Metal Musings Blog

After testing my crook knife, I liked it so well I decided to make another, with less “crook” to it.  In the Pacific Northwest, this one is called a “planer” crook knife, since it is used to produce a nice texture on flat or convex curved surfaces (not used so much on the inside of bowls).

Here’s the paper pattern glued onto a sheet of 1075 high carbon steel (the white paper on the lower left corner).  This sheet is 1/16 of an inch thick.

Above is the blade blank cut out of the sheet.

With my micromotor grinder and a small carbide burr, I’ve carved two little divots in the center of the blade where I’m going to drill the holes to fasten the blade to the handle.  These little divots will help keep the drillbit from wandering and drilling off center.

I’ve drilled 1/8 inch diameter pilot holes.  I’m going to “sneak up” on the size holes I want.  Hich carbon steel is a problematic material to drill holes in – if I start with too large of a drill bit, I run the risk of work hardening the steel before the bit cuts through.  This will locally harden the metal and cause the drilling to stall out, causing the drilling to stop, and damage the drill bit as well.  I never have problem with 1/8 inch diameter bits, so I’ll start with that, and drill several times with the next size up until I finally reach the 1/4 inch diameter I want.

Here are the 1/4 inch diameter holes.  They’ll fit the screw posts seen above the blade.

Now it’s time to begin shaping the blade.  I’ve cleaned up all the sides, and now I’m going to use an angle grinder to put in the initial flats in the business end.  I’ve clamped it in my vise horizontally.  There’s always an “easy” side of the blade, and a hard side.  The top edge of the blade is the “easy” side in this case.

Here the flat ground in the top side.

Now for the “hard” side.  I’ve installed the blade in a vertical position in the vise, and I’ll grind in the flat on the right side of the blade – this is the awkward side for me.

Here’s the blade with both flats roughly ground in.  This is the top side of the blade, and the blade will curve “up” as it sits here when I’ve finished.  I’ll hammer the curve in later.

Now I’m going to use a file to do the fine finishing in shaping the blade.  This is my filing setup.  I use a woodworking clamp held in my woodworking bench, with a long thin piece of wood underneath to keep the blade from bending during filing.

Here’s a closeup of the business end of my filing rig.

Here I’m draw-filing the left hand side of the blade.  Remember when I said there are easy sides and hard sides?  In filing, this is what I find to be the “hard” side.  The red color is what I call the “Sharpie Trick.”  I color the metal with a dark colored (red in this case, it helps hide the blood!) Sharpie permanent felt marker, then file.  You can see here the places where the color is left after filing – those areas are still below the surrounding surface, indicating I need to do more filing.  I’ll re-apply the Sharpie, file, and repeat several times.

Here I’ve finished preliminary filing with a Nicholson Bastard file.  You can see the file marks (I re-colored with the Sharpie, and filed again).  This is a medium file, and leaves a fairly coarse finish.  I’ll use an almost worn out finer cut file for the final finish.

Here’s the final finish on this side (the worn fine cut file).

I finished up the other side, paying special attention to getting both sides even, and meeting in the center of the blade.

Here’s the back side of the blade, just left flat.

Now to start on the handle, here drawn in side profile.  I’ve got a nice piece of Yew wood for this handle.

I cut out the area around the prospective handle.  I’ll leave it square and flat like this until I’ve finished drilling the holes to hole the blade in, and cutting the bottom channel the blade will rest in.

Here’s the blade ready to heat treat.  Above, I’ve dipped the blade several time into a solution of boric acid and alcohol, burning off the alcohol each time, leaving the boric acid behind.  This is an old jeweller’s trick – the boric acid will melt, forming a kind of glass that will help keep the blade from losing carbon when exposed to oxygen when I heat it up to an orange heat.  This blade is so thin, and I have to leave the cutting edges so sharp that decarbonization is a real problem if I overheat the blade.  I can’t leave the blade a little thicker as I normally do, since once I’ve hammered the blade into a curve, it will be really difficult to do much grinding on it after heat treatment, and it will be way too hard to file.

And here is the blade after hardening it in warm vegetable oil.Here’s the procedure I use for heat treating a blade:  I put a railroad spike in the forge and start heating it.  When it is at a nice glowing orange, I’ll dump it into a coffee can of vegetable oil (to heat the oil).  Remember to keep a fireproof lid on hand just in case things get a little excited – the red hot spike will flame the oil a bit, and there is a possibility of catching the oil on fire if it gets too hot.  The lid will smother the flames!

In the meantime, I’ve held the blade in the flame emerging from the propane gas forge, watching it carefully to a low red heat, and then tapping it with a small ball peen hammer on the horn of my anvil to put in the amount of curve I want.  This will take just a few light taps.  Then, I’ll “normalize” the blade at least once to refine the grain in the steel.  Normalizing is heating the blade up to “critical” temperature, in this case a low orange color (and non-magnetic – the point where the hot part of the blade will not stick to a magnet), then allowing it to cool slowly down to black heat in still air.  If I’ve done a fair amount of forging, on a blade, then I’ll repeat this process at least three times.  Once is probably fine here, since I’ve not really done any forging on it with the hammer.

After normalizing, I’ll heat it back up to the dull orange color (and non-magnetic) critical temperature, then QUICKLY quench it in the warm vegetable oil.  This will make the blade as hard as glass.  I check that with a file (it should skate off the metal, not cutting into it at all).  This is WAY too hard to use as a knife, since it is also as brittle as glass.  the blade can shatter if I bump it hard, drop it to the concrete floor, and sometimes just for spite sitting around.  To fix that, I IMMEDIATELY popped it into an oven at 425 degrees F for several hours to “temper” the blade.  Tempering will draw back some of the hardness, and reduce the brittleness of the blade.  I adjust the temperature of the tempering cycle, depending on the use I intend for the blade – for instance, on the first crook knife, I tempered it at 450 degrees F, since I wanted it less hard, but more “springy” considering it has much more “crook.”

Above is the completed blade, cleaned up after the hardening and tempering cycles.

I’ve clamped the blade into position for marking on the bottom of the handle.  I traced around the blade, and will be removing wood for a channel to set the blade in.


And here’s the handle marked, and ready to cut.

OK, I’m cheating again with my Sherline mini milling machine.  I could have done the same thing with knife and small chisel.  I just don’t believe in suffering just to build character, especially when I have a milling machine just sitting there, all lonely and unloved…

Above, I’ve used a knife to trim everything up for a nice close fit, and set the blade into position, ready for drilling.

Using a 1/4 inch diameter drill bit, I used the blade as a drilling jig.  This keeps everything lined up, guaranteeing a good fit.

Another little trick I use is to insert the screw post into the first hole as a locating pin, making certain the second hole will be drilled accurately.

I left the handle nice and square up until this time to make milling and drilling easier.  Now, I’ve cut the top profile of the handle with a bandsaw.  It’s important to think the order of these operations through, or you will work yourself into a corner…

Next, I cut the two sides off.  Still easy to do, because the bottom of the handle is stille nice and flat.

Lastly, I cut the bottom of the handle to shape.  This is the trickiest cut, since there isn’t a flat spot, and there is risk of the handle twisting during cutting, and the cut isn’t square to the handle…but, here it doesn’t really matter if the cut isn’t really accurate, I’ll just adjust during the final shaping of the handle.

I installed the blade and the screw posts.  Notice the front screw post is too long, and will have to be trimmed below the surface of the wood.  I did discover something I hadn’t anticipated – the threads in the screw post don’t go very deep, so when I trimmed the screw post shorter, the top screw bottomed out before it should have, so I had to trim the screw shorter as well.  I’ll avoid this problem next time by buying a shorter screw post to start with.

Above, I’ve test assembled the blade and screw posts.  Everything fits just fine…

I used a little cold gun blue to darken the blade, this will help with rust prevention, and I like the color better.

Above, I clamped the handle in various positions in my filing rig, minus the stiffening board, for shaping.  I did it all with a couple of small spokeshaves.

Here’s the handle shaped and ready to put together, along with the two little spokeshaves.  These inexpensive spokeshaves are some of my favorite tools.  They are available from LeeValley.com.

And, finally, there are both of the crook knives, finished with some Watco oil finish (mostly linseed oil).  I still need to sharpen the new one, then I’ll see how it works.

What’s a noggin?  It’s a small, carved wooden cup carried by frontiersmen, often carved from pretty burl wood.  This one is just cherry.  It will hang from a chunky button on a string that goes under a belt, so the prospective Dan’l Boone has it handy to get a drink out of the crick with, and doesn’t have to take his eyes off the dangerous woods by lying down to get a drink.

Bandsawn blank out of a piece of thick cherry I had on hand.

Using a Forstner bit to begin roughing out the hollow.  No reason to make this painful…  Do the drilling now while the bottom is still flat…  An old timer would have carved this out of a burl while it was still green (wet) so the carving would be easy.  I’m using well seasoned cherry so carving this strictly by hand would take many days of painful hand labor.

Smaller Forstner bit to continue the hollowing.  Don’t let the drilling get too close to the bottom or sides, and remember there is a small brad point on the Forstner bit that penetrates farther than the flat bottom of the hole…we don’t want the cup to leak.

I’ve been wanting a set of crook knives for a number of years now, and made one to test out the concept.  It works great!  Now I need several more with differing amounts of “crook.”

Speaking of painful, I decided to cheat a bit and roughout the noggin with my Foredom flexshaft grinder and several shapes of Typhoon carbide burrs.  This makes mountains of fine dust, so a good dust mask is mandatory.

And, starting to refine the interior with the crook knife.  It works really well, and leaves a nice texture.  I’ll be carving the inside and outside off and on for the next week or so.