs will weirdly refuse to do simple stuff, like export an STL file.
    - The interior holes seem to print smaller than shown in CAD, so I need to drill each parts' axis pin hole out with a #7 drill bit (5.1mm), and the disconnector with a #20 drill bit (4.1mm).  This leaves a cleaner hole than anything I can print, and I don't even have to measure the hole location, I just let the drill bit follow the existing undersized hole, so it only takes five minutes.  This would certainly be more repeatable with a drill jig, but print-and-drill works fine.
    - Plastic is not as strong as any metal, and it can't take much abuse.  In particular, I'd expect putting the safety on and pulling the trigger really hard would bend or break the trigger, possibly firing the gun.  Wear is also probably an issue, but at $0.60 each, who cares?
    - Plastic tends to 'creep' under sustained stress, and in particular the trigger hooks are under constant stress anytime the rifle is cocked.  I'm worried the hooks will slowly open up while cocked, possibly enough to let the hammer go and SPONTANEOUSLY FIRE THE GUN even with the safety on and nobody touching the trigger.  This isn't a problem if you carry and store the gun with hammer down on an empty chamber, but a chambered and cocked gun could be deadly, especially in a hot environment like a car in the sun (creep is temperature dependent, and ABS softens noticably at about 200 deg F).

Overall, an interesting technology, and a fast way to get functional prototype parts!  I've got a "creep test" running right now, and if the trigger hooks survive another few days under stress, I'll post the STL files for ya!
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  Have Blue

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Posted: October 08 2012 at 12:03pm | IP Logged Quote Have Blue

Very impressive!  I really did not think that printed FCG parts could actually work - nicely done!

Make sure to keep your filament as dry as possible - that will limit the 'spiderwebbing' from filament ooze.  Also, have you tried infiltrating the 25% infill parts with resin to strengthen them and add mass?
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  orions_hammer

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Posted: October 08 2012 at 1:16pm | IP Logged Quote orions_hammer

Thanks for the filament drying trick!  I had no idea that could be a problem, but it makes sense that tiny steam bubbles would cause filament to continually dribble out of a hot extruder like I'm seeing.  Seems like the microbubbles wouldn't do any good for the material properties either.

I did actually consider filling up the low-infill parts with castable urethane or epoxy.  This would definitely save on printing time and expensive filament, and with a cheap dense filler might be a good combination of strength, speed and cost.  But it'd definitely take more time and hassle overall, and I now have a psychological aversion to bubbles...

Glad you're on here!
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  Tanner Frisby

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Posted: October 08 2012 at 6:08pm | IP Logged Quote Tanner Frisby

Before I starting toying around with firearms, I used to build Rubik's Cube like puzzles using 3d printing. I picked up CAD in 2 years to the extent that I think I can design anything on it (with the exception of mobius type solids).
I do most of my 3d printing through Shapeways. My neighbor has, among other things, a cupcake desktop 3d printer. It works fairly well, but doesn't have the greatest object resolution.

If you don't mind me asking, how much did your 3d printer run you? The Cupcake printer is $750 to $1000, and does not have support material (which limits objects being printed at angles greater than 45 degrees relative to the horizontal). Does your printer use support material?

I too saw the possible benefits of 3d printed gun parts. I tried to implement such parts in my beltfed .410 shotgun (which, due to a design flaw, will never be completed).

I am very interested to see the final results of this testing.



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  weaponeer

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Posted: October 09 2012 at 2:11am | IP Logged Quote weaponeer

Have Blue wrote:
Very impressive!  I really did not think that printed FCG parts could actually work - nicely done!

Make sure to keep your filament as dry as possible - that will limit the 'spiderwebbing' from filament ooze.  Also, have you tried infiltrating the 25% infill parts with resin to strengthen them and add mass?


I was actually thinking more of lead bird shot...

BTW..  It's nice to see you back on the site
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  orions_hammer

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Posted: October 09 2012 at 2:24am | IP Logged Quote orions_hammer

Cool parts, Tanner!

My parts were printed on a Cupcake-esque machine (circa 2009), which has been both electrically and mechanically updated to be more like a Thing-o-matic (circa 2010).  But it retains the main limitations of the cupcake--the 4 inch cube build volume, and the single extruder (so no support material).  Print parameters were 0.2mm per layer, 1 shell, 100% infill.  This seems to only lose a few layers, a fraction of a millimeter, when printing horizontal objects extending over empty space, so I don't really worry about overhangs in the design.

Support material would be nice, and give cleaner results, but the cheapest dual-extruder I know of right now is the Stratasys uPrint, which is not only something like $15,000, they've proven they're not a hobby-friendly company by pulling Defense Distributed's lease.  And as fast as this technology is maturing, I'm not tempted to dump a bunch of money in a top-notch machine.

But I am definitely thinking about upgrading to a Printrbot Plus, which has an 8 inch cube build volume, and the kit only runs about $700 new.  Then I could experiment with printing buttstocks and lower receivers and such!
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  Have Blue

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Posted: October 09 2012 at 8:24pm | IP Logged Quote Have Blue

weaponeer wrote:
I was actually thinking more of lead bird shot...

BTW..  It's nice to see you back on the site


Hi Gary - yeah, haven't done much gun stuff the past few years.  Have a lot of parts kits just waiting for attention (perhaps that BREN kt might be buildable someday).  Lead or steel shot would be a good filler on printed parts, but since raw ABS plastic seems to be working fine for a hammer, run with it!

A few more thoughts on printed parts:

This post on using printed PLA masters was very interesting, and I naturally wondered about using the technique for casting receivers or other parts: http://3dtopo.com/lostPLA/  I wish my Stratasys could process 4043D PLA without barfing it all over!

Moisture is certainly not given the attention it needs when it comes to Makerbot/RepRap level printing - my own machine recently clogged on the extruder due to moisture, and there's a paper on the issue:  http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=62 68999

The high speed video is phenomenal!  I have access to a B&W unit that can do a maximum of 500 fps (yet with a shutter speed of 1/20000s), but it's over a decade old, and the Exilim cameras put it to shame - I'll certainly have to look into getting one.

On the split trigger and disconnector - yes, inter-layer bonding is indeed a weak point.  I brushed on Weld-On #3 to critical areas on my lower to better fuse layers together.  You might try printing with more 'shells' (on my machine/software they're called 'contours') to give your parts thicker exterior skins.

My interior holes printed ever-so-slightly-smaller than modeled as well, but I will gladly take that over having them printed as oversized holes!  Much easier to drill/ream to size than to try and add material.
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  weaponeer

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Posted: October 09 2012 at 10:05pm | IP Logged Quote weaponeer

Have Blue wrote:


Hi Gary - yeah, haven't done much gun stuff the past few years.  Have a lot of parts kits just waiting for attention (perhaps that BREN kt might be buildable someday).  Lead or steel shot would be a good filler on printed parts, but since raw ABS plastic seems to be working fine for a hammer, run with it!


Last time I saw you, you had a mini CNC.  which was long before I finally bought my Grizzly X3 mill (which I bought due to all the X3 CNC conversion kits).  I just was never able to afford the CNC etc.

I can totally relate...  I have not been able to do anything build related since 2008, because I had to move in 2009, and then ended up homeless for the 2009/2010 winter for 6 months, and finally found a place to live, but I have no place to put a shop, so it's all still in storage, alone with my kits except my Sterling, but I cannot finish that because I have zero spending money the last couple years (Army Disability Pay SUCKS) and since I can work, my life is on hold until something changes.

Was it your Mateba Model 6 Unica 44 Mag that I fired (or bump-fired) at one of the early build parties?  

That darn video (which is terrible by today's standards) has something like 100,000 views  as well as one of the most hotly contested videos on YouTube due to Bump-firing a Revolver ..  lol

talk about hateful posts..   with crap loads of people calling me an idiot because "there is no such thing as a semi-auto revolver" ...  lol

Now I have a high speed camera, and not a Mateba Model 6 Unica to be found...  lol   not that I could afford one anyway..  they pretty much have doubled in price when you can find them.

I think I'll have to just resort to buying a plain Chiappa  RHINO 4" REVOLVER when my disability pay rises, which automatically happens when hell freezes over.
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  weaponeer

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Posted: October 09 2012 at 10:14pm | IP Logged Quote weaponeer

As far as 3D printers, the only one I follow is the MakerBot Replicator 2 Desktop 3D Printer, which is about $2200 right now, which they say it has a resolution capability of 100 microns (about as thin as copy paper) and a massive 410 cubic inch build volume (which does not sound all that massive to me.. lol)

https://store.makerbot.com/replicator2.html

They also state that it can use "PLA, the renewable bioplastic that sticks to the platform reliably with practically no peeling, curling, sliding or shrinking. With MakerBot PLA Filament you will make professional-quality prototypes and huge pieces with dimensional stability, meaning that what you design is what you get. What’s more, you will use 32% less energy than building with ABS plastic. "

Print Technology: Fused Filament Fabrication
Build Volume: 11.2 L x 6.0 W x 6.1 H in [28.5 x 15.3 x 15.5 cm]
Layer Resolution Settings:
Fine100 microns [0.0039 in]
Medium270 microns [0.0106 in]
Fast340 microns [0.0133 in]
Positioning Precision: XY: 11 microns [0.0004 in];
Z: 2.5 microns [0.000098 in]
Filament Diameter: 1.75 mm [0.069 in]
Nozzle Diameter: 0.4 mm [0.015 in]

Software

  • MakerWare™ Bundle 1.0
  • .stl, .obj, .thing
  • Windows (XP/7),
      Ubuntu Linux (10.04+),
      Mac OS X (10.6+)

Physical Dimensions

  • 19.1 x 12.8 x 14.7 in  [49 x 32 x 38 cm]
  • 19.1 x 16.5 x 14.7 in  [49 x 42 x 38 cm]
  • 23 x 21.5 x 17 in [59 x 55 x 43 cm]
  • 25.4 lbs  [11.5 kg]
  • 32.0 lbs  [14.5 kg]

Temperature

  • 15°—32° C [60°—90° F]
  • 0°—32° C [32°—90° F]

Electrical

  • 100—240V, ~2 amps,50—60 Hz
  • 24V DC @ 6.25 amps
  • USB, SDcard [included]

Mechanical

  • Powder coated steel
  • PVC Panels
  • Acrylic
  • Wear-resistant, oil-infused bronze
  • 1.8° step angle with 1/16 micro-stepping


You can download MakerWare BETA here

http://www.makerbot.com/makerware/




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  orions_hammer

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Posted: October 10 2012 at 12:35am | IP Logged Quote orions_hammer

There are a *bunch* of 3D printers out there.  Sorted by price, infodump!

http://store.solidoodle.com/index.php?route=product/product& amp; amp;path=59&product_id=56
    Solidoodle 2nd gen
    $500; $550 with heated build platform
    Folded sheet metal frame; ships fully assembled
    6x6x6 inch build area
    RepRap Sanguinololu v1.3a Electronics
    Recommends ABS
    Not quite actually shipping yet; 8-10 week wait.

http://printrbot.com/shop/plus/
    $700 for full kit
    Made of wood in a Prusa "overhead X" style.
    8x8x8 inch build volume, ABS centric
    Project is by Brook Drumm, of Lincoln CA.
    Single-board "printrboard" driver, with Marlin firmware
    19V laptop power supply (not 12V PC PSU)
    Pretty darn good forum here:
      http://www.printrbottalk.com/forum/

http://www.makergear.com/products/3d-printers
    $800 Prusa Mendel kit, from Makergear
    8x8x4 inch build volume
    Many parts can be built on an existing 3D printer
    (an evolutionary "Mendel" replicating rapid fabricator or reprap)
    Most structural parts are threaded or smooth rods.
    You don't even really need the kit; just source steppers & electronics,
    add rods, print brackets and go to town.

https://3dprinterhub.com/3d-printer-store/index.php/felix-3d -printer-26.html
    Felix, designed by Guillaume Feliksdal
    $1140  (900Euro at felixprinters.com)
    +$140 shipping from printerhub.com
    Some soldering required during assembly
    260x200x200mm build envelope
    Clean design bolted from aluminum extrusions.
    It has a carry handle, but no spool holder.
    1.75mm filament, mostly for PLA, though ABS works if you build on acrylic

http://sumpod.com/sumpod-buying-options
    SUMPOD
    760GBP delivered to USA ($1200)
    240x240x150mm build area
    Heated bed
    Aluminum extrusions
    *Really* slow, but also can mill MDF.

https://shop.ultimaker.com/en/ultimaker-kits/ultimaker-kit-n ew.html
    1200euro all-in, with custom Arduino Mega control board (no soldering).
    Laser cut plywood kit; 21cm^3 (8 inch plus) build volume.
    Prints PLA or ABS.
    They put the extruder motor off next to the spool, pushing the filament through a "Bowden tube".  This makes the print head very light and *insanely* fast, up to 5m/s!
    You do have to assemble the thing yourself; a solid weekend of work, mostly putting in tiny 3mm screws.
    Ultimaker is quoted at 0.66cc/minute (40cc/hour, almost 3x Prusa speed).
    Shipping is pretty bad: DHL worldwide for 122euro

https://3dprinterhub.com/3d-printer-store/index.php/creatr-3 d-printer.html
    Leapfrog Creatr (sic).  Seriously, people: VOWELS!
    $1570, or $1900 with dual extruders.  Again, insane $300 shipping.
    Comes preassembled, actually looks like a printer.
    Huge 25cm x 27cm x 30cm build volume.
    Supports ABS, PLA, PVA
    200mm/min extrusion speed (though definitely not the spiderspeed of the Ultimaker)
    1.75mm filament

http://store.makerbot.com/replicator-404.html
    $2000 for dual extruders, fully assembled.
    Makerbot Replicator 1
    Built from wood panels (version 2 is metal).
    8.9"x5.7"x5.9" build volume
    24cc/hour deposition rate
    Unlike version 2, this one supports PLA or ABS.
    MakerWare is a *very* slick software solution.

http://www.uprint3dprinting.com/3d-printers/3d-printer-uprin t.aspx
    $16,000 to buy; $300/month to rent (but not to make gun parts!)
    Extrudes expensive proprietary ABSplus plastic filament.  Cheap version only prints in white (?!).
    Extruder somehow automatically switches to printing water-soluble 'support material', which makes it possible to print overhangs without the spaghetti effect.
    8x6x6 inch build volume
    With the high price, tiny build volume, expensive consumables, and fairly mediocre 0.25mm layer spacing (my old cupcake does 0.18mm, ultimaker folks are pushing 0.02mm!), I don't see why this Stratasys gets so much press.

Edited to add: looks like there's a *huge* list at http://www.3ders.org/pricecompare/3dprinters/.
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  northumbrian

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Posted: October 10 2012 at 3:20am | IP Logged Quote northumbrian

Another alternative, is Selective Laser Sintering (SLS), this process uses both plastic and metal to produce some very elaborate designs.

The metal version of the process is also called Direct Metal Laser Sintering.

It is being used to rapid prototype anything and everything small enough to fit in the printer. From medical/dental products to turbine blades for jet engines.

The process uses a powdered metal/plastic which is built up layer by layer. A laser melts the powder to form the part. When it's finished the powder is simply brushed off to use again later.

There are loads of rapid prototyping companies over here, I am sure you could find one over in the US which would do gun parts.


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  Have Blue

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Posted: October 10 2012 at 9:43am | IP Logged Quote Have Blue

weaponeer wrote:
Was it your Mateba Model 6 Unica 44 Mag that I fired (or bump-fired) at one of the early build parties?


Sure was!  You're still the only person who's managed that feat on it.  If you ever do another build party, I'll definitely bring it along again, as I'd love to see high speed video of it.
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  Have Blue

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Posted: October 10 2012 at 10:15am | IP Logged Quote Have Blue

orions_hammer wrote:
With the high price, tiny build volume, expensive consumables, and fairly mediocre 0.25mm layer spacing (my old cupcake does 0.18mm, ultimaker folks are pushing 0.02mm!), I don't see why this Stratasys gets so much press.


The main draw of the Stratasys machines is that they're the most 'turn-key' printers of the FDM variety.  Soluble support means you can make very complex objects with no cleanup needed.  And while they may not have the layer resolution of other machines (which is purely a software limitation, BTW - that's part of how they differentiate their product line, by artificially crippling the lower end machines), they do have really good accuracy, and you can be pretty certain that a printed part will be to within a given tolerance.  The heated build chamber also helps a lot in that respect - even heating keeps warp to a minimum.  Finally, the software is very easy to use (they've been developing it for decades), and you don't have to keep a computer connected to the printer all the time - you just send your file over the network to it.

Of course, those reasons are why the Stratasys machines are so expensive and are sold to industrial designers.  Stratasys does not like hobbyists very much - when I have a look at the machines at trade shows and tell the sales reps that I'm just a hobbyist, they pretty much immediately stop talking to me.  When I got my used machine and wanted to get updated software for it, I was told that I'd have to pony up $2000 for a service contract.  Oh, and since my machine was being dropped from support at the end of the year, I'd actually be getting less than 6 months of usable 'support'.
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  weaponeer

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Posted: October 10 2012 at 3:30pm | IP Logged Quote weaponeer

Have Blue wrote:
weaponeer wrote:
Was it your Mateba Model 6 Unica 44 Mag that I fired (or bump-fired) at one of the early build parties?


Sure was!  You're still the only person who's managed that feat on it.  If you ever do another build party, I'll definitely bring it along again, as I'd love to see high speed video of it.


I moved out of the area I was living in, so now I'm nearer to  Wisconsin Dells.  (Actually Wonewoc, WI) and all my tools are in storage, and I have no garage. so I no longer have any room, or access to the old build party location.

It may be possible to rent a building at the gun club to host a build party, but unless everyone else brought the tools and jigs, it would not be of a lot of help.

Last year I hosted monthly Camp & Shoot's..  people came to do some camping, learn some fire starting skills etc, at night, and headed to the range 8 miles away during the day for a day of shooting...   but this year I didn't have anyone interested in the camp & shoots.

But, if you are ever in the area and interested in heading to the range for shooting as well as shooting some high speed video, just let me know...
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  orions_hammer

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Posted: January 07 2013 at 3:27am | IP Logged Quote orions_hammer

Inspired by the "lost PLA metalcasting" project, I decided to try "lost ABS", and it works!  The basic idea is:
  1. Make your part in some soft, melty substance like 3D printed plastic.
  2. Surround the part with a castable "investment" (I used plaster of paris).
  3. Burn out the part, leaving a part-shaped hole in the investment.
  4. Pour in metal, filling the hole and making a part.
Here are some castable AK FCG sets, with 3D printed hammer and disconnector (at low infill), and a wax gear as a control.  The vertical fill/vent tubes are just straws, which I hotglued onto the cast parts.  The plastic, straws, and hotglue will all melt away.  The can and plaster will remain.


The steel dogfood cans are my "casting flasks", which keeps the soft plaster from cracking during burnout. 

I mixed up some really thin plaster of paris, with some sand added for high-temperature strength.  I also tried adding concrete, but I think the sulfates in the plaster kept it from curing, so I can't recommend it!  You can also just buy commercial casting investment (jewelers and dentists use it).

Caution: plastic floats!  I had to weigh down the parts to keep them from floating out of the investment.  I let the plaster cure for about three days on the radiator, then set them inside my boiler's firebox for a few hours to burn out all the plastic.  Here's the flask after burnout, ready for casting:



Note the carbonized remnants of the tall plastic sprue.  Clearly I don't have good temperature control in the boiler firebox, since only one side is white.  Evidently burnout is supposed to hit 1350F, basically glowing a dim orange; I wasn't patient enough to let it get that hot, and didn't know if my ghetto investment would survive anyway.  I've tried this without the steel can, and cheapo plaster investment tends to crack up if it's not securely surrounded.

Next I poured in molten metal.  I didn't know what was going to work, so I tried some low-melting zamak (90% zinc, 10% aluminum) and my standard die-cast aluminum (which includes some silicon for fluidity).  I can't melt steel yet, and you need ceramic investment to take the heat, but evidently this process scales to steel too. 

The moment of truth: breaking open the flasks.


They both worked!  And they worked *great*!


The detail is really incredible, especially in the zamak.  You can see every 0.2mm filament layer.  You can see the smoother spots where I plugged holes with a swipe of hotglue.  You can see where tiny air bubbles clung to the original part.  I'm impressed!


The 5mm disconnector hole cast fine, even in aluminum.  This hole is as-cast!


Here's a comparison with my "best" sand cast hammer on the left, and my "first" investment cast hammer on the right.  The investment cast hammer has way better small feature detail, although the sand gives a smoother finish.


Overall:
    - Detail is *amazing*, clearly showing the 0.2mm 3D printed layers.
    - Everything filled 100%, and out of three tries in two materials (zamak and aluminum), all three produced usable castings.  This is way better than sand casting, which still fails more often than it works for me.
    - Holes, undercuts, and overlapping parts seem to be castable without issues.  This is also a huge advantage over sandcasting.

There were some drawbacks:
    - The tiny 1/8" spring hole in the disconnector was filled with metal.  I don't know if the plaster failed to fill the hole, or if it broke off and the little plaster slug floated off somewhere.
    - A few tiny air bubbles left holes in the plaster on the bottom side of each part, which then filled with metal.  These are easy enough to scrape off as a postprocess, but I should vacuum debubble the investment; just tapping the sides wasn't enough.
    - A few carbonized fragments of burned ABS still remained on the top sides of each object.  Evidently you're supposed to flip the mold over halfway through burnout to let these burn off, or blow air through the mold, or something.
    - The overall process has lots of steps and waiting.  You print the parts (1 hour), glue them up (10 minutes), mix the investment (10 minutes), dunk and let cure for several days, burn out for several hours, melt metal, pour metal, let cool, and finally break out the parts. 

But when you see gleaming metal parts appear all shiny and perfect, it's worth it!
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  ymg200

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Posted: January 08 2013 at 9:50am | IP Logged Quote ymg200

orions_hammer,
I like the drawing of AK trigger group. The plastic model looks very clean.
I wish you get your hands on a metal prototyping machine.
Google "slm 3d printer" - that's what you need! Although, I would imagine that both the machine and materials are expensive.
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  Slowhand

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Posted: January 12 2013 at 3:04pm | IP Logged Quote Slowhand

Very interesting posts, wish I were as literate in CAD and design functions as you fellows.  How much if any shrinkage is involved in the plastic 3D final castings from the mold?  How accurate are the final dimensions after the buildup is done?  This sure sounds like the way to go in the future, especially in restoring old guns where parts are not available and you want a functional gun as opposed to a collectible all numbers match.  I shall keep checking back to see how you are progressing with methods and techniques.
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Posted: January 12 2013 at 7:33pm | IP Logged Quote weaponeer

These are always some of my favorite topics, and the results are really looking good
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  ymg200

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Posted: January 12 2013 at 11:44pm | IP Logged Quote ymg200

Slowhand wrote:
where parts are not available and you want a functional gun as opposed to a collectible all numbers match.


Technically, CAD software does not prevent you from "stamping" a serial on a part being reproduced....

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