How does a swordsmith repair a chipped blade?

You have to assess the damage very carefully. A crack may propagate further than is visible at first.
This crack doesn’t look fatal, even at this distance. Notice the edge bevel is quite large, and there’s a good amount of ‘meat’ left in the high carbon edge steel. You can see a line delimiting the edge steel and the less hard steel of the body of the blade. You can polish this by grinding the visible bevel, retaining the bevel’s angle, but moving it up a little. A skilled polisher would keep this blade functional without destroying the overall shape. It would just have a little less meat on it, as if it had been resharpened many times. What you can’t see here is that the back of this blade is probably hollow, and it is made so that the user can sharpen the blade by hand using a flat stone on the hollow side. Some of this hollow side will be lost too, in order to maintain the edge geometry while getting rid of the chip.

This crack looks fine! Moon shaped, or chips that are rounded are the best kind, if such a thing exists. You’d be best off with a chip like this. This blade does not have great shape, probably from user abuse, but reshaping this by moving the edge bevel up a little more won’t break the overall shape of the blade. An expert might know it was repaired, or again it may just end up looking older than it is, having lost some of its meat during the repair.

This is the kind of crack we’d rather not ever see. This is a stress crack, and it’s fatal to the blade. Something happened here, either during heat treatment, or after heat treatment it may have been bashed on the spine and caused this to form.

This is a straight crack that starts at the edge and extends 1/3 of the way into the blade. It doesn’t appear to extend further into the blade, and blades like this were made to stop catastrophic crack propagation by the Japanese for hundreds of years. A polisher would look at this blade and say there’s nothing to be done but retire the thing. The hardest part of the blade has been cracked through, and there’s no way to repair it as is. If you grind away the edge until there’s no more crack, you lose the 1/3 of the blade that is made of the kind of steel that can be hardened.
All of the above are assessed and repaired or retired by a polisher, not a sword smith or anyone involved in “hot work”. Once a sword is forged and ground, further hot work is not worth the trouble. If a sword is chipped, and the polisher can’t grind out the chip without ruining the shape of the blade, a smith can’t do anything with it either. There are some historical examples of swords that have been repaired by carefully forge-welding steel into the blade, but that takes significant amounts of careful work that will ultimately not result in a useable blade. The hardness will be lost in that area, and the whole blade would need to be heat treated again, which would most likely fail. With a blade like the 16th century William Wallace sword in Scotland, there is a “patch” forge welded into the side of the blade which is not usually on display. I’m not sure how that got there, but it would have required some very careful forge work. It does not effect the ‘working’ part of that blade, as the first 1/3 or so of it is square and not sharpened at all. So if a polisher can’t fix it, it’s not usually worth fixing.

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