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Thread: Honing the flat edge of blades

  1. #1
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    Default Honing the flat edge of blades

    Good morning,

    Iíve got a sorby draw knife Iím pretty fond of. Iíve used it for a while with the factory edge or minor touchup.

    It needs a proper sharpening at this point.

    I am trying to polish up the flat side first. Itís got tool marks from when it was originally ground. What Iíve noticed is that the edges polish up before the middle. And I recall something about when you rub two flat surfaces together, they become convex.

    Is that a thing? And is there a way to get a flat surface? Iíve got a variety of sharpening equipment, all of which I use badly.

    Iíd like to do a proper job on this. The same question applies to when I want to hone the flat side of plane blades.

    Thanks

    Tom

  2. #2
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    Default Re: Honing the flat edge of blades

    Ensure that the blade is clamped firmly down, use a FLAT stone or diamond plate, and do not allow the stone or plate to rock.
    I suppose that you could relieve the middle of the flat surface the way that Japanese chisels are relieved with a slight hollow so that you are only honing close to the sharp edge.
    It really is quite difficult to build an ugly wooden boat.

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  3. #3
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    Default Re: Honing the flat edge of blades

    Tom, I'm probably worse of a 'hack' at tool sharpening than most, but I did a Google search on ' sorby drawknife sharpen ' and saw some youtube videos on how to sharpen it - That may be a place to start.

    Others should be along soon with better information.


    Rick
    Last edited by hawkeye54; 10-10-2020 at 09:36 AM. Reason: Cross post with Nick - slow typing

  4. #4
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    Default Re: Honing the flat edge of blades

    If the edges are honing before the middle wouldn't that be a good thing? If I'm understanding you it sounds like the blade is already slightly concave? I'd call that pretty much perfect geometry. If the center of the blade was coming off first I'd be correcting it, but...

  5. #5
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    Default Re: Honing the flat edge of blades

    Quote Originally Posted by Hugh MacD View Post
    If the edges are honing before the middle wouldn't that be a good thing? If I'm understanding you it sounds like the blade is already slightly concave? I'd call that pretty much perfect geometry. If the center of the blade was coming off first I'd be correcting it, but...
    Exactly.
    Tom, can you post a photo?
    It really is quite difficult to build an ugly wooden boat.

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  6. #6
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    Default Re: Honing the flat edge of blades

    In the case of a drawknife, it doesn't matter if the back is flat. A slight convexity will not impede its use because the tool is used freehand. On the other hand, a tool such as a paring chisel or shipwright's slick wants to sit as flat as it can on its back in order to present it's cutting edge cleanly to the work.

    That notion about rubbing two flat plates together will eventually result in two convex surfaces is right. Because any deviation from absolutely flat by the operator (an imperfect human, after all) will result in a tiny amount of greater pressure/grinding at the edges. Most of us, not being machines, will create these convex surfaces. I know that I do and I've been sharpening my tools for a long time. This is why grinding in a concavity to the bevel is helpful. Those fancy sharpening jigs, of which I have several, help in that they slow down formation of the convex surface. But given enough time/use, it will still happen.

    In an infinite universe, everything becomes a sphere. (I just made this last bit up... it might be true!)

    Jeff

  7. #7
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    Default Re: Honing the flat edge of blades

    You want the back face to be hollow ground. It's a draw knife, those teeny weeny tool marks don't matter anyway, but they will be ground away when you gently and conservatively flat hone the back to remove the burr created by honing the beveled face.

  8. #8
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    Default Re: Honing the flat edge of blades

    Agreed. The marks don't matter. But I could use the practice getting a nice finish. Thanks all.

    IMG_20201010_115009510.jpg
    Last edited by bluedog225; 10-10-2020 at 01:01 PM.

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    Default Re: Honing the flat edge of blades

    If you can go to a glass shop and get a piece of plate glass cut to fit a sheet of wet and dry sandpaper it will be cheaper than buying diamond plates. A draw knife that has been hollow ground on either front or back will not be as good a tool as one that has flat ramps on both front and back. Here is my friend Izzy Esnard learning about the handeling of a well sharpened
    draw knife. Note how clean the chip is.
    Jay

  10. #10
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    Default Re: Honing the flat edge of blades

    Quote Originally Posted by jpatrick View Post
    In the case of a drawknife, it doesn't matter if the back is flat. A slight convexity will not impede its use because the tool is used freehand. On the other hand, a tool such as a paring chisel or shipwright's slick wants to sit as flat as it can on its back in order to present it's cutting edge cleanly to the work.

    That notion about rubbing two flat plates together will eventually result in two convex surfaces is right. Because any deviation from absolutely flat by the operator (an imperfect human, after all) will result in a tiny amount of greater pressure/grinding at the edges. Most of us, not being machines, will create these convex surfaces. I know that I do and I've been sharpening my tools for a long time. This is why grinding in a concavity to the bevel is helpful. Those fancy sharpening jigs, of which I have several, help in that they slow down formation of the convex surface. But given enough time/use, it will still happen.

    In an infinite universe, everything becomes a sphere. (I just made this last bit up... it might be true!)

    Jeff
    That is why I clamp the knife down. I find it easier to control the stone than the knife. Put the stone down flat and apply pressure with fingertips over the centre of the blade.
    It really is quite difficult to build an ugly wooden boat.

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  11. #11
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    Default Re: Honing the flat edge of blades

    hat notion about rubbing two flat plates together will eventually result in two convex surfaces is right.
    If anyone fancies an internet rabbit hole - google DIY astronomic mirror grinding.....
    I'd much rather lay in my bunk all freakin day lookin at Youtube videos .

  12. #12
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    Default Re: Honing the flat edge of blades

    Quote Originally Posted by bluedog225 View Post
    Good morning,

    I’ve got a sorby draw knife I’m pretty fond of. I’ve used it for a while with the factory edge or minor touchup.

    It needs a proper sharpening at this point.

    I am trying to polish up the flat side first. It’s got tool marks from when it was originally ground. What I’ve noticed is that the edges polish up before the middle. And I recall something about when you rub two flat surfaces together, they become convex.

    Is that a thing? And is there a way to get a flat surface? I’ve got a variety of sharpening equipment, all of which I use badly.

    I’d like to do a proper job on this. The same question applies to when I want to hone the flat side of plane blades.

    Thanks

    Tom
    Almost all mass produced tools have their back surfaces ground on a grinder that uses a cup wheel, that wheel is very slightly tilted so the following edge of it is not in contact with the workpiece and this produces a faint hollow in the ground surface. To see an extreme example of this, check out a new Irwin built Stanley or Marples plane, it takes a lot, a LOT of work to get those things halfway ready for use. ( Rubbish in my opinion)

    Its actually helpful in that in order to remove the small grooves that the grinding wheel leaves, its only necessary to polish the surface from the edge back a few mm, I do that with a 1200 grit Japanese waterstone then strop it with green paste on leather. The waterstone gets trued up as needed as they wear pretty quickly.

    John Welsford
    An expert is but a beginner with experience.

  13. #13
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    Default Re: Honing the flat edge of blades

    I thought that rubbing two flats gave you a concave and a convex; that's how telescope mirrors used to be made.
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  14. #14
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    Default Re: Honing the flat edge of blades

    Quote Originally Posted by Peerie Maa View Post
    Ensure that the blade is clamped firmly down, use a FLAT stone or diamond plate, and do not allow the stone or plate to rock.
    I suppose that you could relieve the middle of the flat surface the way that Japanese chisels are relieved with a slight hollow so that you are only honing close to the sharp edge.
    This is the whole thing right here. It is also the hardest to master. Rocking the stone will produce a convex surface.

  15. #15
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    Default Re: Honing the flat edge of blades

    Quote Originally Posted by htom View Post
    I thought that rubbing two flats gave you a concave and a convex; that's how telescope mirrors used to be made.
    Looks like that is correct.

  16. #16
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    Default Re: Honing the flat edge of blades

    Quote Originally Posted by bluedog225 View Post
    Looks like that is correct.
    My dad built a 12inch reflector telescope, ground the mirror himself, two slabs of optical quality glass from the glass company where he worked his winter job ( farmer, in those days they had a non milking season in the winter) and a lot of grinding paste while sitting in front of the fireplace on cold nights.
    It worked pretty well, we got amazing views of Saturn among other things astronomical.

    John Welsford
    An expert is but a beginner with experience.

  17. #17
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    Default Re: Honing the flat edge of blades

    Quote Originally Posted by htom View Post
    I thought that rubbing two flats gave you a concave and a convex; that's how telescope mirrors used to be made.
    They still are by ATMs (amateur telescope makers)...

  18. #18
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    Default Re: Honing the flat edge of blades

    Quote Originally Posted by htom View Post
    I thought that rubbing two flats gave you a concave and a convex; that's how telescope mirrors used to be made.
    This is not about tool sharpening anymore. Nevertheless....

    Just by envisioning the process it seems to me that two plates of more or less the same size will result in two convex surfaces. This is because the edges of each will overlap the other in the rubbing process. In the case of a telescope mirror, one aims to create a concavity. So the non-mirror flat would be sized considerably smaller than the one for the mirror. The mirror's edges will not be allowed to be overlapped by the smaller one. Thus the mirror becomes concave. The small polishing pad is in constant contact over it's entire surface, thus it's edges will become worn.

    Jeff

  19. #19
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    Default Re: Honing the flat edge of blades

    Quote Originally Posted by jpatrick View Post
    This is not about tool sharpening anymore. Nevertheless....

    Just by envisioning the process it seems to me that two plates of more or less the same size will result in two convex surfaces. This is because the edges of each will overlap the other in the rubbing process. In the case of a telescope mirror, one aims to create a concavity. So the non-mirror flat would be sized considerably smaller than the one for the mirror. The mirror's edges will not be allowed to be overlapped by the smaller one. Thus the mirror becomes concave. The small polishing pad is in constant contact over it's entire surface, thus it's edges will become worn.

    Jeff
    With correct pressure and technique you’ll end up with a concave & a convex, here’s a decent explanation from a project looking to reproduce 15th century tech
    https://wp.optics.arizona.edu/falco/...oncave-mirror/
    more modern (17th century) technique is to use a pitch lap for one side, there are advantages for edge polishing with it being oversized.

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