Either way, I am blown away by your talent and creativity.
I wish I went to school as a mechanical engineer, I'd be able to build some serious shit I'll tell you. As is, this is just pure research and nose-to-the-stone measuring.
I've spent the last 3 months (ah shit) assembling my new 3D printer to speed up the prototyping process and check the precision of my model. That, along with the expanded hours I've been working at my job have left me little time to fraternize on this site or work on the CAD. I've spent nearly $1000 on top of the printer on accessories and parts to make the printer more reliable and sturdy and it still needs some TLC. Not even 2 weeks ago I wanted to tear the hairs off my sack with how frustrated this monstrosity made me, but then I managed to correct a few things that led to another. As of today I finally have the second 3D print of my lower and the first fitment prototype.
There is some warping at the magwell, it's the first part of the lower that is actually printed, but it's intact enough for us to get an idea on how it looks like. I'll still have to make adjustments with the printing process so we don't get this kind of warping but the important part is that the main body of the lower is to spec.
The new lower mated to an XCR-L standard upper, as provided by Kermit. Much of the support material can be pulled off but the main mass at the bottom of the lower is gonna take more manual work to reveal the trigger area and grip. A lot of the lines I designed match up with the upper and the mating edges between the upper and lower sit flush with each other.
The main issue is that the model has about an 1/8" gap at the rear mating surfaces of the receivers. The lines at the side cheeks of the lower also don't match. These are being corrected on the next CAD model revision, but this goes to show how important 3D printing is with this process; the model has been this off since the very beginning and none of my notes caught this.
Comparison between the old print and the new one. Notice that the hole locations in the new model have been moved over a bit. The new print is higher quality and shouldn't crack apart when I install the FCG. The mag release slot also seems to match with the slot on the magazine.
Comparison between my XCR and my would-be pistol/SBR build with the printed lower. The front takedown pin is lined up perfectly. The point where the rear of the lower angles up to the radius is also perfectly placed despite the gap at the end. The angle of the magazine isn't quite right and that concerns me. However, the last pic of the lower with the mag seems to have the top of the mag lined up correctly with the lower, so the bolt should be able to strip a round out of the mag without issue. I also noticed that the curvature of the front lip of the magwell isn't as pronounced as that on the original rifle, but that can also be an artifact from the forging process and may not be important to my lower.
Once I have the lower freed of the support material, I'll install my trigger into it and do my best function check with it. I should still be able to use snap caps to test the bolt cycling over the magazine. After I tighten these dimensions, I'll make a third print and use it to do a complete install and function check; it may not be sturdy enough to shoot the rifle with but it's certainly durable enough to dry-fire and cycle rounds with. Currently, the Pic-XCR lower is an arbitrary 10" long, but the presence of the gap in the receivers I saw today has made me consider shortening the entire lower by the length of that gap, just to keep the lower as small a possible. I might round out the length to an even, whole number just to sooth my OCD.
As a bonus, here is an idea what the final expanded lower for the AR FCG will look like:
It looks like the same damn thing, but calculations show the AR-XCR lower could be 10-15% more material than the Pic-XCR lower. I may have to do something creative with the lobe that covers the hammer pin on the OG lower; the AR FCG doesn't have pins that are that long. I also have to decide to go with either .25" or the .29" I calculated on the additional depth for the lower; I can't tell which works best until I print it. Otherwise, straightforward work.
In other news, the CNC machine I was planning to use turned out to be inadequate for my uses upon further research, so I canceled my order for it. With 2022 nearly over and my weekly schedule now very different than it was earlier in the year, I've shifted plans to have the CAD work for both types of lowers done and function-testing 3D prints for both ready by the middle of December. On the upside, there are FFL CNC machine shops in other parts of the country that I can call on to have these made. ZYCI
in Georgia and Versatool & Die Machining and Engineering
in Wisconsin are two places I've found that might be able to machine these for me. I figure it's more than a regular lower from Robinson, but I'll also bet good money they'll deliver a lot faster. Once I've confirmed that the CAD models are producing functional parts, I can ask one of these guys to produce one for me. And if it works really well, I'd be willing to provide the CAD data for other users to send to one of these guys to have these lowers made and transferred to you. I'll leave a tip jar or something out so people can help support me for this work.