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Thread: Contrary Ground Finishing Tool.

  1. #21
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    Quote Originally Posted by DICKEYBIRD
    Isn't the width of the point of actual intersection of tool edge and the work wider? Similar to a large radius on the nose of a conventional tool?
    I'm puzzled by that too Dickey. The back-slanted edge is hitting the workpiece at a tangent, and the cutting interface would be that point of intersection plus the DOC, which is very small. I'm having a hard time picturing how the shear angle works. Unlike a positive or negative rake tool, the 45 shear angle seems to be pushing the metal out in front of the work, towards the headstock?

    Seems like using a endmill would provide a better cutting surface, since it would have a gradual entry into the workpiece.

    As Nick and other have said, this tool is casually mentioned in a lot of the old machinists texts, mostly as a finishing tool for a shaper. I'll try to get time in the shop today to grind one and try it...
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

  2. #22
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    I don't know anything about a "shear plane" effect but you can shave metal of a bar with a sharp carbon steel pocket knife. While watching the video about wood working I saw the direct relationship to a shear cutter on metal.

    Of course you can't use a knife edge as used on wood for metal cutting, the edge would not last as it has no support against the forces. On any metal removal cutter there is a point of diminishing return at to the narrowness of the cutting edge, that is, the less the angle from 90 deg the less durable the cutting edge is and therefore more prone to dulling.

    There is a good reason for the slight flat between the cutting edge and the chip breaker on inserts and correctly ground by hand cutters with a chip breaker. The reason is tip strength because a knife edge will fail quickly on steel.

    Some just go blindly grinding their cutters the same way for ever and others determine why the darn things get dull so darn fast and make changes to how they grind the cutter.

    Machining requires experimentation and willingness to change. As stated else where, sometimes it changes on the next pass on the same piece of metal with the same exact setup and you have to change with it.

    To me the shear cutter is a way to get a good finish on some piece of crap steel when you get close to size and nothing more. In other words, a tool to save your butt when all else fails.
    Last edited by Carld; 02-21-2010 at 10:27 AM.
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  3. #23
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    Quote Originally Posted by Carld
    I don't know anything about a "shear plane" effect but you can shave metal of a bar with a sharp carbon steel pocket knife. While watching the video about wood working I saw the direct relationship to a shear cutter on metal.
    .
    not sure what you mean by that, there's not a shear 'effect'; the cutting tool creating a shear plane ahead of itself in the work is how all machining works, whether lathe bit, abrasive or your pen knife. There is no relationship or analogy to be had between cutting wood and metal.


    Machining requires experimentation and willingness to change.
    agreed. To paraphrase one has to be willing to learn, which i 100% support. Heck i do it all the time. imo its worth learning that first chapter in the machining text on how metal cuts because it underpins almost everything we do....and eliminates the erroneous metaphors between cutting wood and metal

  4. #24
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    Quote Originally Posted by Carld
    I don't know anything about a "shear plane" effect but you can shave metal of a bar with a sharp carbon steel pocket knife.
    Right, positive rake is if you apply the pocket knife head on to the workpiece, with the spine of the knife lower than the edge. That's 90% of the shear angles we use in machining. The old machinist texts describe positive rake as the wedge you form with a snow shovel -- the tip of the positive rake tool is digging under the workpiece and shearing the metal away.

    But in this case, finishing tool is holding the knife at a 45 angle to the workpiece. That seems to push the metal towards the headstock -- it's not positive or negative rake.
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

  5. #25
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    FYI, nice college level introduction to the shear plane effects McGyver is describing:

    http://www.engineer.tamuk.edu/depart...N3349/ch21.ppt

    Lots of pictures, little bit of math, nothing too scary. It does explain things like why positive rake results in lower cutting forces, how and why built up edge forms, and things like that.

    Cheers,

    BW
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  6. #26
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    Well, actually I didn't understand what the term meant but I see what he meant now. Bob, I will read the link you posted, it looks interesting and something on the order of others I have read.

    What the shear tool is doing is shaving the metal off a little like draw filing does with a lathe file.
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  7. #27
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    Okay, I think I see how this thing works.

    First, it is very analagous to standing an endmill vertically and presenting a flute to the workpiece. It's obviously a very high helix flute, and not exactly the geometry the endmill was designed for, but similar. Given the way the workpiece is moving, it seems most analagous to a plunge cut with the endmill. I would guess you could use an endmill to create a similar tool, perhaps mounted not unlike a tangential cutter.

    Second, I drew this thing up in Rhino just to see what the chips look like up close. Here is the solid model I started from:



    Looked at that way, a wide range of tooling could be adapted, perhaps even inserted carbide if you are so inclined.

    Now here is the chip after a difference on the solid objects:



    The chip seems like a pretty nice shape. I have drawn the skivving tool too far off vertical--it is laying down too much. A more nearly vertical tool would be closer to real practice. But, that angle emphasizes the chip shape in a shorter distance.

    We can see the chip will peel up from a nice thin cut to a thicker one, which is the essence of what we're trying to do. The other thing that benefits the skiving tool is there is a lot of meat behind the cutting edge, so it should be pretty steady. The reason it can't take really deep cuts is that much like a threading tool being plunged the triangular chip gets bigger really fast and so do the cutting forces.

    Those who would like to experiment and have a cutter grinding setup (or skill by hand) that is conducive might try to make the edge curved. Viewed looking at the workpiece, it seems that a curve towards the headstock will increase the rate at which the chip gets thicker, and a curve away from the headstock will decrease that tendency. It might be you can get a shape that is amenable to a little deeper cutting. If you were going to try to make this style cutter out of an endmill, it would be interesting to compare performance of a right hand versus a left hand endmill because of the effect of that curve due to which way the helix runs.

    I'll have to try one of these at some point. They're certainly easy to grind and might be handy to have around.
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  8. #28
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    Quote Originally Posted by BobWarfield
    FYI, nice college level introduction to the shear plane effects McGyver is describing:

    http://www.engineer.tamuk.edu/depart...N3349/ch21.ppt
    That's an excellent foilset Bob. Especially foils 17-
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

  9. #29
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    Looking at the pics and diagrams, I think I understand the reason that this makes a nice finish: half of the cutting force is directed towards the headstock instead of tangential to the piece like a normal cutter. This will result in less flexion of the workpiece and half of the flexion in the tool will be perpendicular to the length of the workpiece instead of perpendicular to the diameter.

    A normal cutter, when it flexes, behaves like a cantilevered beam with a fixed end and as a result, the distance between the cutter and the work vary. The distance change is proportional to the length of the cutter cubed. The cutter thus moves downward and away from the work piece. For a conventional 3/8 HSS bit with 3 inches of stickout cutting a 1 inch radius workpiece, I compute a radius change of about .001inches per 20 lbs of cutting force F if I did not make a mistake.

    Due to the angle of the shear cutter, half of the cutting force is now in the direction of the headstock. This cuts the force bending the cutter downward in half at a cost of a force away from the headstock which is also half of the original force. Using the cantilevered beam equations, it can be shown that the distance from the center of the workpiece varies less for the shear cutter when the cutter deflects than it does for a conventional cutter.

    --Cameron

  10. #30
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    There is a thread I started called Shear Tool Update and there are photo's in it that may help you understand it.

    The tool in the photo's is a left hand turning tool in the holder upside down but I am working on some different shapes.
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