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Posted: 11/6/2020 11:58:11 AM EDT
I made it five rounds before it broke. And it seems like the forward force of the bolt slamming home is what broke it, more than the recoil of firing it.
Settings I used, based on discussions with others who were building their own Mac Daddy was 6 shells/skins, and 60% gyroid infill. The thought that was doing it at 100% infill was overkill. Now I'm not so sure. However, the fact that it was only 36 degrees outside probably contributed to making the PLA+ even more brittle. OTOH, the firearm was warm from being in my car and hadn't sat outside for very long before I started firing it.  
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Posted: 11/6/2020 12:50:50 PM EDT
[#1]
Hmm.
How brittle is PLA+? I bought some impact resistant PLA from Atomic, but I haven't tested it against normal PLA. |
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Posted: 11/6/2020 1:47:34 PM EDT
[#2]
PLA and PLA+ can be stronger than ABS or others depending on the application. However, it's also more brittle.
I think it works better in compression than in tension. And when the bolt is slamming forward, it's creating tension at that front receiver lug and bolt. You can see how it cracked behind the bolt. It pulled it's way out and forward, it didn't push it's way out the front of the receiver. |
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Posted: 11/6/2020 1:58:22 PM EDT
[#3]
Yup, that break was certainly from shock load on a brittle plastic.
I'm sure 100% grid infill would help structurally, but that is a weak point, at least for a brittle polymer. |
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Posted: 11/6/2020 2:05:49 PM EDT
[#4]
That location should have a stainless steel insert plate
it would be fairly simple to engineer a void into the print |
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Posted: 11/6/2020 3:14:12 PM EDT
[#5]
Quote HistoryQuoted: That location should have a stainless steel insert plate it would be fairly simple to engineer a void into the print I was thinking a U-piece of metal around the nose. Like the U-bolts used on the AR lowers but wider. Spread out the impact load across more vertical cross-section. I have not been doing anything lately, but seeing these I'm still kinda mad I didn't buy that PPSh-43 kit a while back. |
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Posted: 11/6/2020 11:25:19 PM EDT
[#6]
Looks less like a material failure rather a layer separation. You can see where it follows the layer for a while before breaking across the layers in front of the bolt. Once it delaminated it just kept increasing the leverage until it broke everything else.
Just eliminating that counter bore for the button head would probably fix most of the issue. |
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Posted: 11/8/2020 2:45:27 PM EDT
[#7]
Double tap.
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Posted: 11/8/2020 2:48:00 PM EDT
[#8]
Quote HistoryQuoted: Looks less like a material failure rather a layer separation. You can see where it follows the layer for a while before breaking across the layers in front of the bolt. Once it delaminated it just kept increasing the leverage until it broke everything else. Just eliminating that counter bore for the button head would probably fix most of the issue. Close inspection looks like it didn't follow any layers for any significant distance. The cracks are all angled against several layers. I don't know if the pin and screw would reach without it but even eliminating the counter bote on one side would make it stronger there. |
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Posted: 11/8/2020 9:01:32 PM EDT
[#9]
Is it possible a more flexible material like PETG would work better in this application?
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Posted: 11/9/2020 10:55:27 AM EDT
[#10]
Quote HistoryQuoted: Is it possible a more flexible material like PETG would work better in this application? I think it might, but it's always been a mess for me to print. I might try it again in ABS. |
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Posted: 11/9/2020 11:03:38 AM EDT
[#11]
I want that frame made of metal.
I know this is the 3d printing section but having a mac frame that takes AR components is a win. |
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Posted: 11/9/2020 11:13:57 AM EDT
[#12]
Quote HistoryQuoted: I want that frame made of metal. I know this is the 3d printing section but having a mac frame that takes AR components is a win.  |
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Posted: 11/9/2020 12:09:56 PM EDT
[#13]
Quote HistoryQuoted: Search "lost PLA casting" It would be a somewhat complex mold, I think the magwell, and the big blind square hole at the rear of the receiver where the Mac 11 upper receiver slots in would be tricky to reproduce in refractory plaster. Making the mold a one-time use item that'll have to be chipped out of those areas. Others have gotten seriously more life out of their 3D printed Mac Daddy builds than I, this was primarily an issue of using PLA+, cold weather and seeing if I could get away with something other than 100% infill in the print. |
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Posted: 11/9/2020 12:12:45 PM EDT
[#14]
I wonder how carbon fiber filled PETG would work?
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Posted: 11/9/2020 7:08:57 PM EDT
[#15]
Quote HistoryQuoted: I wonder how carbon fiber filled PETG would work? Always seemed to me that the carbon fibers would be too short and slippery to really add any strength. There is a very interesting older vid of some guys testing different materials by printing full size boat props and running them. Polycarbonate won, and iirc, the only other material that even came close was wood fill PLA. They had these on a boat in a lake. I dont remember any of the others like ABS and nylon even making it beyond the test tank. One of them was carbon. My best guess is the roughness of the wood fibers allowed the PLA to grab on better to add strength. |
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