Precione, por favor, a las partes rojas e azules para el YouTube.
SOLDADURA SIERRA INSTRUMENTO PARA CORTAR
Please go direct to those in red (success) and blue (failure) for YouTube entries.
Additional remarks recorded on 28 September 2015
Blade breakage again. I decided to make some new ones from the coil and also repair those already broken and left untouched. In so doing I obtained new insights and felt the need to keep them on record for subsequent repair occasions.
1. I had left an enormous amount of record about blade repairing, but I very much felt that not doing actual reparing for more than 6 months meant that my repairing skills were totaly inadequate.
I made more than 20 repairing attempts, but all of them were no good.
2. For that reason I stopped my actual repairing attempts and instead decided to do practice sessions using disused old blades. That way, my succes rate got better and in the end 8 or 9 attempts out of 10 were found to be acceptable.
Put concretely, I was trying to weld very short pieces of already broken blades until my skills recovered fully, more or less.
3. Here again, I found that the success rate at the first attempt with the newly cut out blades from the coil was 100%, and yet with the old broken blades which really needed welding the success rate was much lower at something like 70%.
The main reason perhaps is that if the blade had broken again at the point of last welding then that portion had probably become soft iron、making it difficult for that portion to be welded properly.
However, if I cut that portion out then the whole length would be shorter. So, I had to cut them into a lot of shorter pieces for further practice. There was no other alternative.
4. Newest discovery is that I did not need to make the cut facets (blade ends) smooth by the rotating grindstone at all. I simply cut the coil blade by my shear cutter and I did not need to dress up the freshly cut facets at all.
5. I also noticed that in annealing the welded portion I did not need to press the power button in advane so that I can press it further for the current to pass.
What it means is a little involved.
The annealing button is meant to let the current through immediately when pressed once. However, it had been my impression that the current will only pass if the annealing buttn is pressed further from the halfway position in order to reduce idling pressing gap.
In reality, however, initial light pressing of the annealing button in an attempt to eliminate the pressing gap will in fact start the current and actually it is far better from the viewpoint of temperature management as it is not the full current through a large area of contact.
One advantage of this light (or weak and gentle) pressing is that you can complete the entire process of annealing quickly. Prior to this discovery I had pressed the button every 3 seconds or so.
However, I was now pressing at the rate of once every one second. That meant that I was now able to complete the annealing in about 30 seconds. That was great, actually.
Additional notes on 4 March 2015 addition
I did blade welding a few days ago, trying to recall what I had said about bandsaw welding. In so doing I noticed a few new things to note and I am duly adding them to my earlier article as follows.
What folows is the photo of the coil blade I use.
With this coil I can make one blade for as low as JPY 500, very good value for money indeed. I used to buy off-the shelf blades for JPY 3,500 each.
The next picture shows the length jig I made for cutting out the length I need.
I clump one end of the coil blade and pull out the length I need and roughly cut the other end according to the mark I have there.
I use a power cutter with a grind(ing) stone. Length torelance is about 5mm and it is OK to all intents and purposes.
The discoveries I made on this occasion are many fold, as follows:
1. Other web sites say a lot about the batting ends of the blade to be welded. They typically say that the blade gap should be very nearly zero.
Today, I did not care and I just simply did a quick grinding of both ends, but otherwise simply pressed them together without too much attention to the gap width.
As a result the gap width was such that it was recognisable as a simple line, not more than that.
2. Likewise, I did not pay too much attention to the number of times I let the current run, perhaps 7 or 8 times.
I could not see the line because of the incandescent light from the gap heating and stopped the current while the gap line was still there to be seen.
3. I then tried to bend the blade at the welded location and the blade did not snap. So, I decided that it was done and attached the blade to the bandsaw machine. It was OK. I actually cut a few pieces.
All this gave me an enormous confidence about my welding skills.
In particular, I was now so confident that as long as melting does not occur I can let the current run as many times as I want. Other web sites say, typically, instant current, only once, sort of things. It is very untrue from my experience.
Naturally, you cannot start with an abnormally large gap. However, if you put them lightly (and casually) together and find that there is still a very visible thin line you can start your welding.
If you come to think about it you still need a small gap anyway. Without a small gap discharge cannot take place and without the discharge no heating, no melting, no welding. Simple as that.
Realistic criteria may be the simple batting, done casually by hand pressing, perhaps leaving a gap of 0.5mm, say.
0.5mm will easily start discharging process and you have your automatic pressing mechanism in action anyway which comes with the welding machine.
Once melting starts it is maintained. We do not need to care so much in the first place, I think. Therefore, if a hair line is still there visible at the end of welding there is no problem with the tensile strength of the blade.
Conclusions must be summarised as follows.
You can electrify as many times as you wish as long as the blade does not melt. I did not measure it precisely, but melting takes place if conctinusously electrified for 1.5 seconds in the case of 6mm width blades .
That means in turn and in practice that you can repeat the electrification of less than 1.5 seconds as many times as you wish, simply because the end result of electrification is not cumulative.
That is to say that if you repeat 0.5 second electrification 4 times, with the cumulative time of 2 seconds exceeding the 1.5 seconds limit melting will not occur.
My guess is that out of 10 attemps, say, you may stop after 0.5 seconds. Sometimes, you may stop after 1.0 second and you may even stop after 1.4 seconds. There can be many variants.
Therefore, if 10 attempts include just one 1.4 seconds that will be great, because that means you stopped only just before blade melting.
In reality you may stop after 1.2, or 1.4, or even 1.0 seconds, but even those may well mean that you have come very close to the ideal situation.
Statistically, you will be wondering about in the broad Gaussian dsitribution as the number of attempts increases, meaning that you are more likely to succeed if you keep pumping your current in a large number of short bursts.
That is exactly why I keep saying that you never, ever think that you can complete your welding in a single short burst.
If you happen to have an electronically controlled, sophisticated welding machine, the you will be OK, no matter how you do it. But, here, we are only doing it with a simple manual machine. You cannot control your welding time.
About the welding line, my guess is that perfect welding will make it disappear, but it is my impression that you do not need to worry too much about it. Rather, I make sure that I do not press the batting ends too much with my handmade wooden pincher in case deformation sneaks in.
Anyway, my records so far above are all meant for biginners. I wanted to give them small amount of theoretical background and confidence. Blade welding is of utmost interest to many who are engaged in wood work.
In particular, I wanted to remove umbiguity from the process as much as possible.
My practical advice would be that you try and intentionally melt your batting ends several times so that you will come to know the limits.
About the machine
The following photo shows my machine.
This is a simple and cheap machine, newly bought via internet. It is a Chinese made machine, and is very similar to the machine 
made by Tokyo Shin Denki in Nagoya area of Japan.
It is usable. It requires single phase 200 V, not the usual tripple phase supply.
So, I had an electrician to install an extra wall socket especially for the purpose. Having a socket on outside walls will make it more expensive as it will involve external wiring. Here, it is a simple extetion from the existing internal disribution board.
In normal Japanese households either single phase 100V or 200V (newer houses and often for induction heaters and dish washers) is the norm. If your contract current is below 30A, then you normally find three cables coming into your house from the grid.
One of them is for ground earthing, the other two are + and - 100V. They are coloured differently. If it is above 30A, a single thick cable is used.
So, if you want 200V single phase supply then you simply use +100 and -100. That will give you a single phase 200V easily.
Typical expenses, including a 10m cable, will be something like JPY 20,000.
In my case the cable is detachable.
Outline of the functions.
With this machine you cannot alter the amount of current you run (more on this later). You have a large dial on the upper left corner of the machine with a scale notches. The large button upper right is the start button and the button below it is for annealing.
Pressing the large button will run the current. It requires a lot of finger pressing and it also requires subtle pressing. Therefore, my suggestion for those who are using the same or similar machines is that you set the batting (pressing) strength to 1 on the scale.
Welding procedure: (ideal)
1. Make the batting ends (contact ends) completely flat (impossible: NB:1)
2. Make the batting end orientation at the right angle to the blade motion (possible to some extent: NB:2)
3. Arrange the batting blades completely in a straight line (this is extremely difficult to achieve: NB:3)
4. Keep the gap width to as close to 0.5mm as possible (NB:4)
5. Use a dielectric pincher for continued pressing of batting ends during welding (NB:5)
6. Do annealing properly (NB:6)
In actual weldig there are always defects at the batting ends. Small protrusions, dents, scratches, or tilted batting faces, etc etc.
As a result you cannot control the initial point of contact for the currnet to run, not even by careful application of the rotating disk sander. You just cannot predict where the current wants to go.
Naturally, you want to the current to run somewhere down the middle of the contact region, but reality is harsh and more oftern than not the current will run through either end of the contact region. You notice it as the red growing of the blade there.
With experience you will begin to learn where the current is likely to go when the start button is pressed. That requires a lot of failures and practice.
However, it is nothing more than casual prediction and trying to move the initial point of current entry by further subtle sanding will not get you anywhere. So, forget about it. Waste of time.
So, you may wonder how in practice you may go about your welding process.
My advice is to accept that blade welding cannot be done in a single extremely short burst of current, as you often see in Youtube videos.
The reason is that with bat welding you must melt down the whole length of contact and the initial point of red growing is merely the starting position of the entire melting line.
In short, any protrusion will be the first to melt away and that will be naturally leading to one single ideal line of melting contact. That then wll be the start of the true welding.
Tale a look at the following movie. Movie-1
As you see here on the movie the Knack is to apply an extremely short burst of current, meant to melt away any small sticking points of contact to pave way to the welding of the entire contact line.
That will mean more or less an even gap between the batting ends and applying small lateral pressures with your dielectric pincher (handamde) will narrow down the gap even further. You will need to apply the current a few times here.
Here, let me now talk about colour temperature. You should be able to recognise three different colour temperatures. In increasing order to temperature it goes from carrots, navel oranges, then to lemons.
You must immediately let go of your finger on the button the moment the contact line becomes lemon in colour. Otherwise, the contact line will melt down.
Failure welding is shown in the next movie. Movie-2
Working with the grinding stone:
Note 1: Making the batting ends flat
You do not need to worry too much about this. The reason is that no matter how carefully you use your griding stone you can never ever achieve perfect flats. Ideally, you need optically flat surfaces. You cannot do that.
Many things happen. Batting ends may be tilted. They may even consist of a few different mini flat surfaces. These defects can be gradually rectified by applying the current in a few short bursts.
What I am seriously and emphatically saying here is that you can achieve perfect welding of the batting ends even if they are not pre-treated, coming straight from the shear cutting. Small surface defects are not that important.
Note 2: Making the batting faces perpendicular to blade direction
Compared with above, batting face orientation is easier to manage. In reality it is not exactly true, but you can see the faces and making them as closely perpendicular as possible to blade motion is desirable.
You cannot look into the batting faces easily, but orientation setting is a lot easier to achieve.
Note 3: Setting the blabes in a straight line
This is also important. Failing this your bandsaw blade will be making a lot of horrible noise as it rotaes on the machine. It will eventualy lad to breakage again.
However, it is something you can improve on with some ingenuities. There will be ways, such as a guidance plate, which can be either welded to the machine or connected with nuts and bolts.
Note 4: Gap between batting ends
You do not need an almost invisible line of contact for good welding. Perhaps, 1mm is just too large, but even then it can close fairly easily once the current runs through the gap. Besides, pinching also helps to narrow down the existing gap of reasonable magnitude.
Note 5: Pinching (or batting) pressures
You can actually see my handmade pincher in the movie.
I made it myself from plywood pieces. 
You need a pincher like this because once your finger is detached from the electrifying button the batting pressure is automatically removed.
I opened up the casing of this machine and examined the internal mechanism. It is a serious flaw of this machine in that batting pressure is removed even while melting is taking place each time the button is released.
Therefore, your left hand should maintain the pincher pressure whenever your right hand comes off the electrifying button. Keep the batting ends pressurised for a few seconds longer when you do that.
However, you do not need to pinch strongly as the pincher is only meant to stop reverse motion of the batting ends.
Note 6: Annealing
I actually measured my annealing process. In the case of movie-1 I let the current run 20 times in 35 seconds. With the annealing button there is no batting pressure. This buttonis only meant for current running.
In the movie-1 you can see a few heating grows, and you may regard these as failures. You should not be able to see colours at the welded line during ideal annealing.
If you see hotter colours beyond an ambient colour then it means that there are invisible welding defects somewhere and those defects have larger resistance than the rest of the welded portion, leading to hot growing.
In that case, you must redo your welding all over again.
Ideal annealing is no colour changes, just the mechanical sound from the running current. You should remove the slag before annealing with a metal brush so that you can see temperature waves easily.
If welding was uniformly done then you will see metalic colour waves (temperature waves) spreading as if you were opening curtains sideways from the line of contact.
If on the other hand welding was not uniform then dark blue/black radial waves will be seen spreading from the point of initial curent passage.
If you continue with the current several times more then the temperture waves will disappear, leaving a very dumb looking thin brown coloured surface.
If you now look at the welded blade you can see that the colour change s have only taken place in the immediate neighborhood of the batting line.
Portions outside this area remain unchanged in colour. The reason is that the metal parts around the batting end act as a large heat sink.
It is for this reason that you should not further anneal these unchanged portions with a gas burner fire, as it will lead to composition changes in the blade.
You will be making these portions into soft iron. Annealing is only meant to recover the original atomic structure of the blade as faithfully as possibe.
Note7: Colour changes and the number of current passages
There is no rule whatsoever. However, be advised that once is hardly enough. There are many internet movies of that kind, but they are no good for the reasons sated above.
Put simply, if the whole portion between the batting metal pieces of the machine grows uniformly red then that will be the end of your welding.
You often see small red-hot grows of triangular shape spreading at the very start of your welding process. That is not enough. The red-hot grow must spread all across the entire area between the batting metals.
You need to pump as many times as until you see uniform red hot clour between the terminals. However, too much will mean lemon colour and instant melting.
You can actually see this in movie-2. There, I shood have stopped after 2 passages, but I did one more passage and melting took place immediately.
Additional note:
With this machine, you cannot control the amount of current. Therefore, you cannot weld blades of 3 mm width. Current will immediately melt down the contact point. This machine is ideal for blades between 6 mm and 8 mm widths.
I have not tried wider blades, but it may be possible. I have also tried soft iron wires. 2 mm wires are OK, but 1.6 mm wires will melt immediately.
Additional note 2:
You need practice and getting used to. Initially, I did my welding 100 times or more, before serious attempts. I can now succeed 9 out of 10 times. However, you need regular practice to keep your skills.
Button pressing is such a subtle business.
Additional note 3:
You need practice before each welding session. You should aim to produce at least 5, or 6 new blades while your skills are good.
これは連結用の丸棒で、収納容器を作るかどうかは、未定だ。連結は10ｃｍ程度毎になるだろう。この部材で４基のパン切り台が出来る（はずだ）が、すべてギフトとして贈呈してしまう積りだ。