Attn genius bar: Mac repair job

Discussion in 'Sharpening Station' started by ian, Dec 31, 2018.

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  1. Dec 31, 2018 #1

    ian

    ian

    ian

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    Hi everyone,

    I'm trying to fix this MAC 8'' chef knife with dimples, the one labeled as `molybdenum steel'. It was a gift to my parents a few years ago, and at some point it acquired a 5 mm round chip in the edge.

    After taking out the chip and thinning a bit, it looks like this.

    [​IMG]

    I don't really know what I'm doing, so here are some questions:

    1) How thin am I looking for behind the edge? Currently, it's around

    .75mm at 10mm behind the edge

    and parts of it are around

    .5mm at 5mm behind the edge, although some parts are thicker. I'll try to make this more uniform in the next couple days.

    I'd send a choil shot but currently the knife is much thicker at the heel than anywhere else, so the picture wouldn't be representative of the overall geometry.

    2) My thinning plan has been to start by pressing on the top of the dimples (or rather, the bottom in the upside down picture above), and then eventually I'll move to the middle/bottom of the dimples and do that for a while. Is this reasonable?

    At the moment, I'm doing all this by hand on an Atoma 140. I'll then try to get out the scratches on a King 300, then move to a 1000, etc...

    Any comments or advice would be appreciated.
     
  2. Dec 31, 2018 #2

    mikaelsan

    mikaelsan

    mikaelsan

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    i know this is properly not the answer your looking for, but i would thin, till you hit the apex, and then again only putting pressure about 1-1,5 then 2-2.5 cm behind the edge. When you feel like you don't want to go further, quick sharpening and go cut some carrot, potato, or basically anything that shows performance.
    If your not yet happy with the result yet go back and repeat the process.
    In thinning you are always to some degree hitting a compromise between food penetration and release its really up to you where you want to end

    also looks good from the picture, question is if you went as far as you wanted
     
    Kippington likes this.
  3. Dec 31, 2018 #3

    ian

    ian

    ian

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    I hear you there, I guess I was just looking for ballpark estimates so I don't have to go through the sharpening/test-cutting process a billion times before landing on something reasonable.

    I also hear you suggesting to thin immediately behind the edge, *then* further up the blade. Is there a reason for that order? My initial intuition was that it wouldn't matter.
     
  4. Dec 31, 2018 #4

    mikaelsan

    mikaelsan

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    well no, its just how i like to make sure i've gone far enough, then when you test you know the issue is not how thin is is directly behind the edge. I do suppose starting right between the edge and moving up the blade should theoretically yield more convexity then the alternative, unless you go all out with wobble strokes
     
  5. Jan 1, 2019 #5

    Kippington

    Kippington

    Kippington

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    Sounds to me like you're doing a good job! :D
    Best of luck with the polishing, that part is the worst...
     
  6. Jan 1, 2019 #6

    ian

    ian

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    Thanks to you both! I'm a little concerned that I ****ed up the grind with the copious thinning: I'm getting some steering near the heel that's proving nontrivial to correct. Oh well, guess I should have factored in the expense of a couple bags of carrots into the repair budget. Remind me not to buy organic next time.

    And maybe I should have been actually counting strokes or something during the thinning, but I don't know if I can count that high. :)
     
  7. Jan 1, 2019 #7

    mikaelsan

    mikaelsan

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    sounds good, like your getting somewhere :) that's the thing about extensive thinning jobs by hand, especially those that are not done on widebevels. It really is trial and error you sometimes find that you have not done the same thing on either sides, maybe the angle was different a little too high on one side, maybe you removed significantly more material on one side, put pressure different places on the knife who knows. Good thing is even on something like a diamond 120 stone your not really removing metal that fast, so you can always check your progress and go back and adjust
     
    ian likes this.
  8. Jan 2, 2019 #8

    Kippington

    Kippington

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    Oh yeah I forgot to say, you can get a pretty good idea of how the knife goes through food by feeling the edge with your fingers. Have the spine of the knife on the webbing between your thumb and forefinger, edge facing out. Pinch the edge (and behind the edge) with your thumb and forefinger in a way that your fingers are flat, meaning you're using the fleshy part of your fingers instead of making the 'okay' sign with your fingertips.

    You can get a really good idea of how the knife performs using this method. A blade that's laser thin at the edge will be very difficult to pick up - near impossible to feel. Running your fingers back towards the spine will give an indication of food release, but you need a good understanding on how that stuff works to read into what you're feeling.

    Going through real food is only needed at the end, before polishing... kinda like the conformation test to see if you've done what you thought.
     
    ian likes this.
  9. Jan 2, 2019 #9

    Benuser

    Benuser

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    Those figures are exactly what you should aim for. I use a rule of thumb 0.2mm above the bevel, 0.5mm at 5mm from there, and about 1mm at 10mm.
    Just curious: how far are you from the dimples?
     
  10. Jan 2, 2019 #10

    ian

    ian

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    Rather close. I’d say it’s about 1.5mm from edge to dimple on average.

    Thanks for the numbers, too. I just realized also that when I was measuring the 5 and 10mm thicknesses, often my caliper was going into one of the dimples on the back. (Doh.) So, they’re really more like .75 and 1mm, respectively. I’ll try to take the .75 down to .5 and see how it cuts, while trying to correct the steering. Before, I tried getting some sample numbers by measuring some of my other knives, but there was a lot of variation (eg the Shun is like twice as thick as some of the nicer knives) so didn’t know which numbers would be most appropriate for this steel.
     

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