Sharpening at Tosho: Process and Considerations for Repairs

To continue on from the pilot IG post, Soondae Sunday, I’d like to take a slight detour from the culinary side and share a few things about a recent repair work at Tosho. We received this Morihei Inox Santoku blade with a large chip. (10mm+ including the crack going past the chip).

In fact, a chip & tip repair is one of the most common repairs we see. So I thought it would be helpful to share the process and the considerations we contend with. The client explained that the chip occurred when shaving kernels from an ear of corn and the blade bit into the cob; they tried to twist the blade off and… well.

After discussing the two possible repair options with them, they opted for the first option (the one with faster turnaround and the lower cost) of having the chip removed and edge sharpened only. We briefly covered the more complicated 2nd option of ‘thinning-out’ via an Instagram post HERE , please check it out. Now, to the workshop.

When doing a large chip removal like this, the assumption is to keep the same edge curvature. Since this blade has a crack that goes past the chip, where the crack ends is the top of the new edge line.

After tracing the edge shape on the blade to avoid wasting steel while grinding, the prep is done. I prefer tracing instead of eyeballing the profile when removing such a large chip, to save myself from the guesswork during the removal process.

The main thing to watch out for while removing the chip using a grinder is overheating the blade. Overheating a tempered steel changes the heat treatment of the blade which affects the edge holding and taking ability of the blade irreversibly. Best to use light pressure and frequently cool the blade down in water.

It was so cold in Toronto the day before the shoot, the water in our workshop froze over! Luckily, this helps with cooling the blade down faster.

The closer we get to the end of crack removal, the sanding belt gets worn out. This is where it requires the most amount of patience (or switching to a new belt). An overused belt heats up the blade extremely fast, due to increase in friction. In addition, the last few mm takes substantially longer time to remove due to the thickness increase.

Going slowly with patience and checking frequently as you go, is the fastest way to get it done. The more rushed you are, the higher chance of overheating.

After the chip removal is complete and profile has been established the knife needs a fresh edge.

I started the sharpening on a coarse grit belt to define the geometry of the edge with a slight right hand bias just as one of these fresh out of the box would have. With a few belts of increasingly fine grit, some felt and a good stropping the blade has a razor sharp edge and is ready for the kitchen!

Many experienced sharpeners amongst the readers will know that this blade will wedge on dense food ingredients, regardless of how sharp the cutting edge is.And even without experience, one can see the difference in the thinness between the two choil shots. These two would not perform the same way in the kitchen.

So why not thin the blade? Simply, the client does not wish it, yet. They would like to use the blade as is, and seek for additional work if they find it necessary. And that is 100% smart. When I was younger, I would get frustrated when some clients chose anything other than the only way for their blade to be fixed. Pretty funny thinking back.

Now, I focus on asking better questions to understand their circumstances, and explain the available options to them and have them make the choices best for themselves. And just get to work.