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Thread: Carbide sharpness and other favourite fairytales

  1. #1
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    Default Carbide sharpness and other favourite fairytales

    Was fooling around with tig brazing (see the scribing caliper thread) and brazed one used DCGT insert to the end of craptastic screwdriver.
    These particular inserts are really sharp out of the box and seem to sharpen easily so I wanted to see how sharp I can get the carbide insert if it is shapened to more like woodworking tool angles.

    Grind to about 15 degrees, finish with well used 3000 grit diamond wheel to nearly mirror finish ("almost" "glendo" = 5 usd diamond disk on a 20 usd 3000rpm bench grinder)
    Seems pretty sharp so time to test:



    YUP! I'm not much of hairy guy but shaves my arm without any effort (ok, took some effort not to cut myself as the tool shape is far from optimal to shaving)
    Took I test cut on my beard, cuts dry with a bit of effort. Would definetely need shaving cream.

    Wood?
    Yes. This thing cuts dry european Beech like butter:



  2. #2
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    No one ever said carbide couldn't be sharpened, it's that carbide doesn't stand up to USE very well when closely honed.

    The nature of the beast is that tungsten carbide is effectively a powder cemented together with cobalt as a binder. between the brittleness of the TC itself and the fragility imparted by the "sintered" nature of the insert, very fine points and edges simply don't last very long.

    Therefore inserts are designed around what are seemingly "blunt" edge profiles, designed to withstand cutting forces for as long as possible, to maximize tool life. That "honed" insert you just made likely won't last long even for woodworking, and for metalcutting, would likely eat itself in surprisingly short order even on aluminum.

    Doc.
    Doc's Machine. (Probably not what you expect.)

  3. #3
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    Quote Originally Posted by Doc Nickel View Post
    That "honed" insert you just made likely won't last long even for woodworking, and for metalcutting, would likely eat itself in surprisingly short order even on aluminum.

    Doc.
    Was actually surprisingly durable. Even a really good carbon steel wood chisel is at the limit if sharpened to ~15 degrees and used for cutting hard endwood like that.
    Wouldn't work on aluminium but nobody sharpens aluminium cutting HSS tools neither for 15 degrees

  4. #4
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    +1 on Doc's post.

    The books on using older carbide types recommended honing a 3 to 5 thou bevel on the cutting edge to improve tool life for just that reason. There are better carbides, but it is still particles in a matrix. The matrix does not hold tiny thin flakes as well in a sharp edge.
    1601

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    Hashim Khan

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    I frequently use carbide inserts intended for aluminium for finishing steel. You can take cuts of 0.0005" easily when ordinary carbide will just make a mess. I have even used them for machining soft jaws, which is about as interrupted cut as possible. The cheap Chinese ones are the best buy, especially for the doubters to try out without breaking the bank. Even straight out of the box, they could cause a nasty little cut on the finger.
    Last edited by old mart; 01-26-2019 at 04:26 PM.

  6. #6
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    Quote Originally Posted by J Tiers View Post
    +1 on Doc's post.

    The books on using older carbide types recommended honing a 3 to 5 thou bevel on the cutting edge to improve tool life for just that reason. There are better carbides, but it is still particles in a matrix. The matrix does not hold tiny thin flakes as well in a sharp edge.
    Works if you want to actually remove some material. (btw bevel is also sometimes recommended for HSS roughing tools)

    These alu specific inserts come with zero bevel and seem to last pretty well on finishing cuts on a steel. Much better than any of my tooling HSS on 4140PH and yet able to take 0.02mm off from diameter without a problem (ie 5 tenths DOC)

  7. #7
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    Quote Originally Posted by old mart View Post
    I frequently use carbide inserts intended for aluminium for finishing steel. You can take cuts of 0.0005" easily when ordinary carbide will just make a mess. I have even used them for machining soft jaws, which is about as interrupted cut as possible. The cheap Chinese ones are the best buy, especially for the doubters to try out without breaking the bank.
    Yep, this was originally one of the ebay (fake) korloy DCGT aluminIum inserts I use mostly for steel.
    They seem like a excellent alternative for HSS for light cuts.

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    "For light cuts".

    Yep... many things work for light cuts.

    The problem with carbide is that it is not as general use as HSS... when you get doen to the niitty-gritty, you tend to find out that there is a different insert that is deigned to do just exactly what you are doing. In order to do everything with the best insert, you need drawers full of different specially made $5 to $10 per each inserts, and toolholders to fit them.

    Each grade of steel has an optimal carbide, which may be blunt or sharp, round, lozenge, square or triangular. Each type aluminum ditto. And then you get into roughing, finishing, threading, with full form topping, lay down, stand up, etc etc etc. Even left and right hand threading types, and, of course, the TPI means one type per each pitch.

    That's fine for the folks doing repetitive production work, but is far less help to the poor schlemazel in the basement shop.
    1601

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  9. #9
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    Quote Originally Posted by J Tiers View Post

    The problem with carbide is that it is not as general use as HSS... when you get doen to the niitty-gritty, you tend to find out that there is a different insert that is deigned to do just exactly what you are doing. In order to do everything with the best insert, you need drawers full of different specially made $5 to $10 per each inserts, and toolholders to fit them.
    .
    I see no problem with that and my drawers are nowhere near 3-PH lightbulb capacity. 10 usd per insert... these ones I pay less than that per 10pcs box
    This (fake) korloy H01 grade seems to shine on insufficient speeds and very small depth of cuts on steel. (Same what I used for the M 1.4x0.2 threading exercise)

    Problem is that these last too well and I don't have scrapped inserts as much as I would like

    Anyone with used excess nanograin carbide endmills? Lemme know, I'm interested.

  10. #10
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    Quote Originally Posted by old mart View Post
    I frequently use carbide inserts intended for aluminium for finishing steel. You can take cuts of 0.0005" easily when ordinary carbide will just make a mess. ......
    You're not the only one by a long shot. When I posted asking about carbide a year or so ago a lot of folks came back recommending exactly this for fine finish cuts on steel. And particularly when used in a holder that is angled to create some amount of back and side rake but not remove all the front and end clearance.

    Still haven't got around to ordering anything but that's another story.....

    There's more than a few videos on YT about re-sharpening some inserts so they can be used for general turning too.

    And there's no doubt at all that carbide can be sharpened to a keen razor's edge. Table saw blades attest to that quite well.

    The downside would be if a chisel like edge were used to try to lever waste out of a mortise or groove. I suspect that they might snap the edge off with such loads. Carbide is great when under compression but not forgiving to tension. That's why so many of us clumsy carbide newbies chip the ends when we try to run the carriage back out of a cut or stop the lathe and turn it backwards with the cutter still pressing into the work.

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