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Thread: "Truing" an AR's receiver face?

  1. #101
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    Quote Originally Posted by BSmith View Post
    Please don't take that as me questioning you. There's no dispute that they are machined that way.

    It also amazing that, whoever in the other thread, is making lowers that are as off center as they are. But at least that's different setups.
    Do what you like. Different strokes for different folks

    I have found lapping the face of the receiver has several benefits and no detrimental effects. Sure you could do it better with a lathe but do you really need to.

    Then I glue my barrel extensions in with blue locktite. Does make a difference.

    Many upper builders follow the same procedures and build some accurate AR's

    If you were building a cheap SHTF gun doesn't really matter but if you want an MOA gun every little bit helps

  2. #102
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    Sounds like more minutia than minute of angle.
    "A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects."
    — Robert Heinlein

  3. #103
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    Put it on a hydra lock and skim the face with a grinder.

  4. #104
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    I will see if Bryan is willing to fixture a test run on the CMM comparing one of our uppers before (Out of our machine) and after (Running the lapping tool) and let the Zeiss decide.

    Ryan
    Founder and co-owner/operator at 2A-Armament

  5. #105
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    Aluminum doesn't grind well, unless something has changed.
    "A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects."
    — Robert Heinlein

  6. #106
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    Ground aluminum, magnesium, and titanium for years on helicopter transmissions.

  7. #107
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    Quote Originally Posted by BSmith View Post
    No, we do mold work. But if you are machining aluminum .002" out of square over an inch when the machine is square within 80 millionths over the entire travel... With it being done on the same op, you should be a LOT more square than .002 over 1".

    I know, there's a tolerance, and if you hit nominal within a couple tenths (ten thousandths of an inch) every time when the part only needs +-.002 then you're wasting time. But, this is a squareness issue on a setup that is inherently square.

    FWIW, I can indicate a 15" part that was hard milled (52-54 HRC), and the perpendicular side is cut 4" down and see no more than .00003" total movement on the indicator and not uncommon to see it within .0001". All that is cut in one op. Deck the top, mill the sides.
    Ok I gotcha now. That kind of variation from what is expected is indicative of possible equipment/tooling issues.

    I work with welding robots. Our issues are almost always work piece driven, since we are dealing with finished component parts.

    I'm learning something from you guys.


    Sent from my iPhone using Tapatalk

  8. #108
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    Sorry I didn't intend to make our Blue Loctite process sound bad, I use it myself. I don't think using it as a giant gap filler on a sloppy mating is something I would encourage.
    However based on what I'm reading above the looser fitting build could benefit more from lapping over the highly sought tight fit? .002-.003 of center equates to a significant amount at the muzzle and cross hairs.
    Truing the upper could then help bring things closer to center than just doing nothing. Ryan may have a good point in a fixture to check overall point to point measurements as a baseline, lap then recheck? Putting an assembly between centers could help answer the overall question I think.
    Last edited by smihtp; 01-27-16 at 13:00.

  9. #109
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    I just ordered the Brownells tool. I have a Bushmaster flat top that I use on a blaster gun. This weekend I put another barrel in the upper and noticed that it too required a lot of windage to zero. I'm hoping that truing up this turd will fix this.
    "You people have too much time on your hands." - scottryan

  10. #110
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    Quote Originally Posted by markm View Post
    I just ordered the Brownells tool. I have a Bushmaster flat top that I use on a blaster gun. This weekend I put another barrel in the upper and noticed that it too required a lot of windage to zero. I'm hoping that truing up this turd will fix this.
    I'll be curious to hear the results.
    Steve

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