Kanna Help You?
Posted: Tue Nov 18, 2014 10:04 am
Here in Massachusetts, the seasonal swing in relative humidity is fairly extreme, typically ranging from 34% (comfortable) to 98% (very humid) over the course of the year, rarely dropping below 16% (dry) and reaching as high as 100% (very humid):
The blue line indicates average daytime high relative humidity, the blue line the night time relative low humidity.
Wood and wooden tool parts, like plane bodies, are continually adapting to the changes in humidity. This can make hammer and chisel handles loose when the wood shrinks, and a wooden plane body can also shrink to the point where it runs into the blade, and the result can be a stuck blade, or worse, a crack in the plane body. The way the dai (the Japanese term for the wooden part of the hand plane, 台, which, strictly translated, means 'platform') shrinks, I might add, associates to the way the grain in the block is oriented. Dai with the most movement across the width are flatsawn.
I try to set up a new plane when the relative humidity is somewhere around 'average', a sweet spot between the driest and most humid times of the year. This way, when the dai swells or shrinks to its maximum extent, the fit of the blade should still - hopefully - be reasonable. If one set up a new plane when the dai was its most swollen or most shrunken, then the fit at the time of year when humidity is at the opposite end of the range may result in the blade either being swimmingly loose, or too tight to be capable of adjustment. The looseness can be rectified somewhat by shimming under the blade with paper, however the tight fit scenario means either you are opening up the fit to accommodate the blade, a cycle which, over time, is likely to eventuate a poor blade fit most of the time. Or, the tool gets put on the shelf for a while and you use another tool which has a blade you can adjust properly. An overly-tight blade fit, combined with a metal hammer used to adjust the iron, combines to result in a mushroomed over or deformed blade head after a while.
The rational answer, for those that can afford it and who live in a place with large seasonal fluctuations in relative humidity, and who wish to have the best fitting blades throughout the year, would be to buy two of each plane you use, setting one up at the humid time of year and one at the dry time of year - what might be called 'summer' or 'winter' planes, though it is not always the case that high/low humidity levels correspond perfectly to those seasons. East and west coast of North America, for example, are reversed to one another in terms of which season is the most humid.
It's not a bad idea, when you obtain a new Japanese plane, to let it sit on the shelf for a full year or more, blade backed well out and protected from rust, to allow the dai to get a run through the highs and lows of your local relative humidity. That's not always practicable, but is an ideal. I have a number of planes which I have purchased in the past while, one of which has been on the shelf untouched for 3 years, the others anywhere from a year down to 8 months. As we climb into May, I have decided to set up some of these planes. My patience has evaporated - I want to try some of these delectable works by various smiths, see how the cutting 'tastes'.
It might be presumptuous on my part, but I thought that some out there who have a new Japanese plane, or who are contemplating getting one at some point, might be interested in seeing how I go about setting up a plane. I would hardly call myself a 'plane guru' by any stretch, but I have picked up the odd useful bit of information here and there over the years -- and I'm sure I've done it wrong in every conceivable way as well, mistakes being the greatest of teachers.
I view each new plane as a chance to set the works up as perfectly as I can, and believe me, 'perfect' is a vanishing horizon that moves away as fast as you can move towards it. I may never attain it, because it is probably impossible, but I enjoy trying for some perverse reason.
Japanese planes remain, it seems, the poor stepchild of the Japanese tool family, as least insofar as being accepted into the arsenal of tools by woodworkers across the globe. Japanese saws and chisels are part of many craftpeople's sets outside of Japan, but the plane remains somehow forbidding, remote, and there are few who delve in. Many who do so emerge frustrated. Yet, the plane, kanna, is the central tool in the Japanese woodworking tradition, even today. Why nibble at the side dishes when you can experience the delights of the kanna?
Well, the reasons that the kanna hasn't been more widely adopted are apparent enough, at least to me. First off, few woodworkers use handplanes of any type, even if they happen to own one or two. Most automatically reach for the random orbit sander when it is time to prepare wood to a finishing point. That's too bad. At any rate, the pool of people who do try to use planes in their woodworking is a minority these days.
The plane, western or eastern, remains a problem to use for many, knowingly or otherwise, because it works well only when finely set up and finely sharpened. Sharpening is something most woodworkers seem to avoid, for some reason, or do not delve into to a significant extent. A poorly set up or poorly sharpened plane can make a mess of a surface in a hurry, and for some, once bitten twice shy in this regard.
On top of this, unlike most better western planes, even the finest Japanese kanna comes to the new owner as a kit of sorts. You can't simply take it out of the box and start taking shavings after a preliminary adjustment. It is designed this way in purpose in fact, not because the smith wants to offload the labor onto the buyer.
While there is information in English available on the topic of how to turn this kit of parts into something usable, as well as DVD offerings, Youtube tutorials, etc., I would by no means describe it as complete, at least as far as what I have come across. If there was a complete source of information, there wouldn't be much point in rattling on further on that topic, now would there?
I also tip my hat to those who have put information out there in one form or another, in the effort to bring this knowledge to a wider group, and to those who have helped me along the way (knowingly or otherwise). A shout out to Mike Laine, Harrelson Stanley, Des King, Werner Weis, T. Kunimoto, the folks at Japan Tool, Kamijo sensei, to say thanks - - if I've left out a name, no offense intended.
I'll work through the procedure of setting up a brand new plane (kit), and try to be thorough, though I imagine I'll overlook a detail here and there. Please let me know if you think something could be added. There are quite a few steps, and I imagine it will take a few posts to get to the end of the list, so try and remain awake.
Here goes nothing then....
Step One: Protection
Working on a plane involves sharpening and the use of ink and/or charcoal to provide telltales during fitting, and both processes can make your hands dirty. With a nice white oak dai, it is unfortunate to get to the end of set up and have the thing looking all grubby, so I like to cover the dai in painter's tape prior to getting underway:
Cut the tape away carefully from the mouth on both sides so you have full access for making adjustments:
One can consider this step optional, though it seems to me that the idea, at the end of setting up a new plane, is to have a new-looking plane, clean, shiny and all ready to go. Would you deliver a piece of furniture to a client with marks and dirt on it? I hope not. This is your tool and it is your 'gift' once it is complete, so taking a few moments to protect it from grime during set up is well worthwhile.
I'm working on a few planes at the same time so please don't expect absolute consistency in the photos representing the same tool, step after step.
Step Two: Trim the mimi
Invariably the width of the new plane blade is greater than the width of the mouth and it needs to be adjusted to fit. In the following photo, note the two red circles, within each of which you can see that the blade edge's corner protrudes slightly beyond the edge of the ramps in the mouth opening:
If the blade corners or 'ears' - mimi in Japanese - were left long, then shavings can get stuck in the corner between the blade edge and the ramp, and this throws off planing as you slightly lose registration to the wood surface. There needs to be a slight gap between the end of the blade edge and the inside corner of the ramps in the mouth opening.
There are various ways to trim the ears back, but I prefer a metalworking vertical belt sander. It has a large abrasive surface and platen which makes it quite hard to overheat the blade. A grinder can also be used, but take extra care not to overheat the blade.
I hold the blade like so, carefully aligning the bevel of the mimi to the platen:
I opted to not try and take a photo with the machine running, holding the blade with one hand - -seemed like discretion was the better part here. By holding the blade as shown above, most of the metal is removed back away from the cutting steel surface, thus creating little or no burr. If a burr does form on the cutting steel when you trim the mimi, you will need to dress the burr(s) off on a finishing stone before proceeding further.
As you sharpen a blade, the edge recedes back little by little, and as the ears are trimmed on an angle, the blade edge gets progressively longer as the edge is taken back. This means that you'll be revisiting this step in the future, after many sharpenings have caused the blade edge to once again be wider than the space between ramps in the mouth. If you consider this problem, you can see that it is better to grind the mimi on a 30˚ angle (relative to the side of the blade) rather than a 45˚, as this prolongs the time until the ears will need trimming once again.
Here's the result after trimming, red circles now showing the clearance near the mouth opening:
If your mimi grinding was a bit overzealous, it is not the end of the world, it just means that the blade is going to be narrower than it could be. No biggie. Ideally you want about a 0.5~0.8mm gap at each side.
Step 3: Check the main blade (kanna-mi) for twist
The process of forging a Japanese laminated blade, where a hard and brittle cutting steel, hagane, is 'glued' onto a soft iron backing, jigane, makes for a happy marriage in many respects, but like unions between opposites, certain stresses can manifest early on and need to be dealt with before they fester. As the forge weld cools, the steel and iron shrink to differing degrees, and the iron has less resistance to bending than the steel. Essentially it is a tug of war which the steel will win every time. The result, if both backing iron and cutting steel were flat to begin with, is a cupping of the blade towards the cutting steel side (the ura, or back of the blade). The removal of metal by grinding or scraping to form a hollow on the ura afterwards can also affect the finished shape of the blade in terms of the degree of cupping. It is not an uncommon outcome as well for the blade to develop a certain amount of twist during this process of accommodation between the hagane and jigane. You need to check the blade for these two conditions carefully before proceeding to do any sharpening.
With a twisted blade left uncorrected, the process of flattening the back will be screwed up, and it can mess up the fitting of the blade to the dai afterwards even if you do manage a half decent flattening of the back of the blade. Unfortunately, sometimes the person who cut the dai for the plane blade - the dai-ya san - did not notice that the blade was twisted and as a result has performed an initial set up to a deformed blade shape. Or, maybe they did notice but it is not their task to deal with this issue so they chose to only barely fit the blade into the dai.
Here's how to check for twist: start by placing the blade ura side down on a reference flat, so that only the cutting steel portion of the blade is on the flat surface:
Start with one finger in the middle, as shown above, and then use the first finger on your other hand to tap the two corners of the blade, right where you recently trimmed the mimi. You're looking for any gata-gata - a clattering sound indicating that the ura side is not completely touching or in plane with the reference flat. If the blade contacts in the middle, but not at either corner, then the bevel area is bowed across its width and both corners will therefore need to be raised a little. If the blade contacts at both corners of the edge, it is still possible that it is raised in the mid-portion - one can check using feeler gauges.
You can also swing the blade down on the corner of the reference flat (I use a Starrett Lab grade 'A' surface plate), using the upper end of the cutting steel line as a 'hinge', until the cutting edge meets the stone surface at some point, and then tap on each corner to see if one of the corners is lifted from the reference flat:
In the case above, I found that the blade had a fairly pronounced twist, and the high corner (i.e., the corner not contacting the reference flat) is indicated by my finger in the above photo. I say 'high corner' in regards to the above view of the blade, however if we flip the blade onto its other side, ura side facing up, then this same corner would be the 'low corner'.
Note: unless the blacksmith has totally fubar'ed the blade, one can normally expect that the blade will touch the flat somewhere along the edge portion. A bad situation would be where the edge does not contact the reference surface anywhere, and the blade is therefore meeting the surface somewhere in the middle, an indication that the blade's ura is bowed convexly. If you find this situation, it is probably best to consider returning the blade to the store you bought it from. A convex ura can be dealt with, but it's much more of a hassle.
Generally the blacksmith engineers things so that after the forging and grinding is done, the ura side (the cutting steel side) is slightly concave lengthwise. I'm not talking about the blade's hollow - the concavity I'm talking about - sori as it is termed - can be measured along the portion of the blade at the edge which is outside of the hollow. The degree of sori reflects the blacksmith's skill, his philosophical attitude as to how meticulously finished the tool 'should' be, the price point of the tool, and how much work was done to the blade after it left the blacksmith by the sharpening person, or sei-ken-ya. More on that later.
To get rid of the twist involves a bit of pounding. First make up a copper shim, about 1/16" (1~2mm) thick - I used some 5/8" (15mm) copper pipe strapping that was laying about, folding a couple of bits over one another:
Now for the nerve-wracking fun part. Put on some ear protection and place the blade upon an anvil. I use the anvil on the back of a larger Japanese metal working vise that I have. You want to place the blade ura side up, so the blade edge is well away from the anvil, and place the low corner of the blade upon the piece of copper. You will then place a short piece of wood (or a large brass drift, or a non-hardened hammer head, etc.) against the middle of the ura and, using the piece of wood as a drift, strike it as hard as required with a hammer:
You might find it helpful to tape the copper shim down so it does not fly or fall off of the anvil.
A heavier hammer is better than a light one for this step. You can strike the blade in the center with the drift, for the most part, and you may wish to also strike it on the mid-potion of the blade side, directly below the place you have shimmed. Please do so at your own risk, and hammer judiciously. Do not hammer the drift anywhere close to the blade edge. It wouldn't hurt to put the blade in a glass of hot water before any whompin' but I haven't found it necessary to achieve results (though it may speed up the process). Patience certainly helps!
Depending upon how twisted the blade is, you may have to give a few gentle whomps, or you may have to get a bit more, shall we say, determined. It should not reach the level of red-faced Hulk Hogan-esque shirt tearing and bellowing - be clear about that. Try a test hit or two, then go over to the reference flat and re-check the blade to see if you have removed any twist yet. My blade was quite twisted, so I had to hit the drift pretty hard, and through many rounds, probably spending 20 minutes on this step until I had the twist out. You will know when you have the twist out as the blade will sit flat on the reference surface without any gata-gata at either corner.
By bringing the twist out, however, you also intensify one of the other side effects of the blades forge-welded nature, increasing the sori on one side of the blade commensurate with the amount of twist removed. Further, it makes the blade's bevel surface a bit more convex. But one thing at a time - first we get the twist out, then we deal with the other deformities. That's the order in which things have to happen.
The blue line indicates average daytime high relative humidity, the blue line the night time relative low humidity.
Wood and wooden tool parts, like plane bodies, are continually adapting to the changes in humidity. This can make hammer and chisel handles loose when the wood shrinks, and a wooden plane body can also shrink to the point where it runs into the blade, and the result can be a stuck blade, or worse, a crack in the plane body. The way the dai (the Japanese term for the wooden part of the hand plane, 台, which, strictly translated, means 'platform') shrinks, I might add, associates to the way the grain in the block is oriented. Dai with the most movement across the width are flatsawn.
I try to set up a new plane when the relative humidity is somewhere around 'average', a sweet spot between the driest and most humid times of the year. This way, when the dai swells or shrinks to its maximum extent, the fit of the blade should still - hopefully - be reasonable. If one set up a new plane when the dai was its most swollen or most shrunken, then the fit at the time of year when humidity is at the opposite end of the range may result in the blade either being swimmingly loose, or too tight to be capable of adjustment. The looseness can be rectified somewhat by shimming under the blade with paper, however the tight fit scenario means either you are opening up the fit to accommodate the blade, a cycle which, over time, is likely to eventuate a poor blade fit most of the time. Or, the tool gets put on the shelf for a while and you use another tool which has a blade you can adjust properly. An overly-tight blade fit, combined with a metal hammer used to adjust the iron, combines to result in a mushroomed over or deformed blade head after a while.
The rational answer, for those that can afford it and who live in a place with large seasonal fluctuations in relative humidity, and who wish to have the best fitting blades throughout the year, would be to buy two of each plane you use, setting one up at the humid time of year and one at the dry time of year - what might be called 'summer' or 'winter' planes, though it is not always the case that high/low humidity levels correspond perfectly to those seasons. East and west coast of North America, for example, are reversed to one another in terms of which season is the most humid.
It's not a bad idea, when you obtain a new Japanese plane, to let it sit on the shelf for a full year or more, blade backed well out and protected from rust, to allow the dai to get a run through the highs and lows of your local relative humidity. That's not always practicable, but is an ideal. I have a number of planes which I have purchased in the past while, one of which has been on the shelf untouched for 3 years, the others anywhere from a year down to 8 months. As we climb into May, I have decided to set up some of these planes. My patience has evaporated - I want to try some of these delectable works by various smiths, see how the cutting 'tastes'.
It might be presumptuous on my part, but I thought that some out there who have a new Japanese plane, or who are contemplating getting one at some point, might be interested in seeing how I go about setting up a plane. I would hardly call myself a 'plane guru' by any stretch, but I have picked up the odd useful bit of information here and there over the years -- and I'm sure I've done it wrong in every conceivable way as well, mistakes being the greatest of teachers.
I view each new plane as a chance to set the works up as perfectly as I can, and believe me, 'perfect' is a vanishing horizon that moves away as fast as you can move towards it. I may never attain it, because it is probably impossible, but I enjoy trying for some perverse reason.
Japanese planes remain, it seems, the poor stepchild of the Japanese tool family, as least insofar as being accepted into the arsenal of tools by woodworkers across the globe. Japanese saws and chisels are part of many craftpeople's sets outside of Japan, but the plane remains somehow forbidding, remote, and there are few who delve in. Many who do so emerge frustrated. Yet, the plane, kanna, is the central tool in the Japanese woodworking tradition, even today. Why nibble at the side dishes when you can experience the delights of the kanna?
Well, the reasons that the kanna hasn't been more widely adopted are apparent enough, at least to me. First off, few woodworkers use handplanes of any type, even if they happen to own one or two. Most automatically reach for the random orbit sander when it is time to prepare wood to a finishing point. That's too bad. At any rate, the pool of people who do try to use planes in their woodworking is a minority these days.
The plane, western or eastern, remains a problem to use for many, knowingly or otherwise, because it works well only when finely set up and finely sharpened. Sharpening is something most woodworkers seem to avoid, for some reason, or do not delve into to a significant extent. A poorly set up or poorly sharpened plane can make a mess of a surface in a hurry, and for some, once bitten twice shy in this regard.
On top of this, unlike most better western planes, even the finest Japanese kanna comes to the new owner as a kit of sorts. You can't simply take it out of the box and start taking shavings after a preliminary adjustment. It is designed this way in purpose in fact, not because the smith wants to offload the labor onto the buyer.
While there is information in English available on the topic of how to turn this kit of parts into something usable, as well as DVD offerings, Youtube tutorials, etc., I would by no means describe it as complete, at least as far as what I have come across. If there was a complete source of information, there wouldn't be much point in rattling on further on that topic, now would there?
I also tip my hat to those who have put information out there in one form or another, in the effort to bring this knowledge to a wider group, and to those who have helped me along the way (knowingly or otherwise). A shout out to Mike Laine, Harrelson Stanley, Des King, Werner Weis, T. Kunimoto, the folks at Japan Tool, Kamijo sensei, to say thanks - - if I've left out a name, no offense intended.
I'll work through the procedure of setting up a brand new plane (kit), and try to be thorough, though I imagine I'll overlook a detail here and there. Please let me know if you think something could be added. There are quite a few steps, and I imagine it will take a few posts to get to the end of the list, so try and remain awake.
Here goes nothing then....
Step One: Protection
Working on a plane involves sharpening and the use of ink and/or charcoal to provide telltales during fitting, and both processes can make your hands dirty. With a nice white oak dai, it is unfortunate to get to the end of set up and have the thing looking all grubby, so I like to cover the dai in painter's tape prior to getting underway:
Cut the tape away carefully from the mouth on both sides so you have full access for making adjustments:
One can consider this step optional, though it seems to me that the idea, at the end of setting up a new plane, is to have a new-looking plane, clean, shiny and all ready to go. Would you deliver a piece of furniture to a client with marks and dirt on it? I hope not. This is your tool and it is your 'gift' once it is complete, so taking a few moments to protect it from grime during set up is well worthwhile.
I'm working on a few planes at the same time so please don't expect absolute consistency in the photos representing the same tool, step after step.
Step Two: Trim the mimi
Invariably the width of the new plane blade is greater than the width of the mouth and it needs to be adjusted to fit. In the following photo, note the two red circles, within each of which you can see that the blade edge's corner protrudes slightly beyond the edge of the ramps in the mouth opening:
If the blade corners or 'ears' - mimi in Japanese - were left long, then shavings can get stuck in the corner between the blade edge and the ramp, and this throws off planing as you slightly lose registration to the wood surface. There needs to be a slight gap between the end of the blade edge and the inside corner of the ramps in the mouth opening.
There are various ways to trim the ears back, but I prefer a metalworking vertical belt sander. It has a large abrasive surface and platen which makes it quite hard to overheat the blade. A grinder can also be used, but take extra care not to overheat the blade.
I hold the blade like so, carefully aligning the bevel of the mimi to the platen:
I opted to not try and take a photo with the machine running, holding the blade with one hand - -seemed like discretion was the better part here. By holding the blade as shown above, most of the metal is removed back away from the cutting steel surface, thus creating little or no burr. If a burr does form on the cutting steel when you trim the mimi, you will need to dress the burr(s) off on a finishing stone before proceeding further.
As you sharpen a blade, the edge recedes back little by little, and as the ears are trimmed on an angle, the blade edge gets progressively longer as the edge is taken back. This means that you'll be revisiting this step in the future, after many sharpenings have caused the blade edge to once again be wider than the space between ramps in the mouth. If you consider this problem, you can see that it is better to grind the mimi on a 30˚ angle (relative to the side of the blade) rather than a 45˚, as this prolongs the time until the ears will need trimming once again.
Here's the result after trimming, red circles now showing the clearance near the mouth opening:
If your mimi grinding was a bit overzealous, it is not the end of the world, it just means that the blade is going to be narrower than it could be. No biggie. Ideally you want about a 0.5~0.8mm gap at each side.
Step 3: Check the main blade (kanna-mi) for twist
The process of forging a Japanese laminated blade, where a hard and brittle cutting steel, hagane, is 'glued' onto a soft iron backing, jigane, makes for a happy marriage in many respects, but like unions between opposites, certain stresses can manifest early on and need to be dealt with before they fester. As the forge weld cools, the steel and iron shrink to differing degrees, and the iron has less resistance to bending than the steel. Essentially it is a tug of war which the steel will win every time. The result, if both backing iron and cutting steel were flat to begin with, is a cupping of the blade towards the cutting steel side (the ura, or back of the blade). The removal of metal by grinding or scraping to form a hollow on the ura afterwards can also affect the finished shape of the blade in terms of the degree of cupping. It is not an uncommon outcome as well for the blade to develop a certain amount of twist during this process of accommodation between the hagane and jigane. You need to check the blade for these two conditions carefully before proceeding to do any sharpening.
With a twisted blade left uncorrected, the process of flattening the back will be screwed up, and it can mess up the fitting of the blade to the dai afterwards even if you do manage a half decent flattening of the back of the blade. Unfortunately, sometimes the person who cut the dai for the plane blade - the dai-ya san - did not notice that the blade was twisted and as a result has performed an initial set up to a deformed blade shape. Or, maybe they did notice but it is not their task to deal with this issue so they chose to only barely fit the blade into the dai.
Here's how to check for twist: start by placing the blade ura side down on a reference flat, so that only the cutting steel portion of the blade is on the flat surface:
Start with one finger in the middle, as shown above, and then use the first finger on your other hand to tap the two corners of the blade, right where you recently trimmed the mimi. You're looking for any gata-gata - a clattering sound indicating that the ura side is not completely touching or in plane with the reference flat. If the blade contacts in the middle, but not at either corner, then the bevel area is bowed across its width and both corners will therefore need to be raised a little. If the blade contacts at both corners of the edge, it is still possible that it is raised in the mid-portion - one can check using feeler gauges.
You can also swing the blade down on the corner of the reference flat (I use a Starrett Lab grade 'A' surface plate), using the upper end of the cutting steel line as a 'hinge', until the cutting edge meets the stone surface at some point, and then tap on each corner to see if one of the corners is lifted from the reference flat:
In the case above, I found that the blade had a fairly pronounced twist, and the high corner (i.e., the corner not contacting the reference flat) is indicated by my finger in the above photo. I say 'high corner' in regards to the above view of the blade, however if we flip the blade onto its other side, ura side facing up, then this same corner would be the 'low corner'.
Note: unless the blacksmith has totally fubar'ed the blade, one can normally expect that the blade will touch the flat somewhere along the edge portion. A bad situation would be where the edge does not contact the reference surface anywhere, and the blade is therefore meeting the surface somewhere in the middle, an indication that the blade's ura is bowed convexly. If you find this situation, it is probably best to consider returning the blade to the store you bought it from. A convex ura can be dealt with, but it's much more of a hassle.
Generally the blacksmith engineers things so that after the forging and grinding is done, the ura side (the cutting steel side) is slightly concave lengthwise. I'm not talking about the blade's hollow - the concavity I'm talking about - sori as it is termed - can be measured along the portion of the blade at the edge which is outside of the hollow. The degree of sori reflects the blacksmith's skill, his philosophical attitude as to how meticulously finished the tool 'should' be, the price point of the tool, and how much work was done to the blade after it left the blacksmith by the sharpening person, or sei-ken-ya. More on that later.
To get rid of the twist involves a bit of pounding. First make up a copper shim, about 1/16" (1~2mm) thick - I used some 5/8" (15mm) copper pipe strapping that was laying about, folding a couple of bits over one another:
Now for the nerve-wracking fun part. Put on some ear protection and place the blade upon an anvil. I use the anvil on the back of a larger Japanese metal working vise that I have. You want to place the blade ura side up, so the blade edge is well away from the anvil, and place the low corner of the blade upon the piece of copper. You will then place a short piece of wood (or a large brass drift, or a non-hardened hammer head, etc.) against the middle of the ura and, using the piece of wood as a drift, strike it as hard as required with a hammer:
You might find it helpful to tape the copper shim down so it does not fly or fall off of the anvil.
A heavier hammer is better than a light one for this step. You can strike the blade in the center with the drift, for the most part, and you may wish to also strike it on the mid-potion of the blade side, directly below the place you have shimmed. Please do so at your own risk, and hammer judiciously. Do not hammer the drift anywhere close to the blade edge. It wouldn't hurt to put the blade in a glass of hot water before any whompin' but I haven't found it necessary to achieve results (though it may speed up the process). Patience certainly helps!
Depending upon how twisted the blade is, you may have to give a few gentle whomps, or you may have to get a bit more, shall we say, determined. It should not reach the level of red-faced Hulk Hogan-esque shirt tearing and bellowing - be clear about that. Try a test hit or two, then go over to the reference flat and re-check the blade to see if you have removed any twist yet. My blade was quite twisted, so I had to hit the drift pretty hard, and through many rounds, probably spending 20 minutes on this step until I had the twist out. You will know when you have the twist out as the blade will sit flat on the reference surface without any gata-gata at either corner.
By bringing the twist out, however, you also intensify one of the other side effects of the blades forge-welded nature, increasing the sori on one side of the blade commensurate with the amount of twist removed. Further, it makes the blade's bevel surface a bit more convex. But one thing at a time - first we get the twist out, then we deal with the other deformities. That's the order in which things have to happen.