With the amount of bodged cuts that we make when were learning it's fairly easy to see how on occassion we end up with a chip or a ding in our blade. I've done it myself and I did it to my favourite blade too which wasn't much fun. I decided to neaten it up as best I could and carry on using it. Clearly I didn't do a very good job because although I was able to carry on for a couple more months, it eventually succumbed to the stresses that I didnt even realise I was subjecting it to.
It snapped cleanly across the blade, leaving me with 2 very sharp pieces of metal and a very distinctive pinging noise ingrained into my memory. As it happens I should not have carried on cutting with that blade and doing so was very dangerous, to myself and to people around me. Fortunately my cuts were quite controlled and the broken piece only made it about 3 foot to my right hand side before sliding to a stop.
This piece in fact
This is a lesson that I wont soon forget and now I know how dangerous it can be, if I sufficiently damage a blade again it will get retired. It's just not worth the risk, learn from my mistakes please! Not every dent or ding to a blade renders it instantly useless or dangerous however. If this was the case then you'd end up going through swords at a rate of knots because every single time you cut with your sword, you sustain to some degree or another a variety of different types of edge damage. The types damage that I will mention here are the main contenders for blade snappage and these are known as 'nicks' and 'dings'.
- Nicks (and chips) - A nick in the blade is where it has met with something hard enough to gouge out a chunk of the metal. This sort of damage could probably best be visualised by thinking about the 'edge to edge' contact that was sometimes incurred in a real sword fight. In truth this was a horrific way to fight and warriors were taught how best to avoid exactly that sort of damage, but if you can imagine that scenario you understand what a nick is. In cutting, this sort of damage is usually incurred by hitting a random screw or nail that has been used to secure a cutting post. Do yourself a favour and don't use nails or screws to secure your post as a fluffed cut and an errant nail can ruin a blade (and your day) in a split second. Chips are similar to nicks but are where a piece of metal has actually 'left' the blade never to be seen again.
- Dings - A ding in a blade is where it has impacted on another hard but dull surface causing the metal to minutely buckle under the stress. This happens on glancing blows against harder objects such as a poorly chosen cutting post or even in some cases the target itself. Some steels and/or geometries aren't meant to cut certain things, for instance I wouldn't trust 1045 to cut bamboo reliably. It might do in a perfect situation but we're real people and our cuts aren't perfect. Even the lid of a particularly tough bottle can ding some blades. On knives, dings can usually be pushed back into place with a steel but on a sword because of the massively increased speed of the blade as it hits the target, 9 times out of 10 sword dings aren't this simple to fix.
So, nicks and dings are the most common edge damage. Theres also flattening and rolling and a handful of other types but these two are the most common and worst ones that will require your attention.
Unfortunately with edge damage like this, the integrity of the blade may have been compromised. The important thing first off is to stop cutting with it until you've decided what youre going to do. The further down the blade the damage occurred, the more likely and quickly it is to fail. If we're talking about a small chip on the kissaki for example, then you're probably going to be fine. If you've placed a nick in any area below the monouchi, then it's only a matter of time before it breaks.
Ultimately it's up to you to decide whether or not the damage to the blade looks bad enough to retire it and I suggest that the best thing to do would be to show the sword to someone who has some knowledge of the subject, in person and ask their opinion. It has been my experience however that as long as the nick or ding is small (1mm-ish or less) then it can usually be polished out.
Seriously, stop cutting until it's sorted
Yep. put it down and get it sorted. If you need more motivation to not cut with it until it's been fixed then I should mention that the success rate of this sort of repair is directly proportional to the amount of cutting that you do or have done with it after you've noticed the damage.
Just briefly I'll elaborate on this so you can see what I mean. As soon as you've damaged the edge you have also created another leverage point along the blade where different amounts of stresses are being applied directly to the metal that sits behind that damage, slowly increasing the severity of it. With the repeated stresses from cutting being applied here, the metal being affected can also harden up more than the surrounding metal, becoming more brittle. Eventually the sword splits at the chip, along the line of this hardened metal. This is sometimes called a stress fracture. Remember I'm not a metallurgist and this is only how I understand it, but it makes sense.
This can be the result of not tending to your sword properly once it's been damaged.
(The usual disclaimers apply here of course although they're implicit in any advice I give, I feel the need to remind you that if you've got a family treasure or it's someone elses or you don't feel entirely comfortable making irreversible and potentially damaging changes to your sword, then I suggest you have a damn good think about whether or not you want to continue... ;)
This damage to the edge of a katana can in my mind only be effectively repaired in one way, and that is to take the entire edge back past the point that was damaged. Yes, that means a large amount of the blade will have to be worked on. You cannot simply polish out just the small area surrounding the ding or dent and expect the rest of the blade to be fine that a section of it is now significantly and disparately thinner.
So, if you are going to remove this ding/dent then a little prep work is needed. You must decide the best line to use to bring the edge back past the damage by looking at the blade and marking it. This should be done slowly and carefully with a pen such as a permanent fine line marker, so you know exactly how much metal you are going to be looking at taking out. You should know that this is just to help you get an idea of what you will be doing before you actaully go ahead and do it. The marker itself will come off very quickly when you start removing metal from the blade but it's important to have a visual plan in your mind.
The line should follow from the very top of the blade, down its length following it's curvature, past the underside of the damage and onward as far as is needed to meet back up with the edge. this line should not wobble backwards and forwards. It should be a natural curve in one direction only.
If you are fortunate enough to have a decent quality water stone then I will assume you also know how to use it and it is probably best you do use that. Being as most of us don't though, we will be using the next best thing which is wet and dry sandpaper affixed firmly to a very flat and very hard surface. Smooth marble tiles or small but thick glass sheets work best but you can use ceramic tiles as well as they are significantly easier to source and much cheaper. The main thing is that the object that you are using as a surface for your sandpaper is flat and hard.
So, What you're going to do is to remove enough metal from the has to shift the cutting edge backwards, past the damage thats been done to it. When I'm doing this I usually start with paper in the 800 grit range being as it cuts fast enough to not infuriate you but not so coarsely as to leave huge gouges out of the blade, increasing the time you'll spend polishing them out.
Take some paper glue and use it to affix a sheet of the paper about A5 size to your flat surface making sure all the edges are down and there are no air bubbles. Once dry, prepare the surface of the paper with a lubricant. Rather than water which rolls off easily or evaporates, I have found that oil works best and I use bike chain oil as it's very light, non corrosive and suits the purpose.
Placing the blade on the paper, press down firmly with your fingers on the very edge of the blade. This pushes the side of the actual cutting edge down and rolls the back of the blade upwards. Carefully and slowly you will need to work the blade backwards and forwards, slowly removing metal. Work the blade for 10-15 strokes on one side and then turning it over, repeat the process on the other side. This way, no matter how you alter the edge geometry, it will be even on both sides.
Angle the blade using one hand with pressure to the ha and use your other hand to keep the angle correct. Do not use that other hand to alter the angle.
The direction that you move the blade in is important as well. There are 3 main directions that you work the blade on. These are diagonally one way, diagonally the other and also along the length of the blade. Alternating these evenly will lead you to a much more even polish.
It is crucial to remember the line you are trying to maintain from the tip of the blade all the way down past the actual chip, the one you drew with the marker pen. Don't try to work too hard on the chip, work on the entirety of the blade because if you keep that up, the chip will take care of itself.
With all the work you're putting in, the sandpaper will probably stop working as well at some point and need replacing. Simply soak and remove the paper, wipe down the surface, make sure its smooth again, reapply a new sheet and start over. That's one advantage of a wetstone over the sandpaper method, you don't need to keep stopping to replace it.
There is no problem with taking a break, it doesn't all need to be done right this minute. It is better to do a good job slowly than a crappy job quickly. Sometimes it takes hours just to bring the edge of the blade back past the edge damage. If you do decide you need a break, wipe down the blade, put it somewhere safe and take one.
Once you have removed enough metal from the edge, the damage will no longer be visible because you will have pulled the edge far enough back to be past the point of the chip. However, the blade itself wont look fantastic because of the coarseness of the grit used in reworking it. This is your cue to start working up to the higher grits using the same process. I usually skip straight to 1000, 1500 and then if I have it, 2000. This is of course up to you. This whole process takes time but it will be worth it.