The welding of the limb irons went well.  No porosity and plenty of fillet on the inside.   Everything has been well stress  relieved to prevent  cracks forming from any of that pesky old hydrogen embrittlement.

The weight on these irons is coming in at 6 1/2 lbs each.  That is 1 1/2 lbs heavier than the previous set, but much of that weight has been shifted back in towards the fulcrum.  The main thing is that they will be many times stronger than our previous attempt.  I have acquired a reliable little heat treat thermometer that will make all the difference when it comes to hardening them.  I had thought to send them out for heat treat but shied away when I heard the cost.

I am contemplating some lightening cuts that would further reduce the weight by almost a pound.  Because these cuts can be executed after the limb is completed,  it seems reasonable to first test the limbs in this slightly heavier configuration just to see how much velocity is gained when we shed a pound at the base of the limb, later on.    I’m guessing  no more than 30 or 40 feet per second.  We’ll see.

2 Responses to “Embrittlement verboten.”

  1. Captn harpoon says:

    The new irons are looking good Nick. An old adage comes to mind tho, “if it aint broke,don’t fix it.”

    If you would like to shed the effects of the extra 1.5 lbs. mass at the fulcrom, an alternative might be to shed a few ounces toward the tip or at the tip by simply removing one inch.

    If you review “the little experiment” blog entry, fps loss was very minimal if any as moving the string back a few inches effectively added the weight of the extra wood to projectile weight.

    To save a little time, I thought perhaps you might also make one extra arm, and use that arm to find the sheering point of the design. The arm might withstand 4500 or 6500lbs of force. A simple test would tell you real quick.
    – since most of the stress will be toward the fulcrum or at the fulcrum point, you should not even have to shape the wood.

    It would be really great if you could make a second set of limbs only a few inches shorter, and re-test my shorter limb theory.

    In any case if both the irons and the arms make it to 5000lbs pull, I would think that 425-465 fps will be attained, with a max range of about 3000 feet.

    If you get to fine tuning for a few more fps, then perhaps consider a new custom string.

    Good luck with testing.

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