Sunday, January 20, 2013

another Hooked Knife

Back at home and in the presence of the forge, I took the opportunity to make a proper hooked knife.  For this one, I used a chunk of steel from a leaf spring.

After annealing one end of the spring in the forge, I cut a section off, then started cutting it lengthwise with a hacksaw.  I got about a third of the way in from both sides before the blade began to dull.  Slow work, but a good way to stay warm on a cold day.  I finished it up sloppily with a reciprocating saw.

I made some sketches to help keep the goal in mind.

After a lot of grinding, I got the pits out of both sides of the steel.  Next, I slowly drilled holes in the piece and fastened it to a wooden block for sanding.
I started with 36 grit to remove the grinding wheel scratches, then moved up to 80, 120, 220, 300, 500, 1000, and finally 1200.  The last few went very quickly, as all the big scratches had been removed earlier.  I only sanded one side (the bottom) as the top would need to be dressed with a file.  I made sure to sand it very smooth, as scrapes or scratches could weaken the thin blade.
I took some shots of the bottom gradually getting smoother.
a furry, ferromagnetic friend followed the screw from the hole.
I started shaping a handle.  The wood came from a fallen limb of a tree in the back yard.  Dad says it's a Styrax japonicus.  It's a light, hard, and brittle wood.  It carves nicely.
I was excited to find some spalting in the wood.  It reminds me of blue cheese.
I chiseled a slot for the blade in the handle
then continued to shape the blade with coarse file.

a bit more sanding to remove the scratches

 testing the fit in the handle, followed by more shaping

I warmed some canola oil over the stove.  It is the quenchant.
One torch wasn't enough.  I had to use two to get the blade up to temperature.  Because of that, I had no hands left to take pictures of the quench.
After quenching, I annealed the blade by placing it at the edge of the coals in our woodstove with bits of newspaper around it.  The annealing temperature I was after was 450 F, about the same as the temperature at which paper begins to burn.  It worked, but the temper was not as even as I would have liked.  I will use an oven in future.

I had fun carving these spoons to varying degrees of completion with my new knife.  The three on the left are apple and the other is mulberry.  Mulberry is quite hard!  It's a relative of the osage orange.

I eventually bent the tip of the blade while trying to dig out a knot in the wood.  I feared this would happen, as I'd suspected that the oil quench had not been hard enough.
I also feared that I might have burnt the alloying out of the steel while bringing it to annealing temperature in the fire.
Vacation was up by now, so I brought it to MITERS with the intent of case hardening it.
I read about case hardening, and found an old recipe for a case hardening compound consisting of charcoal and bone meal.

First, I annealed the blade inside the electric furnace at MITERS.  I put the blade inside a soup can with some charcoal, hoping that the charcoal would burn, acting as a getter.

Next day, I flattened the hook and sanded the blade thoroughly -- a fresh start.

 After getting it smooth enough, I put the hook back in.  I made the radius tighter this time as I'd had trouble carving smaller spoons before.

 I made a vessel of some 1" steel pipe with cap welded on one end.  On the other, I cut a slot, and slid in a piece of thin steel sheet.  I packed the vessel with the blade along with a mixture of charcoal and bone meal (favoring the charcoal) and put it in the oven at about 1500F for four hours.

Here's a cross section of the pipe after sitting in the oven for four hours.  I cut it open to check if the case hardening had worked.  I sanded it and dipped it in a ferric chloride pickling solution to make the difference in alloying apparent.  If you look closely, you can see that a thin layer on the inside of the pipe is darker than the rest.  I hoping this is an indication of different carbon content -- meaning some carbon diffused into the steel.

Everything seems to be dandy as of 1/20/2013.  The blade holds an edge and hasn't twisted or chipped.   I'm slowly learning how to handle and sharpen this unique tool.


  1. I don't mind but it would be nice if you'd give us a little credit.

    1. Whoops, I'll make my links a little brighter. Thanks and I hope you and your folks are doing okay.

  2. Sorry missed the link. I'm getting older, grumpier and blind.