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Thread: Modified Sear Fixture

  1. #1
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    Modified Sear Fixture

    The Brownells sear fixture works well for its intended purpose, which is basically cutting a 90 degree primary surface, or trueing up an existing surface. The fixture is supplied with a .020" shim for spacing a cutting stone up off the surface of the fixture. An adjusting screw for positioning the sear is preset to hold the sear at the proper angle for stoning.

    I wanted the ability to precisely set the sear primary face at an angle other than the standard 90 degrees. The sear can be positioned at any angle with the adjusting screw, except that on mine, the screw was frozen in position with a thread locking compound. Alternately, different shims could be used, along with some calculations. With a lot of heat, I was able to loosen the adjusting screw so I could remove it and clean off the compound. I could now adjust the sear within the fixture, but the setscrew is only accessible while the sear is rotated out of position. So it was trial and error to get the sear located properly. Another problem was that the sear was free to flop around within the fixture, as there was nothing to hold it against the adjustment screw (you have to hold the sear during use).

    So I made some modifications.To provide solid working material for the new components, I flipped the fixture over to use the opposite side. I cleaned up the working surfaces to make them flat and square. A new hole was drilled and tapped for a socket head cap screw. The screw head diameter had to be turned down a bit, but still provides better bearing surface than the original adjusting screw, which was just a setscrew. The angle and location of the new adjusting screw allows access while the sear is in position. To maintain the sear in position against the adjusting screw, I added a tiny spring loaded bar to provide counter tension.

    With the fixture working surface dimensions and sear pin center determined, the sear primary angle can be precisely set by a simple measurement between the sear rear edge and fixture. To make it even simpler, I made a brass gauge with two machined steps on it. One step locates the sear for a 90 degree primary. The other step locates the sear for an 87 degree* primary that I am experimenting with. The 87 degree primary face, if correctly centered over the sear axis, can approximate a true radius sear face to within .00012" (+/- .00006") over a primary face length of .020". This is as close to neutral as one can get with a flat primary face.

    The sear is positioned so that the target primary surface will be parallel to the top surface of the fixture. For the standard 90 degree face, this can be simply done using the gauge. Then, to keep the cut parallel to the fixture top, a shim is selected that's equal to the sear protrusion above the fixture surface.

    The 45 degree secondary surface was cut using a .125" shim. The Brownells instructions describe using a .020" shim, which is much too thin to provde the 45 degree secondary angle.

    *actually 87.17 degrees, based on a .030" sear face.


    These two CAD drawings show the original and modified fixture designs. All work was done on a LMS micro mill.

    sear_fixture_original.jpg

    sear_fixture_modified.jpg


    These three photos show the separate components, and the sear in position for stoning.

    sear_fixture_mod_01.jpg

    sear_fixture_mod_02.jpg

    sear_fixture_mod_03.jpg

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    Likes (2) :
    John (26th November 2020), Rick McC. (26th November 2020)


  2. #2
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    This is the sear and hammer engagement with the finished sear. The hammer hooks were cut to 90 degrees and .025".

    9mm_sear_hammer_01.jpg

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  3. #3
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    Quote Originally Posted by megafiddle View Post
    One step locates the sear for a 90 degree primary. The other step locates the sear for an 87 degree* primary that I am experimenting with. The 87 degree primary face, if correctly centered over the sear axis, can approximate a true radius sear face to within .00012" (+/- .00006") over a primary face length of .020". This is as close to neutral as one can get with a flat primary face.
    Those are some ingenious modifications, and I'm awed at the work involved both in figuring out all the angles involved and the work in making the modifications to the jig. Beyond that, however, I have to respectfully disagree that your 87 degree face angle (or any face angle other than 90 degrees) in any way approximates a true radius sear face.

    With a true radius sear face/tip, assuming a 90-degree hammer hook, the contact angle between the hammer hook and the sear tip is always 90 degrees -- a true perpendicular. That means there is no force vector that contributes (or can contribute) to the sear tip sliding off the hammer hooks. Once you hone the sear tip to a negative angle, the sear tip essentially and effectively becomes a ramp for the outside corner of the hammer hooks to slide on. This is illustrated by your close-up photo. While this might be tolerable in a pistol dedicated to bullseye shooting, IMHO it's dangerous and not a condition I would want in a pistol used or carried for self-defense. Since I don't shoot bullseye and I regard all of my firearms as potential self-defense weapons, with all due respect I would not want that sear in one of my pistols.
    Hawkmoon
    On a good day, can hit the broad side of a barn ... from the inside
    Last edited by Hawkmoon; 26th November 2020 at 15:40. Reason: typo


  4. #4
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    Thank you for sharing your work, and the great pics!
    "Sights are for the unenlightened."

    Rick

    IDPA Certified Safety Officer

  5. #5
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    Quote Originally Posted by Hawkmoon View Post
    Those are some ingenious modifications, and I'm awed at the work involved both in figuring out all the angles involved and the work in making the modifications to the jig. Beyond that, however, I have to respectfully disagree that your 87 degree face angle (or any face angle other than 90 degrees) in any way approximates a true radius sear face.

    With a true radius sear face/tip, assuming a 90-degree hammer hook, the contact angle between the hammer hook and the sear tip is always 90 degrees -- a true perpendicular. That means there is no force vector that contributes (or can contribute) to the sear tip sliding off the hammer hooks. Once you hone the sear tip to a negative angle, the sear tip essentially and effectively becomes a ramp for the outside corner of the hammer hooks to slide on. This is illustrated by your close-up photo. While this might be tolerable in a pistol dedicated to bullseye shooting, IMHO it's dangerous and not a condition I would want in a pistol used or carried for self-defense. Since I don't shoot bullseye and I regard all of my firearms as potential self-defense weapons, with all due respect I would not want that sear in one of my pistols.
    First, thanks for the compliments!

    You would be absolutely correct if the engagement was as negative as it appears.
    But it's actually just the opposite.

    The standard 90 degree sear is slightly negative. In a photo, 90 degrees would look even worse (more dangerous) than my 87 degree sear above. The sear axis is not directly below the sear tip in the photo, but is far to the left. So the sear tip moves left and up while disengaging, not straight towards the left. And without knowing the actual sear axis location in the photo, it's impossible to judge the engagement "polarity".

    The 87 degree face is less negative than 90. In fact, once the center of the primary face has reached the tip of the hammer hooks, the remainder of the disengagement is very slightly positive. If anything, 87 degrees is safer than 90. That is the purpose of my experimenting with it.

    True, a flat primary cannot duplicate a true radius sear. But it can approximate it to a high degree. If you draw a .405" radius arc through the 87 degree .020" flat surface for a "closest fit", the flat will deviate from the arc by only +/- .00006". Or a total deviation of .00012". This means that the hammer hooks will drop by .00012" during the first half of sear face disengagement travel, and then rise by .00012" during the second half.

    The real question is, how does that small error from true radius affect trigger pull? The error might be insignificant if friction is the dominant factor.

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  6. #6
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    Quote Originally Posted by Rick McC. View Post
    Thank you for sharing your work, and the great pics!
    Thanks!

    I always had trouble taking photos, especially with lighting.

    Things improved when I recently discovered that my camera had a built in flash.

    The micrograph was made with a very low cost USB microscope. Once again, lighting was a problem. I combined top lighing and substage lighting (backlight).

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    Likes (1) :
    Rick McC. (27th November 2020)


  7. #7
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    Excellent work!

  8. #8
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    Quote Originally Posted by slohunter View Post
    Excellent work!
    Thanks!

    With range shooting less accessible and more expensive, it's a good time to work on things like this.

    -

  9. #9
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    For comparison, these two micrographs show the difference between 90 and 88 degree primary sear faces. An 88 degree sear primary is a bit less approximate to a true radius sear than the 87 degree; but it is still much closer to neutral (less negative) than the 90 sear primary.

    This sear that has been cut to 88 degrees:

    Cmdr_Sear_87_45_Hammer_90_025.jpg


    This sear has been supplied precut to 90 degrees and is setting in the same hammer:

    Sear_90_Hammer_90_025.jpg

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