TAPPED OUT – Short note on a helpful idea

by Jay van Arsdale (adapted from The Wooden Post, vol 9, December 2017)

TAPPED OUT - Jay van ArsdaleMy short talk/demo about URA DASHI [at the Kezurou-kai USA 2017 event in October] was basically showing what was needed to be able to produce more ‘land’ at the front edge of the backside, ura, of plane blades and chisels. This process is not what ‘tapping out’ sounds like, maybe an incorrect translation. This is not done on western single steel blades, so maybe that’s where the confusion comes from. It is not possible to tap out the hollow grind on the ura side of the blade to establish more land near the cutting edge of the blade. This area is worn away by the repeated sharpening of the blade.

It is necessary to reestablish this flatness, so the geometry of the cutting edge has support on both planes establishing the components of the edge. This is necessary for plane blades because just grinding off the ura to get more land results in the taper of the blade being reduced. This changes where the blade compresses to hold the blade in place. Making the blade taper thinner, making this compression happen farther down in the DAI –making the cutting edge extend too far below the gliding surface. This results in not being able to adjust for much finer shavings. Obviously, how far the blade extends below the gliding surfaces determines how thick shaving can be made.

Successful URA DASHI, happens by gently stretching the soft iron, jigane, of the blade by tapping this area over the point of an anvil. This gentle stretching of this malleable steel, pushes the hard steel, hagane, down past the plane of the ura side of the blade. This deformation gives reflattening of the ura material that when ground down reestablishes a broader area for edge geometry. The hollow grind of the ura must be supported when this tapping is being done. Not supporting this hollow will result in a cracked blade. That is why you want to use a corner to support the area being tapped on. Spanning this arch in the back of the blade on a flat anvil edge is not supporting the area being hit upon.

I usually cover the area of the soft iron (jigane) with tappings marked by the indentations of the hammer being used. I like a rounded peen, not a sharp point that turns up a burr. Don’t expect once over will be enough. This is a patient, delicate process that will require more than a once over. Each blade has a different malleability that can only be known by trial and error. Don’t try and to it all at once…you are asking for trouble. This is a gentle process, not a mashing out of the hollow grind.

A word of caution about striking the sweet spot with a metal hammer…remember you are trying to gently bend the edge over, not beat the hollow grind down. This striking should be done just over the point of the anvil at an angle, not direct hits down on the anvil. Hitting directly down can and will cause compression stress cracks in the blade. This striking should be directed slightly above the anvil-stretching the soft steel which gradually pushes the hard steel edge down past the plane of the ura, which is exactly what you want to happen. This deformed edge allow the land to be reformed without decreasing the tapered fit of the plane blade in the dai.

Obviously you can strike the thicker part of the jigane near the heel of the bevel stronger than the thinner soft steel closer to the homon, the lamination line between hagane and jigane. I have found it much more efficient to do this a little at a time, instead of waiting till the hollow grind is exposed in the edge requiring much more work. This technique is also useful in removing chips from the blade edge more quickly than grinding them off. This means that the deflected edge is removed by reflattening the ura after ura dashi because the edge deflected contains the chips that need to be removed. Why grind off the hagane when that is the most special and expensive reason to buy and use laminated steels?

After each tapping process grind the bevel with a rough stone to remove most of the marks left by hammering. Check the ura side with your rough stone lightly to detect what movement, if any, you achieved with your tapping. This flatness at the edge should grow with more repeated tapping. Not much land is necessary for the geometry to be successful. One eight inch is more than enough exposure. Become aware of the sounds that the tapping makes. When you are on the sweet spot, there is a solid sound that doesn’t rattle as you strike the blade. I like a hexagonal piece of soft steel, not the high pitch ring of a bell. I hold the blade in my left hand with my finger beneath the blade, riding against the edge point of the hex anvil to tell me where that sweet spot is. This also stabilizes the position of the blade in relation to the anvil so that you don’t hit the hard steel and cause a chipped or cracked blade.

I suggest you practice on a wide chisel first. If a mistake happens it is easier to repair than a plane blade, which may require the dai to be adjusted for proper fit because the blade got thicker than its previous adjustment position.