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Thread: Stopping Barrel Fall At Linkdown

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  1. #81
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    25th September 2006
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    Vis

    Hi GBW: Now you got me to thinking about my use of the term "VIS". My (only) 1911 is a WWII-vintage GI model that has a distinct "bow tie" which extends about 0.010 inch forward of the flat bottom of the hole below it. My receiver "blueprints" are fuzzy copies of scans of original Ordnance Dept ones. They show this bow tie, but I can't decipher how far (if at all) it's supposed to protrude forward of the surface below. One place seems to show it 1/16, but another place on the same print seems to show it as ".005 -.005", implying zero protrusion (a flush surface) is allowable.

    I recall reading somewhere in this forum that some guns don't have a bow tie, so for these I guess the VIS extends from the bed down to the bottom of the recess for the barrel lower lugs, a vertical distance of about 1/2 inch.

    Anyway, the bow tie is what I refer to as the VIS. I always thought the area below it was recessed a bit to avoid the possibility lower lug contact too far down.

  2. #82
    Join Date
    16th July 2005
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    Hi N: I confess I don't know either. To me the VIS is the entire surface, but you have a point. And with your definition, the feet hitting the vis would be pretty much a non-issue, provided a dead flat end mill cut for the lower portion. (There is a set of receiver drawings on one of the CNC gunsmithing sites, I think, but they may be the ones you already have.)

    My Caspian frames have a bow tie cut and the Essex don't. Not sure about commercial Colts - I have 3 older S70s, I'll check them tomorrow. I don't have any newer Colts or other manufacturers. As for the originals, I'm just not sure.

    In any event, only the uppermost part of the lower lug should hit the vis to minimize rotational shock on the lower lug. Clearly the bowtie, when it exists, will insure this.

    Also, the bow tie cut imposes a limit on the contact area - whether that's significant or not, and whether that smaller area is subject to setback by peening over hard use, which could change the timing somewhat, are also questions.

  3. #83
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    25th September 2006
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    Barrel/Slide Clearance

    In Post #80, this thread, GBW wondered if it was possible to obtain the 0.015 inch minimum clearance between the barrel and slide when doing the Schuemann timing tests on a mid-spec M1911. For this test, the slide is about .25 inch out of battery, the barrel pushed fully rearward and the barrel aft forced upward to place the link under tension.

    Dimensions and tolerances from Army Ordnance (Springfield Armory and Rock Island Arsenal) blueprints were used to make a series of calculations leading to the barrel/slide clearance under these conditions. The only assumptions made were:the barrel axis was parallel to the receiver top; the barrel link pin was a press fit with no change in its diameter; the tolerance for the .278 standard link was +/- 0.001 inch.

    The clearance worked out mathematically (rounded to 3 places past the decimal point) to be 0.010 +/- 0.020 inch. Mid-spec clearance twixt barrel top and slide is thus 0.010 inch, which is 0.005 less than the minimum of 0.015 inch specified in the Schuemann timing tests.

    Note that the minimum-spec clearance works out to be -0.010 inch. This implies the barrel upper radial lugs are that far away from disengaging from the slide. However, because the math involved about 16 different individual specifications (dimension and tolerance) among 6 different gun parts, the chances of any gun having a tolerance stacking problem like this are probably quite slim.

    The chances of Niemi24s making a misteak are, however, not quite as slim. Went over the stuff twice, but it sure would be nice if somebody'd volunteer to independently check my work. All that's needed is a set of blueprints & some algebra and trigonometry skills. Any takers?????
    Last edited by niemi24s; 26th December 2006 at 19:56. Reason: Correct typo


  4. #84
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    Finally Got It!! [I Think]

    After 3 weeks of digesting all the sage info in this thread and connecting it with Schuemann's timing test instructions (barrel fully rearward and AFT END UP) the little light finally came on in my head.

    In Post #1 I said a mid-spec GI 1911 would have it's downward motion halted by the bed. Tuner said that's OK as long as first contact is made with the VIS (which halts rearward motion). The key seems to be that the link is in tension when the barrel (its feet or chamber bottom) makes first contact with the frame - like Schuemann says. Ahhhhh, honorable masters, grasshopper finally get big picture.

    So, how far is the barrel bottom above the receiver bed when VIS contact is first made in a mid-spec USGI 1911? After getting reliable specs for the link, it worked out mathematically to a tad more than 0.006 inch at mid-spec. With rearward motion first halted by the VIS (with the link in tension), the 0.010 inch slack in the link system then allows link tension to be relieved as the barrel drops straight down 0.006 inch to the bed, leaving the link loose (and 0.004 inch away from being in compression). All this in a mid spec gun.

    Oddly, the max-spec distance was almost 0.027 inch which seems (to me) too much to even allow the barrel to unlock from the slide!

    Odder yet, the min-spec distance was a tad more than -0.010 inch. That's negative 0.010, indicating the barrel's mathematically below the bed - a physical impossibility. This must mean the barrel hits the bed first, while the link is in tension, and the feet would never touch the VIS (until the link breaks)!!

    Encountering either the max- or min-spec situation would require about a dozen dimensions on 5 parts all at one tolerance extreme or the other. IMHO, the probability of this occuring is about the square root of zero point diddly squat.

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