Gunsmithing Machining a non-rotating flush cup

samb300

GCP Rifle Co. Accuracy Obsession Vision Products
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  • Feb 22, 2013
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    Milwaukee, WI
    I'm trying to figure out how one would best machine an anti-rotational flush cup.

    Does anyone have the specs (if there is a standard) for the inner diameter of a flush cup, and the diameter/size of the inner non-rotating feature?

    I'm thinking of something like the Magpul bolt-on flush cup, if that helps visualize what I'm going for.

    Thanks!
     

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    If you are capable of machining one you should be able to measure one. You can buy one to measure for four bucks!!

    Is anything you post ever helpful or useful in any way? Just shut the hell up if you don't have anything related to the threads you are posting in.

    I can measure a fucking $4 flush cup, that doesn't mean it's a standard.
     
    Seriously consider machining the main part you are trying to produce but with a threaded pocket to accept a limited rotation cup.

    The cups from GrovTec are stainless with black oxide finish. Very high quality part and are inexpensive enough that it may be worth your while to purchase that part already made then loctite in place with rotation clocked to position you desire. The thread is a standard, easy to machine thread and the cups have a 1/4" hex drive in the bottom. . . .

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    Seriously consider machining the main part you are trying to produce but with a threaded pocket to accept a limited rotation cup.

    The cups from GrovTec are stainless with black oxide finish. Very high quality part and are inexpensive enough that it may be worth your while to purchase that part already made then loctite in place with rotation clocked to position you desire. The thread is a standard, easy to machine thread and the cups have a 1/4" hex drive in the bottom. . . .

    ./

    Terry,

    Thank you very much for the insight and the information.

    Do you think there is a downside to using 6061 or even 7075 Aluminum for the actual QD pocket? Will it wear or potentially fail? (I know there only a small rim of material providing the support for the sling to not pull out)

    The design I'm working on is similar to the Magpul 1-piece shown in my original post, but thinner, so I'm not sure the threaded pocket will work as I'm looking at it right now. I'm wondering if making the entire piece out of Stainless would be a better choice if I can't get a threaded pocket to work?

    Thanks again!
     
    I feel like even with a brand new part, you could pull hard and rip the swivel out of an aluminum QD hole... never mind wear. I could be wrong.
     
    Is anything you post ever helpful or useful in any way? Just shut the hell up if you don't have anything related to the threads you are posting in.

    I can measure a fucking $4 flush cup, that doesn't mean it's a standard.

    Good thing it isn't rocket science. It would be way over your head. The $4 flush cup I referred to is actually sold by McMillan Stocks. Not sure if that standard is good enough for you.
     
    Good thing it isn't rocket science. It would be way over your head. The $4 flush cup I referred to is actually sold by McMillan Stocks. Not sure if that standard is good enough for you.

    Please troll elsewhere. Like I already said, I was wondering if there were specs. Go measure ten "standard" picatinny rails all from different companies and tell me if they are all the same, and all match the 1913 standard drawing.
     
    Terry is right....the GrovTec cups are the easiest to buy and highest quality. However, since we are all looking for info rather than BS opinions by some folks....here's my 2 cents. I work in the aerospace industry where we machine every type of alloy you can imagine. My gut says the way I use flush cups to sling my rifle, the amount of times I "quickly" and "agrressively" mount and dismount my slings with mud and sand in them.....I think any type of Aluminum would wear quickly. As you said, the inside rim of the flush cup has little material to retain the detents....and even with something like 7075 heat treated to achieve hardness....I just don't think there is enough material to sustain a "work rifle" and the abuse it must mitigate from someone like myself. Aluminum just doesn't have the integrity for the bearing surface that a flush cup has. I would definitely recommend stainless, or Tool Steel if you can machine it.
     
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    I can also advise that Terry has spent a lot of time looking at the design and mechanics of the flush cup in general....that's another topic....but he's been down this road and I would personally take his advise at gospel. Then again....I'm always trying to make shit myself when there's a $10 part already out there.....I just can't help myself....so, turn the mill on, get some bar stock and start making chips !!!
     
    Thanks gents.

    I agree, Terry's advice is to be taken with extreme confidence!

    I'm planning on just making the part from SS at this point, as the ~0.5oz weight savings Aluminum offers is pretty insignificant. The "original" part I'm working off of is aluminum with a steel insert for the cup (similar to the GroveTec), but for simplicity and making the part slimmer I want to make it one piece.
     
    For the Primary flushcups where a sling is most likely used I'd keep it steel-ish material. That said, for light-duty locations 7075 should work alright as it's tensile strength rivals that of many flavors of stainless steel. It does not stand up to the elements very well though without anodize and then the way they'd be used the anodize would certainly wear off over time. If you can make them you can choose whatever material you want to suit your needs.

    I will say that at one point I was suspicious of installing the Grovtec 9/16-18 threaded cups into wood for fear they'd tear out. I got a thin piece of walnut and D&Ted it and installed a cup, used the HD QD sling swivel and hung 90lbs off it. No problems whatsoever. If the standard SS cups can hold 90+lbs then an aluminum one should hold up a light rifle easily.

     
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    I’m coming in here a little late but I thought I’d add a tidbit about the fears of using properly manufactured aluminum push button base. I read some of the previous post and it got me curious so I thought I’d perform a pull test on a GrovTec GTSW-278 rail mount aluminum base using a Chatillon Digital Force Gauge mounted on a test stand which allows linear testing and recording of peak force measurements. The Chatillon gauge is calibrated in accordance to ISO 2001 requirements. The swivel base is made from 6061 aluminum with a Type III hard anodize finish. I got up to 360lbs of tension without any distortion of the rail, base, or swivel. At 404lbs, I pulled the rail through the hold down screw, the base stayed clamped to the rail, and the swivel loop just started to show an ever so slight sign of distortion but the aluminum base was holding the swivel firmly in place. I removed the push button swivel and inspected the ball bearing groove in the base and there were four tiny, shallow dimples where the ball bearings were pulling against the base. But even with the dimples, the base was still fully functional. In short, with the ability to handle that kind of tension, you can use properly manufactured aluminum bases without fear of your rifle becoming a lawn dart and getting free engraving courtesy of rocks and gravel.
     

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    Please troll elsewhere. Like I already said, I was wondering if there were specs. Go measure ten "standard" picatinny rails all from different companies and tell me if they are all the same, and all match the 1913 standard drawing.
    Amen. What a cluster. Have a couple high quality rifles or rails and have to adjust bipod or night optic mounts when swapping between them
     
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    Please troll elsewhere. Like I already said, I was wondering if there were specs. Go measure ten "standard" picatinny rails all from different companies and tell me if they are all the same, and all match the 1913 standard drawing.

    Amen. What a cluster. Have a couple high quality rifles or rails and have to adjust bipod or night optic mounts when swapping between them

    I even found this on 2 different lengths of fore end pic rails for my Atlas with QD mount, both rails from B&T…..
    Built solely to clamp bipods to. 🤷🏿
     
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