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Thread: Smoothbore project - repro cast sear spring shaping/tempering

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    Tom Magno, 9269V's Avatar
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    Question Smoothbore project - repro cast sear spring shaping/tempering

    My original H&P shoots very well, but has a horrendous trigger pull. I have managed to get the pull to a barely useable 7 pounds by adding a shim to the tumbler. I want to get the pull down to a competition value somewhere in the 3.5-5 lb range. Next step was to shorten the sear spring. But rather than touch the original sear spring, I ordered a repro - but it came as-cast, so needs to be fitted/shaped.

    My question for you metal workers out there - please check my procedure - I am rank amateur at metal working, so would like advice to ensure I have it correct before I begin. Did some basic Google research -

    1. Annealing - I assume I need to anneal it before I begin to shape it or it will be too hard to file? Do I do this by heating cherry red and letting air cool? Or can I start shaping it as-cast?
    2. Shaping - using a file to get it to fit the lock, ensuring final shaping using a fine/draw file to remove filing marks, keeping strokes always lengthwise to the spring.
    3. Hardening - heat cherry red, then quench in oil.
    4. Tempering - heat to blue/purple, then quench in oil.

    Comments/advice appreciated.
    Tom Magno, 9269V
    29th Wisconsin Volunteer Infantry

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    When I have to shape already heat-treated items, I try to shape them without changing the existing heat treat if possible, so that I don't damage the current heat treat nor have to re-heat treat it.

    You may be able to grind the spring as is if you keep it cool with water as you go. You might consider using a bench grinder or a dremel tool with a cut-off wheel depending on what you are trying to do. It should remove material without the need to anneal the item. Just keep it wet and cool as you go. Remember - if it's too hot to hold, it's too hot. And if it starts to change color on you you are definitely screwing up the heat treat.

    Steve

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    John Bly is offline
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    Shortening the sear spring is the wrong approach. The spring needs to bear on the sear as close to the pivot point as possible. The best way is to reduce the thickness of the spring by careful grinding. Re-heat treating is best left to the experts. The trigger pull on just the sear with no tension from the mainspring should be around 2 lbs or slightly less. Check this with the hammer held to full cock manually with no tension on the sear. Once this is achieved then work on the sear and tumbler surfaces. Work slowly and be patient.

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    Greg Ogdan 110th OVI is offline
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    Tom,

    Best to do as Steve says if you can. BTS, I have been able to file several sear springs. If you go too far and wish to try and salvage the part, you have everything correct up to the last step. You do not want to temper as such what you want to do is draw back some of the hard. To do that, I bring my lead pot up to 800* and drop in the spring. Let it stay in until it is up to temp then remove and let cool SLOWLY in air.


    Greg

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    Rick Ramey is offline
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    Sear spring

    I have had good results by narrowing the springs..slowly remove metal from the sides of the spring with a dremel tool, dipping in water often to keep cool...
    Rick Ramey

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    Eggman's Avatar
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    Is it possible to change the temper of a spring without turning the part red or some other color?
    My Enfield fell below the 3 lb limit --- I raised the trigger pull to 3 1/2 to 4 by running a fine file over the end of the sear spring a few times. This gave a microscopic change in geometry that achieved the higher trigger pull.

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    Is it possible to change the temper of a spring without turning the part red or some other color?
    Absolutely. I make medieval armour. I sometimes use 1075 (though I have switched to 4130 as it is more forgiving) high carbon steel.

    To harden them, I heat them to bright yellow (above its critical temperature) in a furnace, and then quench them in water. This makes them very hard but also so brittle. So brittle that you can drop the pieces on a concrete floor and they will shatter like glass.

    Tempering will happen between 400-800F. I was getting the best compromise between hardness and strength at around 800F. But depending on the temperature you temper at you will get a different ratio between toughness and hardness.

    Gotta clarify "color" though for a minute.

    There are two kinds of colors that you can use when heat treating "by eye". First there is the critical temperature color. This is when the metal is actually emitting light (glowing). This goes from dull red to brilliant white.

    The second kind of color is the oxide color. If you take a piece of steel and polish it bright and heat it up in the presence of air, it will start to change color. This is the result of oxide layers forming on the outside of the metal. These colors start from a light straw color and progress through blue to black. You can use these colors to approximate the temperature of the item during tempering.

    So if you're grinding on a spring and it start to change color (not glowing, though), you are already too hot to hold and you are moving quickly into tempering temperatures that may affect the performance of the spring.

    Steve

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    Eggman's Avatar
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    So what I hear you saying Steve is that the metal will ALWAYS start changing color when getting too hot. If there is NO color change at all, nothing, nada, that means that so far nothing in particular is happening hardening wise so far right? I've done a lot of spring grinding and cannot ever recall seeing a color change while doing it (cautiously). Have never dipped in water while doing this. Am I beating this subject to death?????????

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    So what I hear you saying Steve is that the metal will ALWAYS start changing color when getting too hot. If there is NO color change at all, nothing, nada, that means that so far nothing in particular is happening hardening wise so far right?
    Generally speaking, yes. But remember, in order to see the color change you have to have bright polished metal that is free to oxidize. If it's oily or already oxidized you may not see any color change.

    This is very easy to see - go take a nail out of your shop and sand it bright with some sand paper. Then hold it with some pliers and grind on it with your bench grinder or stick it in a torch flame or whatever. As it heats up you will see a rainbow appear on the surface of the metal right before your eyes.



    I've done a lot of spring grinding and cannot ever recall seeing a color change while doing it (cautiously). Have never dipped in water while doing this.
    If you were holding it with your fingers you'd know if you were getting hot enough for oxide colors to form.

    Steve

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    I'll also point out here that in the case of grinding, you may only elevate the temperatures localized to the area of grinding - not necessarily the whole piece. Thus you are "drawing out the temper" in a localized spot. This is generally (but not always) a bad thing. Typically you want a uniform heat treatment.

    But there are cases when you specifically intend to heat treat different parts of an object differently. For example, with Japanese swords, part of the blade was coated in clay to insulate it and so cause it to take a different heat treatment than the sharp part of the blade. This way the edge was hard (and brittle) while the spine stayed softer (and tougher).

    Metallurgy is fascinating. I personally believe that the ability of steel to be both hard, allowing us to work soft steel, and soft, allowing it to be worked, is one of the fundamental contributions to the advancement of mankind's technology almost on par with fire.

    Steve

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