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Discussion Starter · #62 · (Edited)
Here's the latest version of the lower in CAD. I've added the clearance for the Upper's bridge support and widened the space where the RA rollmark goes on the Lower; the latter has resolved the issue where the space for the mag release arm was disproportionally short.
Font Auto part Automotive exterior Automotive window part Art

Font Auto part Rectangle Automotive exterior Drawing

Of particular note is that I've shifted the entire FCG .050" forward; this seemed to resolve the issue of the safety detent being too far forward (the error suggested that the safety hole was too far back) and kept the hammer pin hole centered on the front lobe. This means that the hammer now .050" forward, but it may have needed that distance to travel far enough to strike the firing pin.
Automotive exterior Auto part Bumper Gun accessory Gun barrel

It looks like at this stage, work for this lower will involve a lot of guessing and checking. It's a good thing plastic is cheap. I have to be sure to adjust my print settings to make a more durable fit test.

Seeing Jordan636's XCR with the Zhukov stock has led me to believe that some people might need the slightly longer rear picatinny rail that the Lucky Irishman adapter provides. I may try the alternative rail design when the next print gets made and see how that handles.
Air gun Trigger Machine gun Gun barrel Gun accessory
 

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Discussion Starter · #63 · (Edited)
Small update, I've expanded the rear rail to match more like Jordan's old rifle. Should be helpful for people who need the extra length. I don't think it looks too bad either, it's a compromise between the really long rail I had in the beginning and the shortened rail that didn't appeal to me quite as much.
Auto part Font Rectangle Automotive exterior Drawing


Auto part Font Rectangle Metal Cylinder


While I was at it, I made a model of the bolt hold open too. Its tiny details might be off but the main details like the length and screw thread location are caliper-accurate. I think it's good enough to make a replica part if necessary.
Font Wood Symbol Metal Column

Making a model of the BHO might be necessary because when we extend the lower to fit the AR-15 FCG, we might need a longer version of the BHO to take its place. I will try to use the original BHO and use a longer screw or plastic extender to make up the quarter inch length difference before I do that however. I think doing that will still make the BHO sturdy enough for the job.
Automotive exterior Rectangle Auto part Font Automotive window part

Rectangle Font Auto part Metal Watch accessory




At this point I need to print another one out and do more guessing and checking. But I think I really got it close this time: cosmetically it's mostly there (and all of the production lowers I've seen seem to have some variance to them anyway) and it's only a matter of getting these hole locations right before we have a functioning receiver. The magwell was a little too tight for magazines in the first print, but it could have been due to plastic deformation that was the cause.

Recent events with AWB 2022 being passed narrowly in the US House has been a cause for concern for me, since it (in Section 2, item (40), sub-item (M)) prohibits the manufacture of receivers like this. While whether it can be passed as law is another kind of discussion, it does hold my feet to the fire to getting this thing built. At this rate, I can have both designs finalized by September and something built in metal by October or November. If I don't get a CNC machine for myself by then, there are several FFL machine shops around the nation that I contract to mill this thing out; even if it costs me $600 for one of these it'll be worth it. That should beat out legislation in the chance AWB '22 gets ratified by the end of this year. Being able to produce this thing in numbers for other users, however, gets less predictable even if I can file for an FFL form 7 in time. Even if the window for major production gets slammed shut, the fact that this data exists keeps hope alive. Either way, I need to pick it up a bit to make my target goal; August came by really quick this year, I swore February was last month...
 

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Wow, @Chowwow you are a god of engineering or something!

Guessing by the serial number, there are approximately 14,000 XCRs around. If 5% of them are interested in using AR triggers + 1913 stocks, that's a sizeable order size if you decide to sell this as a product.

Either way, I am blown away by your talent and creativity.
 

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Discussion Starter · #65 · (Edited)
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.

Bicycle part Automotive exterior Wood Motor vehicle Engineering

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.

Trigger Gun barrel Gun accessory Rectangle Air gun


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.

Wood Grey Automotive exterior Line Bumper


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.

Sleeve Wood Camouflage Pattern Font


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.

Air gun Trigger Wood Gun barrel Gun accessory


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:
Font Auto part Rectangle Drawing


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.
 

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Discussion Starter · #66 ·
Update: FCG holes and grip screw hole appear to be in position and trigger seems to be working well. Beaver tail on the lower protrudes too far out and any pistol grip with a beaver tail will not fit. BHO appears to be too far forward and BHO foot won't screw in. Bottom slot for trigger seems to be too far forward and slot currently rubs against the back of the trigger. Magazine catch is too low on the receiver by about .100", so magazines are not quite high enough for the bolt to strip off rounds from the top of the magazine.

Air gun Trigger Gun barrel Wood Gun accessory


I finally liberated the second print from the supports and got to work. The XCR FCG seems to be in just the right place and strikes the firing pin and resets upon bolt pull back. In this photo you can see how the mag catch space seems a little too low to catch the receptacle on the magazine (placed here at the height where the magazine would normally go on a default lower). When the mag is at the proper height, it eliminates the shallow angle of the mag problem I was seeing before this.

Finger Bicycle part Automotive exterior Bumper Gadget
Bumper Automotive exterior Trigger Auto part Automotive wheel system


Had to force the magazine to reach this height. The space for the mag catch was out of spec and a bit too narrow than expected. Still much too low to work.

Trigger Air gun Wood Gun accessory Gun barrel


I might want to bring the slot for the trigger backwards by .50". I can't really tell how the trigger pull is but it feels stiff to pull the trigger like this. The slot for the BHO foot is actually up to spec, it just seems like the hole for the BHO is too far forward for its good.

Wood Trigger Air gun Gun barrel Gun accessory


The rear of the lower protrudes too far out for any pistol grip with a beaver tail to fit on the lower. The placement of the grip attachment itself, including the safety detent hole and grip screw hole are on point, however. I wonder if I should thread the hole with a fine thread like default AR grips or stick to the coarse thread that the XCR uses. If it's being screwed into aluminum, I would like to use the coarse thread screw.

Air gun Machine gun Trigger Gun barrel Automotive tire


Trigger clicks and resets properly. It's just that the magazine is sitting a bit too low for the bolt to strip a round properly.

Bonus:
Air gun Trigger Gun barrel Gun accessory Wood

Air gun Trigger Gun barrel Gun accessory Wood

Air gun Trigger Gun barrel Wood Gun accessory


I bought a Timney Impact drop-in trigger for this project and used it in the mean time to check the default XCR lower. The placement of the FCG holes is precisely like that of the AR-15 FCG, just on a lower profile position. Without that .25-.29" height, the hammer leans further forward than normal and the hammer is so long that it gets caught under the bolt on pull back if you tried to use this trigger on the XCR. This strongly indicates that when I raise the height of the AR-XCR lower, I won't need to move the positions of the holes anywhere to correctly place the hammer.

I've already made corrections on the CAD model. The lower is now 9.75" long instead of 10". I'll sleep on the changes before I decide to hit print on the next one. On the next print, I'll focus on printing the mag catch space accurately. I'm concerned the radius at the mating corner between the receivers might be off but we'll just have to see how the print turns out.

As we get closer to being done I need to think about how this is going to be machined. Right now, the lower emulates the OG lower almost exactly. However, the OG lower is forged (where a piece of heated aluminum is sandwiched between two dies that have the shape of the intended part), which is why it has a seam running along the middle of it on places that have not been machined off and why it has so many rounded and embossed edges. A CNC machined part can also have this many embossed edges but it would take more time and thus more money to make. I was thinking about making the edges square and straight, like that of the CZ Scorpion Evo or the Mac n Cheese 3D printed lower:



It'll take some time to make a mock up, but I can show an updated version later, if people like this idea.

Speaking of CNC machining, our own @SheffieldTactical is an FFL CNC machinist. If I don't get my own machine by then, I'll work with him to simplify the design and have his equipment cut out these lowers. Programming tool paths is just as involved as designing in CAD, and since I know what equipment he's using, I should be able to program the tool paths with my software at my place, which should cut down on the set-up time and labor.
 
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