First ever build was the Spectre SI15. Build guide said to melt a brass threaded insert into the lower. That way the pistol grip would be screwed into metal instead of plastic.
Reading the Ubar3 guide, Ivan chose to tap threads into the lower. I trust Ivan’s judgement because he’s been polishing the ubar for 10ish years. In my mind though… the brass threaded insert seems like it’d be stronger since it’s metal on metal. Harder to strip out the threads etc.
Thing is… with Ivan’s method you can use the actual screw that’s meant for the pistol grip. If you use the threaded brass insert. It’s a much smaller flanged screw. I’m not sure what to think.
Would appreciate thoughts/advice.
I yolo an M6 into the hole. But we should all do what the Alloy(?) does and have a captive M5 nut instead of a heat-set insert or raw plastic tapping.
Realistically, any approach works, but the more you try to muscle a solution, the more you’re gonna stress the plastic in one of the weakest areas of the AR-15 lower receiver.
Yea that makes sense to me. I printed a bunch of test fins and practiced tapping them. Surprisingly worked well. Still seems like metal on metal would be stronger to me. Recent picked up sets of m4/m5/m6 screws/nuts so I can play around with which one fits best while removing the least amount of material.
I like using a pocket for a m5 nut that’s under the trigger. The m5 is smaller than the factory grip screw but if you’re breaking m5 grip screws you’re doing something very wrong 😅
I really avoid using bolts in plastic threads if possible.
So you’re saying modify the STL so that it accepts an M5 nut? I just learned Meshmixer, so I could probably do it. I would just have to figure out how to get the measurements/dimensions right.
I do it with STEP files in Fusion, I’m not familiar with other programs. What frame are you working with?
The ubar3. I checked and it can be done in Meshmixer. I found the nut I need. Is Fusion offline/ free? I only chose Meshmixer because it was.
https://odysee.com/@Sticky:9/UBAR3-with-internal-grip-nut:d
Fusion has a free version.
Could also use a square nut and negative object in your slicer if in a hurry.
Find the nut on https://www.mcmaster.com/ (for example https://www.mcmaster.com/products/nuts/hex-nuts-5~/hex-nuts-2~~/system-of-measurement~metric/thread-size~m5/). Click on the part number to bring up the detail window and you’ll see a CAD Download button. Select “3D-STEP No Threads” and click “Download”. You can load the downloaded STEP file directly into your slicer as a “Negative Part”. Position it where you want it and you’ll get a recess the correct size and shape.
If the fit is a little tight when you install the nut (this method gives you no extra tolerancing), just heat the nut with a soldering iron like you would a threaded insert and it should easily press into place. Alternatively, you can scale the negative part up a few percent in the slicer to create some tolerance. Test by adding the scaled negative to a simple cube and iterating on that until you get the fit where you want it.
Dangggggg McMaster Carr is bad ass. I’ve made a few purchases from them and never noticed you could do that.
I’ll probably just go with a m4 or m5 screw/nut so I don’t risk weakening the lower by removing too much material.
Hypothetically though, If I wanted to use the original ar screw…… From what I’ve gathered, most ar grip screws are 1/4”-28 fine unf. So that means I’d need a 1/4”- 28 fine unf nut?
I’ll probably use m4/ m5 screw/nut to minimize risk of weakening the fin by removing too much material. Smaller screws make me a tad nervous though.
Pretty sure my wrist will fail long before a decent quality M5. A grade 8 M5 is rated for over 2500 lbs of tensile force.
Awesome thanks for the reassurance. Now I just need to check if mine are grade 8.
