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Thread: How Flat is Flat Enough

  1. #1
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    How Flat is Flat Enough

    I'm hoping to pull on everybody's wisdom here. I am in the process of flattening the backs of some vintage Stanley 750's. My question is; What is flat enough? After 220 grit sand paper they have an almost mirror like finish, I was planning on going to 400 and stopping. When I lay the ruler from my Starrett 6" square they are not flat and there is light showing under the ruler. Is this something I need to be concerned about or not? The base I am using is a piece of certified marble for machinist work I acquired. Any thoughts or guidance?
    Andy Kertesz

    " Impaled on nails of ice, raked by emerald fire"...... King Crimson '71

  2. #2
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    Andrew, I'm far from an authority, but a super keen edge is the result of two very flat, highly polished edges meeting. So you need to go higher than 400. I go to 1200 and often an emery polishing paper or a 8000 grit water stone on the backs and the bevel. You only need to do the back once, but it is worth the extra time and effort to get the backs flat and polished (IMHO).

  3. #3
    Flat is good. But I think a lot of the emphasis on flatness comes from the modern mfgs. Seen a lot of old good chisels that are not perfectly flat. When I flatten one it is usually a narrow one that actually rocks a bit . And most of the "machining"is done on the side of the grinding wheel. Do some work with it ,and see if the chisel is working. I have no doubt that for sale somewhere there is something to check if the soup has enough salt, but tasting it works too.

  4. #4
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    The flat side of a chisel must be flat for two main purposes. First flat enough to index correctly to the working surface. That is in the case of a tenon shoulder less than 2 thou. Anything more than that, the separation with the mortise surfaces draw the eye. Second, it must be flat enough to meet what Jake Helmboldt stated for the purpose of creating a keen edge. That usually means substantially less than 2 thou.

    I flatten my chisel backs going from left over sanding belts of 80 ~ 100 grit for really roughing in and through water stones from 220 through 10,000 grit. As previously said, you only have to do that once and there after you just use the 6000 through 10,000 grits. Some folks even go to above 10,000 grit to get a really keen edge but for wood working in general, 4000 and above will do the job nicely.

    I highly recommend David Charlesworth's two videos on chisel sharpening and use. Just really excellent IMO.

    It needs to be flat for the first 3/4 inch or so. Some older chisels were set up with a tiny concavity above the 3/4ish mark. I prefer to get it where no light what so ever shows between a straight edge and the back. No light usually means you are in the scoring range of where you are less than a thou. Over time the refresh on the polishing only stones will continue to make the back flat as possible in the context of a small shop and not a scientific research center.

    A flat, really sharp chisel is a wonderful tool and makes joints and other adjustments a pleasure. Worth the effort so GO FOR IT ! Hoot!

    Enjoy the process.

  5. #5
    Right now I'm watchin ONE video on replacing a heart valve....my chisels are already sharp enuf to do one......BUT SERIOUSLY, folks.....A good research project would be for someone to do a paper on the Wintertur Dominy Shop tools as to how flat they are and what they used to grind and hone them.The WITH HAMMER IN HAND book is good but I don't remember it covering this subject .

  6. #6
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    I agree with the idea that sharp is two polished planes meeting at an edge, but, apart from that, it's much easier to chop straight down if the back of the chisel is straight and flat. I've had more than one chisel with slightly bowed backs.

  7. #7
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    I'd say most folks need a few dead flat chisels for general cabinetry work, paring things flush, riding chamfers and shoulders, dovetailing, etc. it is easy to become focused on and swept away by the very intimate, precise functions of woodworking, (look at how many people turn their noses up at registered firmer chisels, and then go buy bevel edge chisels that have huge flats on the sides anyway) but it doent mean every tool that I have MUST be able to pare perfectly, take sub thousandth shavings, etc. It is easy to go overboard on tool maintenance / selection.

  8. #8
    Flatness is a myth. Tools we think are flat are not. Methods we employ to create flatness do not produce flatness. Last, operations we think require tools that are flat, do not require that. This is perhaps the biggest mistake of our generation. Rubbing tools on abrasive placed on a flat surface doesn't produce a flat tool. Placing a flat backed chisel against a piece of irregular wood does not make the wood flat. Nor does malleting a chisel into wood create a flat surface against the flat of the chisel. We've been wrong about just about everything to do with this subject.

    Polish your chisels and forget flatness. Maintain the edge and the angle and forget all else.

  9. #9
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    Very well said, Adam.
    Where did I put that tape measure...

  10. #10
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    Flat means that the scratch marks are even across the 1 inch on the back. Use the lower grits to flatten the back then you just polish the scratches. If the back doesn't get flat it won't give you a sharp edge. With practice you will be able to see if the backs are truly flat.
    Don

  11. #11
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    Quote Originally Posted by Adam Cherubini View Post
    Flatness is a myth. Tools we think are flat are not. Methods we employ to create flatness do not produce flatness. Last, operations we think require tools that are flat, do not require that. This is perhaps the biggest mistake of our generation. Rubbing tools on abrasive placed on a flat surface doesn't produce a flat tool. Placing a flat backed chisel against a piece of irregular wood does not make the wood flat. Nor does malleting a chisel into wood create a flat surface against the flat of the chisel. We've been wrong about just about everything to do with this subject.
    Polish your chisels and forget flatness. Maintain the edge and the angle and forget all else.
    I can feel a substantial difference between the chisels in my shop that i have "reasonably flattened" and those that i have simply polished as is. The flat ones have less tendency to dive when paring, are much easier to hone and maintain, and are more keen.


    The ones i have left alone are more difficult to sharpen and less predictable. Many times extreme precision doesnt matter to me. Sometimes it does. Perhaps this is a matter of experience or skill, but if all I have to do is spend 5 minutes on a diamond hone to make my chisels more pleasurable and versatile, consider me deluded.

  12. #12
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    I like a flat faced chisel because it is more predictable to me than a random back bevel. Other than the slightly longer initial prep time, I don't see anything wrong with it. Yes, you can get a sharp edge by just polishing (power/manual stropping) both sides.

  13. #13
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    Quote Originally Posted by James Taglienti View Post
    I can feel a substantial difference between the chisels in my shop that i have "reasonably flattened" and those that i have simply polished as is. The flat ones have less tendency to dive when paring, are much easier to hone and maintain, and are more keen.


    The ones i have left alone are more difficult to sharpen and less predictable. Many times extreme precision doesnt matter to me. Sometimes it does. Perhaps this is a matter of experience or skill, but if all I have to do is spend 5 minutes on a diamond hone to make my chisels more pleasurable and versatile, consider me deluded.
    Same here, James. It's too disconcerting to use a chisel that doesn't behave predictably, makes me worry about a slip that cuts skin, to say nothing of wood miscuts.

  14. #14
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    Quote Originally Posted by Adam Cherubini View Post
    Flatness is a myth. Tools we think are flat are not. Methods we employ to create flatness do not produce flatness. Last, operations we think require tools that are flat, do not require that. This is perhaps the biggest mistake of our generation. Rubbing tools on abrasive placed on a flat surface doesn't produce a flat tool. Placing a flat backed chisel against a piece of irregular wood does not make the wood flat. Nor does malleting a chisel into wood create a flat surface against the flat of the chisel. We've been wrong about just about everything to do with this subject.

    Polish your chisels and forget flatness. Maintain the edge and the angle and forget all else.
    Wow, that's quite a statement. I'll have to give that a try. On second thought, nah....

  15. #15
    What about the original advice request? He did not say he had experienced any problem using the chisel.He did not say he noticed any pitting or problem. He has worked on it and sees no distortion except by looking for light while checking with a machinist quality rule.With the flattening system he is using its not going to be perfect. At some point he may spend some money on a diamond hone for some future problem. We have no reason to think there is anything to fix,we just like flat chisels....Let's get a birdhouse, coffee table, gun rack, bombe chest on chest, or something ....FINISHED!

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