Thursday, December 22, 2016

Project #15 - Handle for a Kato 80 mm petty

This project together with the #16 is the first commission work which was agreed under special terms, as I am still learning basic skills and trying new things.

The wish for this particular knife was a nimble, thin handle, than would allow to use the knife as a parer.

I was also rather nervous, because it involved removing a previous custom made handle and it was a strange feeling to undo someone else's work. But hey, whatever makes the customer happy :)

The knife with the original handle and the ironwood block.

Design

The final choice of design and materials was relatively simple - a 3mm thick brass ferrule with ironwood handle. This also implied a challenge - since the bolster was going to be press-fit onto the tang, the handle would be shaped already glued onto the tang. This meant that if anything should go south, I would have to grind the complete handle off the tang and start from the scratch. I am rather happy to say that that did not happen.

Handle removal

As already mentioned - I had to start with removing the original handle which was made from some black wood, spacer and epoxied onto the tang. This was done rather quickly with a coarse belt, but I had to be careful not to scratch the uncovered part of the tang, as otherwise it would have been necessary to re-finish the whole blade - a job I was trying to avoid.

To grind off the old handle took just a few minutes.



Once the handle was removed I have ground very tiny 'shoulders' onto the tang so I would be able to get a nice fit with the bolster. I used #120 belt as I was trying to remove as little material as possible. The job was finished with files.

Grinding 'shoulders' onto the tang.

Bolsters (yes, 2 of them)

Having the experience with the 2 funayuki knives I have rehandled before working on this one taught me that getting a bolster fit on a Japanese kitchen knife is not easy, because the tangs are not ground to shape and thus their cross section where the bolster should sit is irregular.

I have decided on 3 mm brass as 5 mm seemed too much for a handle that was requested to be rather thin & narrow. 

Blade, ironwood and brass - keeping it simple :)

The process was as usual  - cutting out a piece of brass, scribing the centre lines and the shape of the tang, drilling 3 holes with 2 mm drill bit and filing with needle files to shape, keep testing how the bolster fits the tang and hammer-in the last few mm. But I failed. Even though I tried to flatten the tang from sides before I started,  I did not manage to avoid gaps on the sides of the bolster.

Yep, that is a gap. Let's try it again.

I came up with only one way how to assure that I will get the bolster fit without any gaps - and that was to grind shoulders on the sides of the tang as well. It was a bit tricky, because the tang was not too thick so I could only afford to remove very little material, before putting the strength of the whole blade in danger.

So I went back to file guide, sanded a fraction of millimetre of steel from each side and again finished with files. The tiny shoulders on left and right side gave me just enough safety margin so that after the new bolster was fit onto the tag I would get a much better fit.

Once the new bolster was fitting nicely, I would remove it and finish the front surface on a marble stone with sandpaper up to grit #2000.

Bolster with #2000 grit finish.
The front surface was covered with tape and I used the belt grinder to get it closer to its final size. I was trying to minimize the amount of grinding once the handle will be glued as the brass gets blistering hot very fast and I did not want to overheat the epoxy joint.

Mounting the handle

Before the wooden block could have been glued onto the tang 2 steps had to be accomplished - drilling and filing the tang slot and sanding the block until it would fit without gaps on the tang.

The drilling was done with a 4 mm drill bit. This was a bit larger than actually necessary, but on one hand this was not going to be visible, but more importantly I needed to be able to fit a needle rasp in the drilled hole to be able to make the tang slot.

Squaring-up the block by hand with #120 sanding paper on the stone block.

Handle block ready for gluing.

Once all parts were ready I could proceed with the gluing. I have first covered most of the handle with a tape and fixed it carefully in a vice. As usually I have used G/flex epoxy as it has relatively low viscosity (so it will albeit slowly flow inside the narrow tang opening) and long pot time so I did not have to hurry.

Important: During the gluing it is important to allow the air which is trapped inside the tang opening to escape. This can be done by slowly moving the tang in and out and going deeper just step by step. If too much air will remain trapped inside the handle it could push the blade up as the glue cures and you will be left with a gap between the wood and and bolster. This can in particular be an issue with a handle this small & light.

I have then came up with an idea to use 2 clamps to make sure that there will be no gaps - a usable idea in general, but one needs to be careful - if the force (of the clamps) on the two sides is not equal, it may actually induce a gap.

Gluing it all together - watch out for gaps!

Handle shaping

Once the glue cured the next step was to shape the handle. At this stage one should really not hurry. The handle is fixed with an epoxy to the knife - any larger mistake will mean that the whole handle - including bolster - will have to be removed and once can start all over.

Cleaning up the handle after gluing.

Tip: I was told that if one needs to removed a glued handle from a knife, the knife could be boiled (! yes - that is not a typo) and once the handle heats-through the epoxy should soften. It sounds terrible, but 100 degrees Celsius is not enough to negatively influence the blade hardness (since tempering temperatures during heat treat are around 200 degrees Celsius, give or take) and a short boil should not cause rust. But I have not tried this method yet, so use at your own risk :)

Before I would start with rough shaping of the handle I would first check whether the wood (which at this stage is nicely squared up and thus has a rectangular cross section) is in line with the blade, or whether there is some slight relative rotation. If there were any I would adjust the work rest on the grinder and first fix this.

Alignment-grinding. What you can not see is that the work rest is under
angle slightly larger than 90 degrees to the grinder surface.
Once I was sure that the handle is aligned with the blade I would draw centre lines on top and bottom of the handle as well as on the back side. Then I would draw lines that would reflect the taper if the handle width and use the belt grinder with #60 (#40 would be fine too) belt to do the rough material removal and follow with #80 grit disc grinder to clean up and flatten the sides.

About to grind the taper to the sides of the handle.

I would follow in a similar manner to grind the taper to the top and bottom side of the handle. I would of course have to be careful to tough the running blade with the choil of the knife.

Vertical and horizontal taper ground to the handle.

Once the vertical and horizontal taper was ground it was time to check in hand whether the size of the handle is about where it should be. Since the handle at this stage is just a rectangular block it is not easy to judge how it will feel once ground to octagonal shape. Sill - it should be tested in hand. Over time one develops the 'feel'. 

If everything is right than the next step is grinding the octagonal shape. This is for me a tricky part as while one is grinding on the disc grinder - it is not easy to judge the angle precisely so I would keep checking constantly. I would also not try to grind one corner completely before moving onto the next one as I wanted to keep some material as an error margin should one corner be ground under angle that would deviate from the mirrored one (left-right symmetry).

At this stage I would say that I do not aim for a top-bottom symmetry. I tend to grind the bottom corners a little more and under different angle.

Final shaping finished. Next - finishing the surface.

Final shaping finished.

Finishing

Now comes my favourite part. During the finishing the wood starts to reveal its beauty - and this is in particular the case for Ironwood.

Before I moved on with the handle I took a small piece of iron wood (and off-cut) and did a quick test on how it will respond to finishing. I gave it a quick polish with 240, 400 and 600 sandpaper and then applied a coat of True-oil. As it turned out this would have not been a good choice since ironwood is an oily wood by itself and thus the Tru-oil did not want to harden properly. I also found out that the wood reacted interestingly to steel wool. If rubbed with it for too long the grain structure would stand out as the filaments in the wood had different hardness. Still - this was an interesting test to do and, together with some information I collected online I have decided to just sand the handle and then coat it with board butter.

Tools

One important step that I did not photo-document properly was chamfering the edges on the front and rear side of the handle and the tools I used to finish the handle. To sand the handle I have used a small block of hard wood with dimensions about 20 x 60 x 100 mm. On the long narrow sides (20 x 100 mm) I have attaches a double-sided Tesa tape (the side that sticks stronger facing the wood) and then I would use it to attach the sanding paper.  The tape worked so well, that I had to change it only once during the whole process. Since the paper would clog fast with the wooden dust, I would use a small hard brush to give it a quick rub, so I did not have to change the sanding paper too often. This brushing worked of course better with coarser papers as with grit #1000 and finer the brush would not manage to get between the grains quite as efficiently. Still - it did help.

To chamfer the bolster I had to use a much smaller tool as the blade was getting in the way. I use a small piece of off-cut steel of about 3 x 20 x 40 mm (give or take) and used the double-sided tape as above to attach sanding paper.

I would sand the front and rear edges on the handle with every grit that I would use on the handle, but while I rounded the edges on the front side, I sanded the rear side under constant angle (about 45 deg) and did some rounding only with the finest paper. I simply like that look better.


Left: Handle after #240 grit,
Right: piece of sanded wood treated with Tru-oil.
So I have proceeded with sanding the handle. I really enjoyed how every finer grit revealed more and more structure of the wood.

After #240 grit.
After #400 grit.
After #600 grit
In principle I could have stopped after #600 grit, but since the sides of the bolster were also sanded at the same time, I went all the way up to #200 grit so that I would get (at least comparable) finish as I had on the front face of the bolster.

After #2000 grit.
After #2000 grit
After #2000 grit
After #2000 grit.

Warning: When sanding handle with metal parts you may notice that at the finer grits there will be tendency for the metal dust to 'stain' the wood in its vicinity. What happens is that the metal dust is so fine that it clogs the fine pores of the wood. I noticed it happening here at #1000 and #2000 grits. To minimise that I would only sand in the direction from the wood towards the brass and clean or change the paper after just a few strokes.

Once the handle was finished I applied a coat of board butter.


Lessons learned

  • Prepare the tang carefully for the press-fitting of the bolster. Do not allow the thickness change to be too abrupt towards the final position of the tang.
  • If the bolster does not want to fit with light hammering - take it off and work on it or the tang, otherwise it can bend and also the blade could get damaged
  • Be 110% sure that the blade is properly wrapped and will be safe from any damage (scratches, chips, broken tip, pressure marks, etc).
  • Becareful when sanding close to wood/metal boundary with fine grits as the metal dust could stain the wood.

Finished handle





The fine grain of the wood really pops unde the right light.


















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