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Thread: Modified Sear Fixture

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  1. #11
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    Quote Originally Posted by CAWalter View Post
    Because the secondry angle is so sharp the sear has rotated into the hammer hook deeper, increasing the engagement. Hammer hooks of .025" is fine for a long trigger pull, but .018" recommended for a crisper break , with .008" engagement.
    The .025" hammer hook is generous, and not what one would want for a target trigger. The sear tip is also completely captured within the sear hooks (i.e., the front edge of the sear primary surface is behind the tip of the hammer hook; the hammer hooks overhang the sear). That also adds some weight to the trigger pull. For this first gun, I am looking for a safe 5 lb trigger. Although the engagement is large (.020"), there is very little noticable creep.

    I am not settled on a final sear geometry. So I don't want to adjust the hook depth any lower yet.


    Quote Originally Posted by CAWalter View Post
    Guaranteed the hammer cocks further when the hammer hooks pass the the sear. When going to 87 Degrees you have no choice but to further cock the hammer as the spot the hammer hook land on is lower than the the escape edge.
    Not sure why the hammer would have to cock further. The escape edge hasn't changed from that of a 90 degree primary. The tips of the hammer hooks do not have to move up any further to clear the rear edge (the escape edge) of the 87 degree sear. And the front edge of the 87 degree sear is at the same height as the escape edge. Both front and rear (escape) edges are the same distance from the sear axis.


    Quote Originally Posted by CAWalter View Post
    The sear primary when cut flat can not change angles as it rotates, it is the same as at rest, the effects show when pulled.
    Yes, the real change is in the engagement polarity as the sear rotates. With a 90 degree .030" sear face (no secondary relief cut), the engagement polarity is negative at the beginning of disengagement, and drops to neutral at the end of disengagement. When a relief angle is added, the sear escapes before ever quite reaching neutral.

    With my 87 degree sear face, the engagement polarity is very slightly negative at the beginning of disengagement, and very slightly positive at the end of disengagement.


    Quote Originally Posted by CAWalter View Post
    For neutral a true radius primary cut based on the sear pivot center is the only way to geometrically have a neutral engagement.
    I did only say that the 87 degree primary approximates a true radius sear. And it's also not the real purpose of the 87 degree primary. It's mainly to reduce the negative engagement that is characteristic of the 90 degree primary.


    Quote Originally Posted by CAWalter View Post
    However as long as the hammer hook tips land on the face of either a traditional cut sear, or a TR cut the release is a crisp snap.
    Would that be true for any viable primary angle that never went positive during disengagement?

    It is a characteristic of both the 90 degree sear and TR sear that neither ever go positive. I'm also going to try refining the angle of the "87" sear so it drops to neutral right at the escape edge.

    -
    Last edited by megafiddle; 3rd February 2021 at 18:46.


  2. #12
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    Quote Originally Posted by megafiddle View Post
    Not sure why the hammer would have to cock further. The escape edge hasn't changed from that of a 90 degree primary. The tips of the hammer hooks do not have to move up any further to clear the rear edge (the escape edge) of the 87 degree sear. And the front edge of the 87 degree sear is at the same height as the escape edge. Both front and rear (escape) edges are the same distance from the sear axis.-
    It will cock further if the hammer hook tips overhang, because the hook tip face plane is not on the hammer pivot enter. So the tip is below the sear. If the tips land say in the center front to back of the sear they will cock further as the center of the primary plane is closer to the pivot as you have changed the geometry from a right triangle to an isosceles triangle.

    Pythagoras theorem, is easily used here. a2+b2=c2, a can be the distance from the pivot center .404" for instance and b is then the distance from the escape edge to where the hammer hooks land, say .012", considering the sear is .030"and with shorter hammer hooks is actually a lot. So that means c is the distance from the point the hook tips land on to the pivot center. So, c is .404178178, which means the hammer hook tips are landing on a spot that is .000178178" higher than the escape edge.


    Quote Originally Posted by megafiddle View Post
    Yes, the real change is in the engagement polarity as the sear rotates. With a 90 degree .030" sear face (no secondary relief cut), the engagement polarity is negative at the beginning of disengagement, and drops to neutral at the end of disengagement. When a relief angle is added, the sear escapes before ever quite reaching neutral.-
    Not really relative as if the hook tips are on the escape edge it can only be neutral as the escape edge and spot the hooks are on is one in the same. Negative will always be negative and positive will always be positive. Note the hammer hook face plane is not in line with the hammer pivot, the hook tip is comparable to a claw.
    Quote Originally Posted by megafiddle View Post
    With my 87 degree sear face, the engagement polarity is very slightly negative at the beginning of disengagement, and very slightly positive at the end of disengagement. -
    No it is positive, vice the frame and use a dial test indicator on the cocked hammer and you will see as soon as all the tolerances are used up the hammer further cocks, its just the geometry of this arrangement.


    CAW
    “If it ain't broke, don't fix it' is the slogan of the complacent, the arrogant or the scared. It's an excuse for inaction, a call to non-arms.” Colin Powell

  3. #13
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    Quote Originally Posted by CAWalter View Post
    It will cock further if the hammer hook tips overhang, because the hook tip face plane is not on the hammer pivot enter. So the tip is below the sear.
    Ok, but that's different then. It's not dependent of the 87 degree primary face angle itself. That's what I was responding to.

    I should mention that the geometry I'm currently using is like that in the first microphoto in this topic. There is practically no overhang. With some overhang, as shown in the second and third micrographs, the camming force required to move the front edge of the sear out from under the hammer hook tips is about 1 lb. This is for 90 degree hammer hooks. For ordnance 86 degree hammer hooks this camming force is about 1.5 lb.


    Quote Originally Posted by CAWalter View Post
    If the tips land say in the center front to back of the sear they will cock further as the center of the primary plane is closer to the pivot as you have changed the geometry from a right triangle to an isosceles triangle.
    Yes, but this geometry is intended utilize the entire length of the primary surface. I would not want this geometry if the hammer hook tips actually landed in the middle.


    Quote Originally Posted by CAWalter View Post
    Pythagoras theorem, is easily used here. a2+b2=c2, a can be the distance from the pivot center .404" for instance and b is then the distance from the escape edge to where the hammer hooks land, say .012", considering the sear is .030"and with shorter hammer hooks is actually a lot. So that means c is the distance from the point the hook tips land on to the pivot center. So, c is .404178178, which means the hammer hook tips are landing on a spot that is .000178178" higher than the escape edge.
    Looks correct. Pythagoras theorem is exactly what I used to determine my values.

    There are also camming forces involved as the sear primary surface travels under the hammer hook tips. Moving from the front edge of the sear face towards the center, on the 87 degree sear, the camming force is about -.25 lb. This assists in disengagement; the hammer hooks drop slightly; the engagement is negative. At the last bit of movement towards escape, the camming force is about .25 lb. The hammer hooks rise slightly; the engagement is positive.

    Those camming forces are just the reactive component, and due to spring tension. There is a friction component present also on top of that. The camming force was calculated over a sear face travel of .001". This is a reasonably small delta for the x dimension of the dy/dx camming slope.


    Quote Originally Posted by CAWalter View Post
    Not really relative as if the hook tips are on the escape edge it can only be neutral as the escape edge and spot the hooks are on is one in the same.
    That's true at the actual point of escapement, but the sear has to travel some distance to arrive there, even if it's just the last .001" or .0001" of travel. The engagement is in the travel of the sear, not at a single point. This is in reference to the 87 degree sear though, where the degree of engagement is maximum at the front and rear sear face edges.

    For a conventional 90 degree .030" sear face, the engagement does drop to neutral at the point of escape. But the overall engagement is still negative.


    Quote Originally Posted by CAWalter View Post
    Negative will always be negative and positive will always be positive.
    Not for the 87 degree sear. The sear face is the base of an isosceles triangle; the slope is opposite in polarity on either side of center. If the slope changes polirity, the engagement changes polarity.


    Quote Originally Posted by CAWalter View Post
    Note the hammer hook face plane is not in line with the hammer pivot, the hook tip is comparable to a claw.
    Yes, the controling point of the hammer is an edge at the hammer hook tips. The hammer hook face is only significant if there is hammer hook overhang.


    Quote Originally Posted by CAWalter View Post
    No it is positive, vice the frame and use a dial test indicator on the cocked hammer and you will see as soon as all the tolerances are used up the hammer further cocks, its just the geometry of this arrangement.
    If you are referring to hammer hook overhang, I agree.

    -

  4. #14
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    Quote Originally Posted by megafiddle View Post
    Ok, but that's different then. It's not dependent of the 87 degree primary face angle itself. That's what I was responding to.

    I should mention that the geometry I'm currently using is like that in the first microphoto in this topic. There is practically no overhang. With some overhang, as shown in the second and third micrographs, the camming force required to move the front edge of the sear out from under the hammer hook tips is about 1 lb. This is for 90 degree hammer hooks. For ordnance 86 degree hammer hooks this camming force is about 1.5 lb.




    Yes, but this geometry is intended utilize the entire length of the primary surface. I would not want this geometry if the hammer hook tips actually landed in the middle.




    Looks correct. Pythagoras theorem is exactly what I used to determine my values.

    There are also camming forces involved as the sear primary surface travels under the hammer hook tips. Moving from the front edge of the sear face towards the center, on the 87 degree sear, the camming force is about -.25 lb. This assists in disengagement; the hammer hooks drop slightly; the engagement is negative. At the last bit of movement towards escape, the camming force is about .25 lb. The hammer hooks rise slightly; the engagement is positive.

    Those camming forces are just the reactive component, and due to spring tension. There is a friction component present also on top of that. The camming force was calculated over a sear face travel of .001". This is a reasonably small delta for the x dimension of the dy/dx camming slope.




    That's true at the actual point of escapement, but the sear has to travel some distance to arrive there, even if it's just the last .001" or .0001" of travel. The engagement is in the travel of the sear, not at a single point. This is in reference to the 87 degree sear though, where the degree of engagement is maximum at the front and rear sear face edges.

    For a conventional 90 degree .030" sear face, the engagement does drop to neutral at the point of escape. But the overall engagement is still negative.




    Not for the 87 degree sear. The sear face is the base of an isosceles triangle; the slope is opposite in polarity on either side of center. If the slope changes polirity, the engagement changes polarity.




    Yes, the controling point of the hammer is an edge at the hammer hook tips. The hammer hook face is only significant if there is hammer hook overhang.
    -
    Hammer hook tips should have a slight radius, and the angle that the hook tip comes at the sear face makes a difference.

    But I must regress, I had overlooked your desire to obtain a safe #5 trigger, And I'm thinking #1.5-#2, where these points are much more important. A safe #5 trigger is easily obtainable. An unaltered sear from any reputable dealer and hammer along with a new sear spring will provide you with your search.

    CAW
    “If it ain't broke, don't fix it' is the slogan of the complacent, the arrogant or the scared. It's an excuse for inaction, a call to non-arms.” Colin Powell

  5. #15
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    Thanks. All of that makes more sense now.

    I'm working on a couple guns that will have 5lb triggers. I like the Caspian sears that I'm using. There's enough material at the sear tip to allow recutting, and they have been trouble free in all other respects. I'm also using Colt 1991 hammers (just a style preference) which have some pretty rough hammer hooks, so I just mill the hooks for a clean 90 degree inside corner.

    I'm also working on a target gun, which will have a finer trigger. The work on these 5lb triggers might be overkill, but it's a good way to develop the tools and techniques needed for accurate trigger work. I expect it to pay off on the target trigger.

    -

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