This Saturday we decided to test the popular printable lower to failure. All this talk about printable guns and gun parts, and for months, despite being the fitful ecstatics of wiki weaponeering, we’ve been unable to actually experience them.
The lower was printed in Object ABS-like material. It’s only available on Connex printers, but it is designed to simulate ABS thermoplastic. And after reading about how it could do things like this, we thought we might end up with something a little better than an FDM piece. The print was completed in under seven hours, and looked beautiful. Note the pale, spectral glow.
The piece felt strong as well. The print was more solid than the relatively fibrous PLA and ABS FDM prints you run into. Later we would compare the print to an Omni glass-reinforced polymer lower, and see that it definitely wasn’t as strong as the latter. We could also tell the printed, thinner buffer ring was going to be a weak spot.
We had a large choice of uppers, but chose the AR five seven for our first assembly (thinking we might be able to try others if the receiver didn’t fail).
The round is 5.7x28FN. Max pressure is about 50,000 PSI, but recoil in this particular upper is supposed to be relatively light. It was enjoyable putting the rifle together, and we spent much of Friday night discussing the assembly, its vulnerabilities, and how we thought we might already be able to improve it.
Saturday morning the build was completed and we prepared to head out. Daniel‘s thoughts:
Overall, I was even more impressed with the quality of the receiver than I thought I would be. It was assembled with a mil-spec semi auto fire control group retained by KNS anti-rotation pins. All of the lower receiver parts were installed without incident, fitment of all parts was very impressive. To retain the bolt stop, instead of using a roll pin driven into the bosses, we used a prototype KNS product. It is a CNC lathe-turned stainless pin that has a non-interference fit to the receiver bosses, and rather relies on an “E-clip” for retainment. This puts much less stress on the bosses, and also makes assembly of the receiver much easier and less prone to damage.
We did have to tap and thread the hand grip hole. More on this after our results.
And to the testing site…
The first string of fire was just one round, which was fired without incident. The weapon fired, extracted/ejected/returned to battery, and the fire control properly rested, meaning the geometry of the axis pin holes is accurate. After examining the receiver for damage and finding none, the magazine was loaded with ten more rounds. On the second string of fire, the receiver seemed to fail on the fifth round – but may have actually failed earlier.
More from Daniel:
It does not appear that the receiver extension threads failed in any way. The quality of the threads was very impressive, better than I had expected, and the Ace entry-length stock fit perfectly. Instead, it seems like the off-axis force generated by recoil simply “popped” the whole ring area off the receiver in the area of the receiver tube anti-rotation plate.
This result is wholly consistent with HaveBlue’s finite element analysis of the lower, however. The buffer tube was actually flexing downward from rearward force.
There were two obvious fractures (and one not-so obvious) that started on the upper left area of the ring. One started closer to the top and front of the ring and ran almost vertically down, angled slightly to the rear of the weapon.
The other started closer to the joint of the ring and the receiver along the large radius, running almost straight down. This is the break where the ring separated from the receiver. The ring was still threaded securely to the receiver extension tube and was easily removed with no further damage.
Due to the fact that all the dangerous stresses are contained in the bolt and barrel extension – in the event of a lower failure, the only damage the operator faces is to the ego. The spring and buffer simply popped out the the tube and fell to the ground. The buffer detent and spring very lightly jammed the bolt halfway between open and in battery. It was easily cleared and no damage was sustained by the detent, spring, or bolt.
We epoxied the tower ring back together so that the piece might eventually be reused.
Ideas for Improvement
We have decided to modify the lower in SolidWorks, test again, and make our first upload to Thingiverse. There aren’t many other things we can do right now, it turns out. Seems like the Holidays aren’t really when the ATF processes license applications.
The file we’ll have next week will have most of the following changes:
- Slightly thickened magwell
- Reshaped, modernized trigger guard, something similar to the Magpul Enhanced Trigger Guard.
- Hand grip screw threading in 1/4 X 20, which would make the threads in this softer material more durable. The common dimension should be simpler to print as well. This should not require modification of the hand grip.
- Custom markings to the 3D object rather than having to laser etch in a second operation. Safety selector markings are important as well.
- The bolt release bosses are improved already, but there is room to make them a little bigger for added insurance.
- The front take down pin bosses have a great fillet upgrade, but the bosses themselves need a thicker “hoop”. There is plenty of room for this if you carry out the top on the same plane as the top of the magwell, should be enough room to almost double it.
The biggest problem area is obviously the receiver extension tube ring/buffer tower – this is true regardless of material, including metal. The current model’s ring is the same thickness of the original metal version, and should be at least thickened to reduce the large flex, as noted by HaveBlue.
Ideas to strengthen the buffer tower:
Overbuild the ring:
By using the same thickness as the top of the ring, under the charging handle, you can almost double the thickness. More material can also be added to the sides of the ring where there is no other components to interfere with. It would also be beneficial to add thick, sloping reinforcements from halfway up the ring, flowing down and forward along the sides of the receiver, even as far up to – or past – the trigger pins. This could be done with gentle curved surfaces to minimize stress risers and even appear pleasing to the eye. Haveblue noted this still might not be enough to address the problem in his FEA, and there are solutions like this under reinforcement instead: http://wordpress.digitalcrowbar.net/?p=167
If possible, however, we want to avoid a large addition to the print.
Daniel suggests a Webbing:
Extending from the top of the ring sloping forward and down. This webbing could be contoured to clear the shape of the upper so that it “nests” between this webbing. We will have to work around the forward assist on the right side, but still plenty of room to tie the corners together. Could even program some honeycomb cutouts in this webbing for aesthetics and efficient use of material. Simulation in Solidworks could really help us determine the point of diminishing returns as far as strength increase vs. cost of material.
We’ll begin migrating work and development to the Wiki this month. Haroon needs to make some database changes and then we’ll get going.
And that’s really it for now. Let’s talk more after I finish my exams.
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