Tuesday, October 4, 2016

Project #11 - thinning & refinishing Hide 225 mm gyuto

OK, this is not a knifemaking project in the true sense of the word, as I did not make a knife (or even a handle) here, but since I did quite some work on the blade I though it could be interesting to share.

My Friend bought this Hide gyuto in a, let's say 'well used' condition. The knife had some heavy patina on the blade. Also - the blade was apparently never thinned, only sharpened on the edge what left the knife thick behind the edge. At the same time, the knife was ground relatively thick - in particular near the tip (I have not seen such a thick tip on a gyuto before). Also the handle (otherwise a very nice one made from Ichii wood and water buffalo horn ferrule) needed to be touched-up. The knife has a nice Ichii saya, but that one did not require any servicing.

After discussing what should be done with the knife I have agreed to thinning and refinishing the blade. Please do note - this is the first time I have attempted such a work on a traditional Japanese kitchen knife. One may argue that learning on something cheaper than a 500€ gyuto would be a good idea. I would tend to agree.

The ToDo list was the following:
  • Thin the blade behind the edge and up to about 20 mm from the edge
  • Thin the tip (distal taper)
  • Sand the blade to 2500 grit
  • Make finger stones (from a soft natural sharpening stone)
  • Refinish the blade to a kasumi finish with the finger stones
  • Refinish the handle.

Before starting

It is important to check the blade up close and understand what grind it has, whether it needs thinning and decide how the thinning should be done. It is important to notice whether the blade has any sort of bend, what kind of grind the blade has. Failing to notice a bend to a blade would have considerable implications during thinning as because of the very shallow grinding angles even the slightest deviation from the plane the blade would be ground asymmetrically in the vicinity of the bend.

Last but not least - one should have as clear idea as possible what should be the outcome of the refinishing, as there is no way going back once you start to remove metal. This particular knife

This particular blade had an asymmetric grind. The left side was relatively flat with a shallow hollow grind higher up the blade face, while the right had a strong convex. For me it meant that if I did not want to disturb the grind too much, then most of the metal removal would happen on the right side.

Here are a few photos that show the knife before I started to work on it.

Choil shot of the knife before thinning. The asymmetry of the grind is well visible.

The tip of the knife - definitely needs thinning to give it some distal taper.

Some heavy patina, but no rust or pitting.

You can see here that the core steel is more revealed close the tip.
Patina can have such cool colors.

Thinning of the blade

I have used Atoma 140 as I needed to remove a lot of material - over large area the blade needed to 'loose' 0.2 - 0.5 mm in thickness. The price for the speed of Atoma is that it leaves deep scratches and in particular if small pieces of removed metal stick to the surface of the Atoma they may leave nasty deep scratches.

Note: When thinning on a coarse diamond plate make sure to stop before you reach the desired thickness as you will remove more material with coarse stones or coarse sanding paper to remove the scratches left by the diamond plate.

The main point when working with the Atoma was to avoid working too close to the edge as that could leave to unintended material removal directly at the edge what could mess up the profile of the knife.

To get the work done I have used similar technique like in the Project #3 - applying pressure at constant distance from the edge and moving the point where the pressure was applied step-by-step from the heel towards the tip. I was measuring the thickness of the blade regularly with a digital calliper.

About to start. Stone holder, Atoma 140 and digital calliper.

Removing steel up to about 20 mm from the edge.

Right side after some work.

Left side after some work.

Moving higher towards the tip and grinding the distal taper.

Left side after more work - the concave part of the blade can be seen.

Once I was 90% where I wanted to get with the blade thickness and distal taper, I have moved to JNS300 and started to remove the scratches left by the Atoma.

Once the worst scratches were removed I moved to #180 sanding paper. To be sure that I was going to remove all remaining scratches from Atoma and the JNS300 and, at the same time, to be able to proceed as fast as possible, I have sanded the knife parallel to the spine.

The thinning removed quite some amount of cladding close to the tip on the right side of the blade what also made that part of the blade hardest to get the scratches out.

After 180 grit sandpaper. Still a long way to go.

After 180 grit sandpaper. The tip shows that more of the
core steel was revealed during the thinning.

Once (after about an hour) I got all (well, nearly) the scratches removed I moved to #240 grit paper. Again - I have changed the direction so that I could be sure that once I am finished all #180 scratches are gone.

One point worth mentioning is - because of the concave grind on the left side of the blade I had to find a way to get inside that hollow part. I have used wine cork wrapped in sanding paper to do so. While it was awkward to hold, the curved surface of the paper did sand the steel rather fast.

Sanding with 240 grit. The cork serves as semi-stiff support tu get
 inside the concave part of the blade.

240 grit finish.
As always - it was harder to remove the scratches from the core steel - on top of that I had to be careful not to 'slip over' the edge and damage it. After all the material removal and sanding the edge had basically zero thickness.

Hagane is hard and thus the scratches are harder to remove.

Some more Hagane scratches. Oh well.

After #240 grit I have proceeded with #320, #400, #600, #1200, #2500. After #600 I only sanded along the blade trying to get as smooth finish as possible. Also - with every step I tried to leave as smooth finish as possible to make it easier during the following (finer) grit to observe the progress. #2500 left semi-mirror finish.

#400 grit finish

#400 grit finish. Interestingly - the core steel seemed to oxidize faster]
than the cladding.

# working on the #600 grit finish

#1200 grit finish

#2500 grit finish

Once I had an acceptable (far from perfect) #2500 grit finish I have moved to finger stones as at this stage there was barely any contrast between the core steel and the cladding.  See my article on the fingerstones to find out more how these were made. Here I would only mention that these fingerstones were made out of soft natural sharpening stone and were thus easy to use, but each of them would last only a few minutes.

Setup for the finger stone finish.

Already after few strokes the fingerstones were creating mud.

The contrast between cladding and core steel starts to show as the cladding
turns cloudy, but the core steel gets polished.

After about 10 minutes with fingerstones.

Figures above: Just to show the difference:
left: blade with #2500grit finish on the left side
right: finger stone finish on the right side

Boosting the contrast in the blade in Lightroom to show its structure.

Boosting the contrast in the blade in Lightroom to show its structure.

After some work with the fingerstones a strange pattern showed on the cladding - like a set of parallel lines (see below). I could not figure it out where did it come from (maybe a pattern I have ground into the blade while sanding it?), but luckily these disappeared with some more work with fingerstones.

Strongly boosted contrast to show the pattern.
It took me about 10 hours of work of grinding and sanding until the blade was finished. Since I did not take much are about the handle - at this stage it was covered and soaked with fine steel mud and not looking pretty. To remedy that I have sanded the handle with 400, 600, 1200 and 2500 and then polished a bit with steel wool. A few coats with board butter and it came back to life :)

Photos below are courtesy to my friend as I was too short of time to take some.

Refinished handle. That Ichii wood is really lovely.

It really does cut now!

I need to get one of those cherry cutting boards.

Lessons learned

  • Complete re-finish of a knife is a LOT of work. You get faster with experience, but it still takes many hours. 
  • Atoma 140 (or any other diamond plate or stone) will tend to tear the cladding leaving deep grooves on some places. Not every cladding reacts that way - test before you go with full power as you could end up with blade thinner than intended (once you remove those deep scratches)
  • When you start thinning on a diamond plate or very coarse stone (like 150 or 220) - do NOT start at the edge, but higher up the blade. If you bring the edge to near zero thickness too early in the thinning process, you risking removing too much of material at the edge (because you just can not hold perfect angle by hand). Once you get the parts of the blade that are further away from the edge finished only then move closer to the edge. The very last 2-3 mm are best made on 300 - 500 grit harder stone with good feedback.
  • While thinning - measure (or otherwise check) the blade regularly and in particular if using corse diamond stone or very coarse sharpening stone - stop about 0.1 - 0.2 mm (thickness wise) from what your target is because you will remove more material during the scratch removal. Process. Once you get to about 240 grit sanding paper the material removal will become very little.
  • If you want to put a nice hand-sanded finish on the blade you need to have '0 scratch policy' before you move to higher grit. This is in particular important if you aim for near mirror or mirror finish - that will make every scratch that you left behind stand out and your only chance will be going back to much coarser grit and thus wasting hours of work.
  • Finger stone finish on san-mai knives needs at least 1000 sanding paper grit finish, but higher is better. Once the blade was sanded nicely, the work with fingerstones does not that long.
  • Thinning produces a lot of fine steel and stone mud and it will get everywhere. Wear surgical gloves if you do not want to have really dirty hands for several days.
  • The same goes for handle - if you want to keep it clean, either remove it (if possible), or oil it and cover it with some sort of easy to remove tape (or food foil) while working. However do not leave it wrapped for longer time as there will always be some water getting onto the handle what could cause problems if left wet for longer time.
Last but not least - a big thank you to my friend Jan who trusted me enough to let me work on this special knife. The biggest satisfaction for me was hearing that the knife cuts much better now - made the whole effort worth a while :)

And as always, your questions and comments are most welcome :)

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