80% lower machining dilemma..

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Hello all,

What if you were to chuck up an 80% lower in your mill that was supposed to be MilSpec. You made clamping plates to secure it, and set up on parallels. You set your Y axis off of one side and used that as your zero. Only to find out the lower is is .005 off...

So now one side has a wall thickness of .100 and the other is .110 in the fire control pocket. The opening is the right dimension, it’s just off center by
5 thou.

If this hypothetical person in the hypothetical story was to assemble this lower, would there be issues with the trigger and the BCG?

Thanks in advanced!
 
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Thanks for the quick responses!
It’s the overall thickness of the lower that was off compared to the blueprint. I don’t have a DRO on my mill so I used one side and tried to keep my Y Axis going in one direction.

I attached some pice of this theoretical scenario 😁66E5CA2B-B2A3-4BA7-A376-A6F794FD0635.jpegEE52A431-EF16-4809-B224-8A8B61019422.jpeg
 

pastera

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Just checked the first one I did - trigger hole is off center by 0.01" and it doesn't give a damn about it...

I did the machining by counting turns and making my own side plates - If you drilled the pins in the correct locations the rest is eye candy (to a point obviously).

This reminds me that I need to get a few more since my mill is in CNC mode...
 

greencobra

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i like to tell the story of a friend of mine...80+ years old, sitting at his kitchen table with his 80% lower snug in the jig and clamped between his knees. he went at it with an old black and decker hand drill. would you believe when the chips stopped flyin' he built up the lower, affixed his upper and the thing worked like a champ. 7 years later he and the rifle are still very much up and running. very true story.
 

pastera

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kope

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Make a plug for the stock( threaded hole ) end and get your Y off of that.
 

allen-1

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You're fine.

I've seen some AWFUL 80% lowers that run just fine. If your pin holes are in the right locations, and the trigger doesn't bind, it'll run.
 

pastera

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Thanks for the quick responses!
It’s the overall thickness of the lower that was off compared to the blueprint. I don’t have a DRO on my mill so I used one side and tried to keep my Y Axis going in one direction.

I attached some pice of this theoretical scenario 😁
Correct datum to find the center line is the machined area between the front takedown pin ears (not certain what their actual name is)
 

jayhitek

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i like to tell the story of a friend of mine...80+ years old, sitting at his kitchen table with his 80% lower snug in the jig and clamped between his knees. he went at it with an old black and decker hand drill. would you believe when the chips stopped flyin' he built up the lower, affixed his upper and the thing worked like a champ. 7 years later he and the rifle are still very much up and running. very true story.
The evil side of me was hoping for a drastic turn in this story.. like the drill bit caught and spun the lower and broke his dick. but the lower works perfectly.. Something like that.
 
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I know the pin holes are good because IV already done a couple with good results. The pocket dimension is ok as well. Just the location of it is off. But I think you guys are right, i may be over thinking it.
I appreciate the feedback, and the story!
Stay safe In the madness we’re going through!
 

Viper22

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As others have said, .005 isn't going to affect anything in that application.

At least you're using decent calipers. [thumbsup]
 

mac1911

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i like to tell the story of a friend of mine...80+ years old, sitting at his kitchen table with his 80% lower snug in the jig and clamped between his knees. he went at it with an old black and decker hand drill. would you believe when the chips stopped flyin' he built up the lower, affixed his upper and the thing worked like a champ. 7 years later he and the rifle are still very much up and running. very true story.
Iit can be done
 

84ta406

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I use the front take down pin area to get X and Y. Indicate off both sides of the piece where the pin goes. Then I bring it over to the fcg pocket and make a small pass in the center with a .250 endmill, measure from there to confirm its zeroed.
 
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You're GTG.

An observation. Does the trigger group fit inside the pocket and move freely with pins installed? No need to put the springs in for this. The front of the pocket needs to be forward enough to allow the hammer to function correctly with the pin installed. I noticed one of the front of the pocket radii is further back from the other. The hammer may fit in fine until you try putting the pin in it. Use an anti rotating pin set to keep the pin holes from egging out.

Apply tape to an anodized lower before installing it in a jig to minimize marring.
 
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Just checked the first one I did - trigger hole is off center by 0.01" and it doesn't give a damn about it...

I did the machining by counting turns and making my own side plates - If you drilled the pins in the correct locations the rest is eye candy (to a point obviously).

This reminds me that I need to get a few more since my mill is in CNC mode...
Did you do your own CNC conversion?
 
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I use the front take down pin area to get X and Y. Indicate off both sides of the piece where the pin goes. Then I bring it over to the fcg pocket and make a small pass in the center with a .250 endmill, measure from there to confirm its zeroed.
I like your method! I’m going to give that a try next go around.


Apply tape to an anodized lower before installing it in a jig to minimize marring.
I learned this one the hard way 😢
 
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What would a good entry milling machine cost to do things like this? Are there desktop models that would do well?
If your getting a killing machine just for this purpose, it’s kit worth it when you can use a jig kit with a router and drill press. That being said, if your getting into machining this then a Bridgeport is the lightest I would go with. I thought it was a monster at first, but now see that even it has limitations.
 

allen-1

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What would a good entry milling machine cost to do things like this? Are there desktop models that would do well?
If you want a (basically) single purpose desktop model - GhostGunner by Defense Distributed is worth looking into. They're currently on Version 3, which is a lot beefier and faster than its predecessors. There's code available to mill out 80% lowers on it, code for the Poly80 (which is kinda silly but interesting), and code to mill out 80% 1911's. There's also engraving code available, and more stuff being developed.

If you're just interested in completing a couple 80% lowers, I'd buy the jigs and do them with a router or find a milling machine you can use.

Question is what do you want to do - and how much money do you have to spend on it?

I've used jigs and I've used a GhostGunner CNC. I've got a friend down the street who has a Tormach and is working on SCAR-18 (?) lowers right now - starting with a block of aluminum. zero percent to firearm. When he finishes that project, he's planning on working out AR-10 lowers.
 

84ta406

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I like your method! I’m going to give that a try next go around.




I learned this one the hard way 😢
Don't forget to measure the actual hole size you make as well in case the endmill has been reground. Also climb milling works best before the final passes. I used a 1/2" rougher last time and it got rid of the bulk material very quickly after I drilled a bunch out.
 

Golddiggie

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What would a good entry milling machine cost to do things like this? Are there desktop models that would do well?
Depends on your budget level. I know people that have done it with mini-mills, it just takes 2-4x the length of time as using a more powerfull/better mill. IF you can locate a solid knee mill for shot money, that would be an option. Just know that you might have work to do on said [used] mill before it's actually good to be used.

I went with a new Grizzly G0761 mill (along with the stand) for all around milling work. I'm space limited, so getting a knee mill (at my current place) just wasn't in the cards. I've added DROs (from DRO Pro's) to the mill so that I have accurate positioning too.

Another of the reasons I went with the mill I did was how the speeds are controlled. I've had not so good experiences with variable speed machines (lathe) in the past. I didn't want to have to worry about a controller going wonky and F'ing up the speeds. With the six speeds selected by gears/levers, it's pretty much worry free. Most knee mills control their speeds via belts and pulleys. That's good until the belt starts to age and needs to be replaced. Or the tension isn't right and it starts slipping. Sure, it will take time, and all that. For me, changing the spindle speed is easy and fast (compared with the belt option).

Whatever you do, I would get a least a full horse power motor on the mill (even after a 3 phase to single phase conversion if you get a knee mill) so that you have enough power to do things up right. Also get something that uses R8 collets since they are pretty much the standard (easy to find items for).

Something else to keep in mind. You'll spend at least half of what you spend in the mill in tooling/accessories for it before you're done with the setup. Between a solid vise, end mill holders/collets, end mills, measuring tools, etc, it isn't cheap. I started off with a 4" vise (on a swivel base) but then upgraded to a 6" vise (no swivel base) and am much happier with that. Precision end mill holders also make things easier on you. IME, Shars has really good products for your machining needs. I bought the 6" vise, end mill holders and more than a few end mills from them. I'm eyeballing a rotary table setup from them as well (not sure if I'll get the 4" or 6" yet). I also like their digital calipers (have the 6" and 12" models now).
 

pastera

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Did you do your own CNC conversion?
Yes - on the super cheap
3d printed stepper mounts
270 oz-in steppers
36v supply
$10 amazon stepper drivers (not a long term solution)
$10 amazon breakout board
3d printer end stop switches
LinuxCNC

The drivers are the short pole in the tent - They overheat very quickly even after cleaning the yak shit they used for thermal compound out, using Dow thermal compound, removing the covers and running fans.

Even on my Seig X1 (150 watt motor), I can skip steps before overloading the motor while drilling a tap hole for #10
Go slow and I get very good finishes
 
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