Anyone familiar with the Modulus Arms HD AR-15 jig ?

PeterC

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Hi All,

I bought this jig several years ago and never used it. So, this past week I break it out, find the instructions on the web and start working on a 80% lower (one with the rear shelf already milled out if that makes a difference). The drilling goes well but as soon as I set up for the router things get a little wonky. I set up the depth using their jig for the first pass and the bit doesn't even touch the metal ! From what I can find on the web (there's a great write-up on ar15.com, and I put this same question there as well) it looks like the tip of the end mill should have been at least a 1/16, or even an 8th, inch below the top of the lower.

So far Modulus has not answered my emails and it appears they went out of business briefly in/around 2017 and may be owned by 80% Arms ?

Anyway, does anyone have any thoughts about what could be wrong ? Should I just finish milling to the max depth of the jig as per instructions and see if everything fits ?

Is my lower already toast ? :)

Thanks for any help,

Peter
 
I don't know about modulus arms but as long as you drilled your pilot hole just set the router to the depth of the top of the pilot hole and work your way down to the bottom of the preset trigger pocket depth.
 
I don't know about modulus arms but as long as you drilled your pilot hole just set the router to the depth of the top of the pilot hole and work your way down to the bottom of the preset trigger pocket depth.

Yeah, it sounds like at the worst case you will have too much material left in the bottom of the trigger pocket. In that case, your solution is to just drop the end mill down another 1/16" and hog away if it's going to obstruct the trigger.

Best case is you're just lining up the depth gauge wrong.

Just make sure your trigger pin holes line up right. If those are off you're f***ed.
 
Yeah, it sounds like at the worst case you will have too much material left in the bottom of the trigger pocket. In that case, your solution is to just drop the end mill down another 1/16" and hog away if it's going to obstruct the trigger.

Best case is you're just lining up the depth gauge wrong.

Just make sure your trigger pin holes line up right. If those are off you're f***ed.

Hi,

Yeah, I like quadruple checked the router setting (I've been a pretty serious woodworker for 25 years) and I'm pretty sure I'm following the instructions correctly.

Anyway, yes, I guess just continuing on is my best move. I don't think it's recommended to take the jig off and then put it back on so I'll have to find a way way to measure the depth as I go and possibly make that one last pass if necessary. Best I can find on the web is that it's supposed to be 1.249in (+/- .01) - is that correct ?

The jig is supposed to be 'fool proof' (hah !) so those holes are going to be wherever they're going to be. There's no adjustment... We'll see how all that goes.

Thanks !

Peter
 
I would honestly just say screw the depth gauge. Lower the bit down into the material in small increments and make your passes to clean out the material. Just make sure you don't do too much at a time. Final depth is 1.250 from the top surface of the lower. I assume you have a set of calipers? If not I have a set you can borrow if near Worcester.
 
I would honestly just say screw the depth gauge. Lower the bit down into the material in small increments and make your passes to clean out the material. Just make sure you don't do too much at a time. Final depth is 1.250 from the top surface of the lower. I assume you have a set of calipers? If not I have a set you can borrow if near Worcester.

Yes, I have several :) Thanks for the offer ! But, I'm in SNH so if what I have doesn't work then, oh well, guess I get to buy a new tool. lol

Anyway, I'm just going to have to double check that I can use one of them going in from the top thru the hole in the top plate. If not then I'll mill up a story stick of some type that let's me measure 1.25" depth without disassembling the jig.

Thanks !

Peter
 
I've seen quite a few people come through the shop that don't know you can use a set of calipers to check depth. As long as you have a set you'll be fine.
 
I've seen quite a few people come through the shop that don't know you can use a set of calipers to check depth. As long as you have a set you'll be fine.

Yep, they're extremely useful for checking the depth (and width) of mortisses, grooves and dadoes :) Checked earlier and the bottom edge should be able to just catch on the edges of the hole. The extension rod might not be exactly vertical but shouldn't matter that much.

I'm much more hopeful that it might all actually work now. lol

Thanks !
 
check their website for the newest version of the instructions. The older version had an incorrect depth setting for one of the passes,(cant remember which one). They corrected it in the newest version for that jig.......
 
check their website for the newest version of the instructions. The older version had an incorrect depth setting for one of the passes,(cant remember which one). They corrected it in the newest version for that jig.......

Their current website only has visible links for their new Extreme jig. I had to do a Google search and found one that pointed back to some folder on their website - best I could do.... Although it would be nice if they answered my emails.... :-(

Anyway, no biggie - checking depth to AR-15 specs is not what using a jig like this is supposed to be about but I can do it no problem. Just adds a bit of caution and time to the whole process :)

Thanks !
 
I'll leave this for reference.
 

Attachments

  • Modulus Arms Heavy-Duty AR-15 Jig Instructions V1.pdf
    1.8 MB · Views: 11
I'll leave this for reference.

Thanks. I won't be doing a line by line comparison but it does have the same date - January 14th, 2016 - as the version I have. So, given the drilled holes, using the supplied guide seem to have come out correctly, maybe the routing will end up at the correct depth as well even though the initial settings are not deep enough? I'll see soon enough ! lol

Thanks again,
 
Not bad. The chatter is really irrelevant to how it will perform, and I know from experience that it's hard to not get chatter along the walls when you're doing it with a hand router.

Not like you're really gonna see it once you put the gun together again anyhow.
 
Looks good. they are pretty fool proof just remember measure twice and don’t blow the bottom out. 👍

Yeah, I was going down by 128th's at the end - probably too careful. lol Ended up with 3/32 or more thickness in the base. Much thicker than one of my stripped forged lowers.
 
Ya I was pretty nervous doing mine. I did it on a milling mach so had pretty good control of depth but still measured after every other pass when I was close to the bottom.
 
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