Thursday, August 20, 2020

Sharpening a Drawknife

My drawknife is nothing special.  I bought it about 5-6 years ago off Etsy.  But the steel seems fine, the handles are firmly attached and it's comfortable to use.

Unknown maker 8" drawknife
The bevel side is fairly heavily pitted.
Fortunately the back side is in better shape.

There are some drawknife sharpening videos on the internet.  Curtis Buchanan has one here and Mike Dunbar shows a different method (with a different sharpening philosophyhere.  Although I've sharpened my drawknife and used it (a little bit), I've never gotten it truly sharp.  But a couple weeks ago I took an online class about drawknives with Elia Bizzarri.  During the class, I learned a few things that really helped.  When I got out to the shop to try out the new method, I was able to get the knife sharper than I had ever gotten it before.

The real trick for me was to create a hollow grind (definitely not Mike Dunbar's preferred method).  But don't stop at hollow grinding the bevel; you've got to hollow the back slightly so it is easier to flatten it right up to the edge.  Of course, I'm doing this with a  hand-crank grinder and that creates challenges of its own.

The grinding setup

The grinder is attached to a board that is clamped into the end vise.  A block of wood, with a smaller piece of wood on top of it that is set back 1/4" from the front of the larger block, is clamped to the benchtop.  That small piece of wood creates a little ledge across which the drawknife slides.  The distance of that ledge from the wheel determines where on the knife the grinding wheel will hit.  The block of wood is at an angle to the wheel - if it was perpendicular to the wheel, the grinder's handle would hit the right side of the knife every revolution.  

Here's a birds-eye view of same

Because I'm using just the corner of the wheel to grind the metal away, I used the wheel dressing tool to create a little flat at the corner.

In this close-up shot, you can see the chamfer on the wheel's edge

Because the part of the wheel doing the work was so small, it needed frequent dressing as I ground the knife.  Grinding the bevel went fine, but was slow because I was very careful to creep up to the leading edge.

Holding the handle, I tried to keep the knife in the same line as I
moved it across the wheel.
The bevel evenly ground.  It's deeper at the right because (for some
unknown reason) the knife is thicker there.
A close-up.  Could this knife be laminated?

Grinding the back was similar, but had to be done in steps because there is a broader surface that must be taken down.

Red marker to guide the way
(ignore the prior grinding marks from a hack attempt long ago)
First quarter of the knife's depth done with care.
Pen points to where I still need to grind.
After two more rounds, moving further towards the back each time

Now it's on to the diamond paddles.  You can see them lying on the bench in the pictures of the grinding setup above.  These things really come in handy and are the right tool for this job.  The hollow grind on the bevel gives two reference surfaces to rest the paddle on.  It's a matter of rubbing back and forth to abrade away a little metal at the front and rear edges of the hollow grind.  But be careful!  A little slip and you could slice your finger to ribbons.

You can see the front and rear edges of the hollow grind getting shiny at left end

The grip of the knife is important here.  In the picture above, one handle is firmly on the benchtop and the other end is held in my hand.  The tool's edge is pointing away from me - this is important for safety.  Another grip that I really like is shown below.  Hold the far handle and pull the near handle into the chest for a nice stable knife.  This allows me to hone the hollow grind easily.  Again, be careful!  The wrist can get a little close to the cutting edge, so pay attention.

Holding the knife handle between hand and chest ...
... allows good control when honing with a diamond paddle

I used the fine and super-fine paddles, on both bevel side and back side, switching back and forth until the burr was ready to fall off.  Then went to the strop.  This was also interesting - you've got to find a way to hold the strop so the drawknife's handles don't get in the way when stropping.

Sliding the knife left to right as I pull back towards myself

I used about 20-30 strokes on the bevel side and a few on the back side.  I typically use a thumb-nail to test sharpness of my edge tools.  This method of sharpening has by far given me the best edge that I've obtained on the drawknife.

That's all well and good - the proof for me was in the cutting.  I just finished a 2/3rds scale "democratic" chair (one of Curtis Buchanan's designs).  And I don't have access to green wood!  But the drawknife cut the (presumably) kiln dried red alder nicely.  I'll post about the chair next time.

12 comments:

  1. Thanks. I will have to watch the video. A year ago I took my drawknife to the local farmers market. There is a knife sharpening person there. He seemed kind of excited to have something different to sharpen. He got it better than I could. I still think it could be better. I need to get a grinding wheel as I think that will help.

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    1. Interesting - I would not have thought of bringing my tools to the sharpener who comes to our farmer's market here in Mountain View, CA. The grinder is a good investment. My hand cranked grinder has been great. I can see how a powered grinder would be nice, but I like using the old one and I also like the challenges it poses.

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    1. Hey, thanks Zach. I was just thinking this morning about your book, "With Saw, Plane and Chisel". I've gotta re-read it (again) - so much good stuff in there!

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  3. I glue my DMT slips to a piece of wood about a foot long, this keeps your fingers away from the edge, I have been sharpening D Knives for 40 years Sussex Trug making, I find a perfectly flat back is difficult to use, small back bevel over the years has worn on the back, Handles bent down 30 Degrees makes using the D Knife much easier, wrist at a natural position

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    1. Thanks for that. Have to admit I had to look up "Sussex trug". Looks like an interesting bit of woodworking.

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  4. Sussex Trugs centred on the village of Herstmonceux East Sussex UK 3 commercial makers, a few part time and two makers in New Zealand

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  5. Yes Matt your blade is laminated. As were the majority of cutting edges tools back then. That tell tale color difference can be very visible at times. If you pay close attention while hand sharpening you can also feel a slight difference in how the softer and harder metals "feels" or abrades. Very small diff, but its there

    Bob

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    1. Thanks for that, Bob. I have no idea how old this drawknife is, so I didn't know if it was from the time of laminated irons. I'm surprised that they were laminating a knife 8" wide. I don't know what technology was used to do it, but that's a lot wider than laminating a plane iron, for instance.

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  6. Same technology used for "bitting" an axe
    Bob

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  7. I think the handles would get in the way of my new slow speed bench grinder, but I won’t know for sure until I try. Sadly though I’ve had no use for my drawknife. I remember when I first got it from the blacksmith I had to flatten the back and then stone the entire bevel. The whole process took 10 hrs and it was sharp and I was exhausted. I finished the sharpening process by stropping it using a buffer on the lathe which you could do on your grinder. This process took the sharpness of the blade to another level. The blade was so sharp that when I did the finger nail test as you do it sliced the nail like butter. I was extremely fortunate I didn’t slice my thumb off. I’ve never experienced this level of sharpness in my life and I understood then what ninja sharp meant. I guess the length of sharpening is probably why I’ve never used it, but I think I will from now. Because if I can grind it on my bench grinder, that hollow will shorten the time of sharpening to only a few minutes like it has for my bench planes. Great post Matt.

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    1. Thank, Salko. If you want a quicker sharpening method, then Mike Dunbar's is pretty quick. But no matter what you choose, don't let it go too long without sharpening - you can "touch it up" to keep it sharp.

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