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  1. #16
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    Re: Why not make a concave grinder platen, for convexes

    I'm kinda boggled by all this, but i would say you're going to need something resistand to wear, I made a flat platen from O1 last year, it's 63 rockwell and looks like a bar code, Shing reported similar results with D2.

    You either get the impression to use something ultra hard or a softish, wear resistant material.

    Good luck


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  2. #17
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    Re: Why not make a concave grinder platen, for convexes

    Quote Originally Posted by FGYT View Post
    dont forget the belt will only run against the curve when you push on it and only where the blade contacts soon as its off the blade it will straight line to the top of the X area
    so you will round the edges and it ill also push back out as you remove the blade

    Convex is a PITA for reproduction as you cant easily measure and quantify it it can be a simple radius or a curve that changes and you cant easily measure or reproduce the rate of change
    and a great fine convex can be ruined by a small slip that fractionally rounds over the edge more than wanted etc

    Ive been doing mine now by flat grinding an inital angle on the side then flat grinding a secondary at the sort of terminal geometry I want ie 30-35 deg then using marker pen to cover the flats i can see how close to the edge i can get and not round it and also how far up the flat its blended in this i have done on the flat platen for a few passes and then blended with the slack between platen and top roller
    still a fair number of variables but i can meaningfully change the 2 flat grinds etc to get different results

    Ive also now found that actually blending the curve may be easier and better looking if done on a wheel so will be experimenting with that

    Ive been doing Ulus side on and have blended large baldes by shooting the edge down the wheel

    the WHeel has the advantage of a softer surface than the Flat platten

    ATB

    Duncan
    I agree here, that spilt second between when you're pushing the blade against the belt and pressing against the backing (and the reverse ie:lifting off) would be a problem. A client suggested a leather backed platten and that worked a charm. I just stuck some relatively "squishy" leather to the flat plate. The leather did harden with the heat and pressure but it worked. Unsure if I'll be doing it again, kinda concerned about the strain on the motor due to the added friction but mine was a homemade botch-job. I simple adjusted the convexity and angle the same way as "normal grinding" by applying pressure differently. To make sure of my angles I mic'ed an example blade at various heights and just aimed for those.



    Maybe a low durometer neoprene backed platten?

  3. #18
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    Re: Why not make a concave grinder platen, for convexes

    Looks like a great finish, as a guess was it a Konosuke or a Carter you were aiming for?
    Did you do all the grinding with the leather backed platten, or did you put facets on first and then blend with the leather? Any chance you could post the measurements?
    Cheers,
    Tom

  4. #19
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    Re: Why not make a concave grinder platen, for convexes

    You can tell that from the photo??? I need to learn more about kitchen knives. It was an amalgamation between two knives. Some of the names stuck in my memory - flay Funyaki profile, taperd Wa handle and the grind was dictated by a HD Kono. Finishing was horrid, this was M390.

    I'll hunt down the little black book with the measurements will PM them rather than derail the thread. I measured edge, quarter, half, 3/4 and spine at 4 points along the blade to create a kind of net of specs. Will post them once the book is in hand. The spine was 2.4 at the thickest and the edge (and tip) was 0.3mm behind the final bevel.
    Last edited by Gareth Bull; 11-07-12 at 02:56 PM.

  5. #20
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    Re: Why not make a concave grinder platen, for convexes

    I had an idea for convex grinds too - imagine a grinding wheel, but with no center, like the letter "o", the blade gets ground on the inside. I got the idea from a guiness world record for some motorbike with a hubless rear wheel (cant remember much about it though)

    also, how about some sort of thick, soft linisher belt (like the scotch-brite ones, maybe, but even thicker)

  6. #21
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    Re: Why not make a concave grinder platen, for convexes

    I imagine that if the top was as a normal, flat platten & the sinusoidal was only at the bottom (& not extreme) then it might work, but proll only for really shallow convex's


    Just my 2p
    Eric & proud!

  7. #22
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    Re: Why not make a concave grinder platen, for convexes

    Would not a simple convexing wheel or dish wheel be a better tool than a flat platen, this is simply a contact wheel with a shallow concave ground in, the belt will follow this contour and you grind with the rotation of the belt. On my old 3" flatbed grinder I had a 10" cancave cotact wheel with some old car innertube stretched around to lesson the chatter. It worked for me over 40 years producing numerous tanto blades in that time.
    For blades with a riccasso you will still need to some filing or accross the belt grinding. A simple test can be done in a few minutes with some gaffer tape or similar, a half inch strip either side of the contact wheel about a couple mil thick should give you some idea.

    Paul.

  8. #23
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    Re: Why not make a concave grinder platen, for convexes

    dont forget the belt will only run against the curve when you push on it and only where the blade contacts soon as its off the blade it will straight line to the top of the X area
    so you will round the edges and it ill also push back out as you remove the blade
    +1

    first of all, what would make a better blade, one that has a perfect radius in it's convex, or one that has 3 bevels, ground together by hand, so you won't see them, but it's still 3 bevels. (regardless of the fact that a perfect radius thins indeed down the spine, making the blade more flexible.) Just regarding anti-stickyness ;-)

    Looking forward for the little black book. A convex grind could also be described as a parabol, like Y=3x^2



    At least an axe grind looks a bit like it. Wonder how far this idea should be used on a kitchen knife, regarding wedging.

  9. #24
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    Re: Why not make a concave grinder platen, for convexes

    I tried this idea with the inside curve of a Studebaker spring, and all that I got was double edge dagger..seems the belt likes to eat away whatever it touches, regardless of what I actually wanted...

  10. #25
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    Re: Why not make a concave grinder platen, for convexes

    I think the ideal tool for this has already been invented. It's a 3 wheel platen to add to your grinder. Giving adjustable tension to a timing belt. This will give nice tension in a soft backing for finishing blades. The trouble with a slack belt is its just to slack for anything but the very last grinder grit.
    Personally I like to get the geometry established on a dead flat platen so as to have a very fine and even edge with no overgrinds. I then switch to a cork platen with graphite webbing stuff. It lasts for ages on a soft platen. Finally a slack belt. But that timing belt system would be ideal.
    I personally like this system, its versatile and allows you to tweak your grind to get results.

  11. #26
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    Re: Why not make a concave grinder platen, for convexes

    The 3 wheel timing belt tool looks great. Can't find any pics on the web right now, but I know what you mean. But i don't think it will be usefull for shape grinding in the beginning at lower grits?

    Do you hand rub you blades? I've ordered grinder belts 180, 240, 320, 400, 800 en 2000. I want to eliminate as much hand rubbing as possible. Got some wrist problems lately ;-)

    What is the graphite webbing thing? Graphite for lubrication? But webbing?

    Would that 3 wheel thing eliminate your cork platen?

  12. #27
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    Re: Why not make a concave grinder platen, for convexes

    thanks zackerty!

  13. #28
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    Re: Why not make a concave grinder platen, for convexes

    Radius Master make a three wheeled thingie...

    http://www.radiusmaster.com.au/

  14. #29
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    Re: Why not make a concave grinder platen, for convexes


  15. #30
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    Re: Why not make a concave grinder platen, for convexes

    I have one of these from kmg ,( go to the kmg grinder page and scrole down to RPA) (thanks Col) and it does the job perfectly with grits from 36 and all the way to 600 etc.
    the belt tension is variable . I have sharpened many many axes on it with a loverly convex edge and also used it to restore 17C wootz sabres you can follow the convex of the originals...
    It is the rite tool for the job. I somtimes rough out the main bevel angles first as a flat platen is more aggressive.
    Last edited by owen bush; 12-07-12 at 06:04 AM.
    please email me if you want to contact me ,my PM box just gets so full .
    owen@owenbush.co.uk

    www.owenbush.co.uk

 

 

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