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Thread: Will using a grinder to sharpen a Kukri or any knife lessen its hardening?

  1. #1
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    Question Will using a grinder to sharpen a Kukri or any knife lessen its hardening?


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    Obviously when a knife maker hardens a blade, they use heat and quenching to make it harder. Well, if you put your blade on a power grinder to sharpen or repair it, then the friction caused by the grinding will heat up the blade, and might that lessen the hardening, make it softer or make it more brittle?

    I dont plan on doing that, but I've certainly seen people use grinders to sharpen or repair larger blades, and even some smaller ones. I guess if you really put a dent or take a chunk out of your blade, you might be forced to use a grinder, but will that have an effect on hardness?

    How do YOU sharpen your Kukris and your other knives?

  2. #2
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    In short, yes, a grinder, belt sander etc.... can tend to anneal a blade if it is not done carefully and the blade kept cool.
    If you are grinding on a blade and you see colors or black (burned) you have messed with the heat treat.
    Myself.... i never like to let my blades get hotter than i can hold in my hand.
    Last edited by Karda; 07-03-2012 at 03:16 PM.
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  3. #3
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    When the blade gets hot just dip it in some water.

  4. #4
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    Quote Originally Posted by KyleGrantham View Post
    When the blade gets hot just dip it in some water.
    Too late. Now if you had some sort of slow speed water cooled set up that would be a different story but if it gets hot, its too late.

  5. #5
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    As usual, Karda has the answer: too hot to handle = messing with the heat treat. The thinner the metal, the faster that will happen. Particular care needs to be excercised with the point of kukri as the rest of the blade has a lot of mass and won't heat up as fast as the thinner metal at the tip.

  6. #6
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    You can completely change the primary grind on a blade without affecting the temper if you know what you're doing. Hell, some knife makers don't even do the primary grinds until after HT. I keep the thumb of my support hand on the edge at the opposite side of the blade when using a grinder. If you move at a consistent speed (I do about a belt-width per second) and dunk in water every four or five passes, you'll hardly ever get to the point where your thumb feels much heat at all. Heat builds faster with finer belts, so always finish reprofiling with coarse, and save the fine stuff for finishing/smoothing.

    It's like most things---if you know what you're doing, it's fine.

  7. #7
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    Quote Originally Posted by bemo View Post
    As usual, Karda has the answer: too hot to handle = messing with the heat treat. The thinner the metal, the faster that will happen. Particular care needs to be excercised with the point of kukri as the rest of the blade has a lot of mass and won't heat up as fast as the thinner metal at the tip.
    Well, it takes quite a bit more than too hot to handle, but if you're working on it and it's too hot to handle...whats the point?
    Not only can't you work on it, but you could anneal your edge, especially with khukuri where the edge tends to heat up alot faster and the mass of the blade holds the heat. I always advocate and practice caution.
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  8. #8
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    As mentioned above, you can use a wet grinder (preferably with a SiC wheel) for reprofiling. For sharpening, paper wheels on a regular (3450 RPM) bench grinder motor can be used quite effectively.

    But regular hard abrasive wheels on a high speed grinder? No, that'll ruin the heat treat faster than your reaction time.

  9. #9
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    I was talking about a belt sander. Dry, stone wheels on tempered steel is definitely bad juju.

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