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Comparing hardness of aluminum oxide to hardened steel......and, how it applies to sanding.

Discussion in 'Woodturning Discussion Forum' started by odie, Aug 25, 2018.

  1. odie

    odie

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    I've been doing a little checking around, and found the MOHS scale of hardness and compared hardened steel to aluminum oxide. The aluminum oxide rates a 9 out of a possible10 and hardened steel rates aroung 7 or 8 out of 10. They are fairly close, but the AO is harder by a small margin.

    Material

    Mohs' Hardness





    Alumina (Synthetic aluminum oxide)

    3.4

    Aluminum oxide

    9

    Alundum (Fused brown Al203)

    9.0

    Amalgam

    4 - 5

    https://www.reade.com/reade-resourc...perty-briefings/32-mohs-hardness-of-abrasives
    ============================

    • Steel – 4 – 4.5
    • Iron – 4.5
    • Palladium – 4.75
    • Rhodium – 6
    • Titanium – 6
    • Hardened Steel – 7 – 8
    • Tungsten – 7.5
    • Tungsten Carbide – 8.5 – 9
    https://www.jewelry-secrets.com/Blog/rockwell-scale-vs-the-mohs-scale/
    ================================================


    The point of this, is I feel many turners use their sandpaper for much longer while sanding, than they would for their typical turning tool.......when the difference in hardness isn't that much.

    Sandpaper, as well as turning tools give a false impression of sharpness when you run your fingers over the cutting edge to judge how much life you have before tossing the paper......or resharpening the turning tool.

    When we learn to sharpen sooner, and toss used sandpaper sooner......the benefits of doing that are great. It's hard to come to doing that with sandpaper, because it'll still sand......just not as effectively. The same applies to the turning tool.....it will still cut, just not as cleanly.

    -----odie-----
     
  2. Emiliano Achaval

    Emiliano Achaval Administrator Staff Member

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    I think of tools, specially gouges as disposable tools, if you worry about how much steel you are loosing every time you sharpen you need to change your thinking, same with sandpaper. I never use a disc twice, unless the piece is on the small side.
     
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  3. Paul M. Kaplowitz

    Paul M. Kaplowitz

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    I read this quote 26 years ago, and I'm sorry I don't remember who said it. Best Advice. " Use sandpaper like someone else is paying for it."
     
    odie likes this.
  4. robo hippy

    robo hippy

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    It is kind of difficult to tell when your abrasives are dull. Well maybe difficult to tell when you can still sand some more efficiently, and when it is time to toss it. I try to judge it by how much dust is coming off. I found with the blue discs from Vince, that I can rub it with my fingers and it doesn't feel sharp, but it is still cutting pretty fast. No idea why. Some abrasive discs would be dull by the time I sanded just the outside of a 12 inch bowl. I can get a couple of bowls with the blue discs.

    robo hippy
     
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  5. hockenbery

    hockenbery AAW Advisor Staff Member

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    To me that is the most reliable method for dry wood.
    If there is a dust sreammtje paper is working - when there is no dust stream the paper is not working.
    It’s kind of that simple.

    Wet wood there is no dust because it really does not sand.
    The abranet will work but is it sort of a mesh screen which I change when it clogs.
    You can inspect visually and there is a slight different feel when it is clogged and not cutting.
    Abranet can be cleaned
     
    Zach LaPerriere likes this.
  6. odie

    odie

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    This is very true, Al........You can absolutely make judgments about the effectiveness of the paper, based on how the dust stream looks. There is one thing that should be considered, here......and, that is the grit doesn't lose it's effectiveness suddenly. It is a gradual thing. It would depend on any turner's individual perception of the dust being created, as to what degree the paper has lost it's effectiveness.

    Turner "A", might lose 50% of the paper's effectiveness before he tosses it out and replaces it with fresh paper. Turner "B" might wait until 75% of the paper's effectiveness has diminished before he tosses his out. Both are capable of producing a nice sanded surface, but turner "B" will probably spend quite a lot more of his time getting there, while saving two bits worth of sandpaper cost.

    For some of us here, the decisions we make might be based on priorities, mixed with a bit of conditioned knowledge......while, for others, it may be a simple innate subconscious instinct to be cost-wise, without the ability to understand the true nature of the time element involved.....:eek:

    -----odie-----
     
  7. Doug Rasmussen

    Doug Rasmussen

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    What exactly happens when hand sanding and the sandpaper no longer performs like new?

    For sure, on cheap sandpaper the abrasive grits fall off the backing. With wood, even with expensive sandpaper, you can have clogging due to resins filling the gap between abrasive chunks effectively "dulling" the sandpaper.

    "Friability – A characteristic of abrasives grains that describes their tendency to fracture or break apart when hit or placed under pressure. Highly friable abrasive cut more easily, but wear faster than other abrasives. Friable abrasives are usually chosen for soft, gummy materials or where heat produced by worn grits must be carefully controlled. Friability is usually related to the levels on impurities in the manufactured abrasive mineral. "

    I'm not sure to what extent you have abrasive grain fracturing in hand sanding of wood.
     
  8. When I use this quote, I give credit to Mike Mahoney. He said it during one of his demonstrations that I attended. - John
     
  9. john lucas

    john lucas AAW Forum Expert

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    I tried to use sandpaper like someone else was paying for it but apparently he is a cheap as I am.
     

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