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Drawknife from a file #2: roughing out the blade form

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Blog entry by RobynHoodridge posted 03-09-2013 09:42 AM 1465 reads 1 time favorited 2 comments Add to Favorites Watch
« Part 1: planning handles Part 2 of Drawknife from a file series Part 3: general assembly »

Having got the concept of how i was going to make my drawknife down in the form of the prototype that i showed in the previous entry of this series, i moved on to general shaping of the file. To turn it into the blade of the drawknife.
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I started with a file that had lost its job and sold its body, buried deep in a scrap yard.

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I wanted to use a file that didn’t taper at the tip (was rectangular) to minimize the material i’d have to remove to get to a dead-straight cutting edge.

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And it’s perhaps even more important that there’s no taper in the thickness of the file, since flattening a whole big face would be more difficult than bringing one of the sides of a file to a straight line. Especially the way i’m doing it – by eye with a hand held grinder.

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I marked out what i wanted to do to the piece of steel with a marker pen, making some tweaks and changes as I considered what I drew. Then went over the finalized lines with a very fine cut-off disc in a dremel-like rotary tool. This shows up nicely in the rusty mileau, doesn’t burn off if the metal gets hot, and is generally more definite since it’s a marking engraved into the steel.

Then i went to work on the marked-up file, with this result:

I think you can see what i’m going for. A flat, straight blade to which handles will be attached. Here’s the roughed-out blade semi superimposed on the prototype to show the idea.

I chose to make the length of the blade a little less than previously planned. There’s this ingrained tendency in me to make the most of everything, like optimal layout of parts cut from sheet stock, etc. But i fought that ‘urge’. The 236 mm (9 1/4 inch) blade that i’ve ended up with is long enough; i have no reason to need longer; and making the edge shorter separates everything that will be going on at the points of attachment on the ends from the blade region. Making it less fiddly to sharpen for instance. Sometimes less is more.
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And speaking of the handle attachment – i’ve opted to rivet on / in handles, rather than trying to weld to the hardened file or using fixings like nuts and bolts. The brass rivet bar will go through holes drilled in the mild steel strips that the handle will be made from, which line up with the more open parts of the grooves that are ground into the file. If welding a hardened file is advised against, you certainly shouldn’t even try drilling through one. So the grooves were my way of opening up a space for the rivets. I might not use all three holes. Two will hold things together, and counter-anchor each other to deal with forces in all the directions that a drawknife will experience. But i put the three grooves in to keep my options open for now.
A 1 mm cut-off disk in the grinder made the grooves by two cuts angled slightly differently.
In order to retain the hardening in the steel, all the grinding should be done without over-heating the steel (which you can tell has happened when it changes colour, like around the grooves for the rivets). But there’s no reason to keep the hardening on parts other than the actual blade part, and i’m not OCD about these things, so i let the cuts get a little hot in exchange for getting it done faster.

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The flat-bar strips of the handles i plan to attach form loops so that there are two ends of it to consider attaching to the blade. One end is going to lay upon the side of the file that has the bevel (on ‘top’ and out of the way for most cuts since i use a drawknife mostly bevel-up). So i’ve cleaned up (ground flat) the contact area for this meeting, as you can see above in the triangular shape. (The handles meet the file at 45 degrees.)
And i’ve decided to make things complicated and difficult for myself by having the other end of the hadle fit into a slot cut in the middle of the thickness of the file.

So the rivets will go through successive layers of handle and blade. Sandwiching and locking them together. Each layer of the file here is only about 1.5 mm thick. But i’m not too concerned about this ‘leaf’ snapping off when they’re pressed together by the riveting, since the gap will be full of uncompressable handle.
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The flat side of the blade also got attention, where i ground down through the teeth of the file to smooth it out.

It’s easy to remove more material from the sides of the face (edge of the file) than in the middle. The “trick” to not doing this is to grind the area near the edges down to the desired depth first, using the lines in the steel that were the bottoms of the teeth as markers of the depth you’ve gone to. Once they’ve only just disappeared, stop. And once you’ve reached this mark at the edge, fill in the rest of the surface (which is less important) by doing the same. In fact i didn’t even go all the way passed the marks near the cutting edge, so that i’d have this little extra to play with when properly (finely) smoothing the face on abrasive paper and removing the grinding marks. And so that i’d still have these depth indicators when doing so, to aid in ending up with a perfectly flat (straight) leading edge.

You can also see that i’m going to need to lose a millimeter or two at the edge, to lose the irregularities of the chipped file. The opposite side of the file actually has no teeth (safe) and i chose to make that flat side the back of the knife so that i could have a nice, easy, perfectly flat (use as a straight edge), reference side on the knife. But i’m kinda regretting that choice now.
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Now it’s time to spend a little energy lapping the face on this guilotine blade with abrasive paper glued to it.

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The region of the drawknife where the blade ends (sides of the blade) is something i haven’t heard much about in my research on this tool. In fact i’ve heard nothing other than that some people like to round off the very corners of the blade because they’re dangerous and don’t see any use anyway.
Even in traditionally shaped drawknives though, there’s a place where the blade section (which needs to be perfectly flat both for good use and to aid sharpening) meets the rest of the metalwork (i call them the shoulders, which lead to the tangs). And so often the blade’s flat face is inset relative to the shoulders. Meaning that a flat sharpening stone can’t pass over the whole of the blade’s surface, or rides up onto the shoulder when you try. Leading to uneven sharpening.
The answer is to have a region adjacent to the blade’s flat surface that protrudes less, not more, than the flat itself. I’ve done this by grinding a light furrow over the very side edge of the blade (see below, right). One could remove metal down to this level from everything outside of the blade. Which might be nice in a version with removable handles so that you could simply lap the whole thing over a stone and not worry about contacting anything other than the face of the blade.
Here are top and bottom views of the area where the cutting edge ends:

I chose to center the furrow over the gap that essentially is the side of my blade, because i don’t mind losing a couple of millimetres at the very tips of the cutting edge (they won’t be sharpened with the rest of the blade and will be set minutely further back in the blade) since they’re more dangerous than useful. This way, the edge of a sharpening stone can move in the space under the furrow, with the whole face of the blade in contact with the stone and getting flattened. And without interference from the neighbouring parts.
But there’s another aspect to a drawknife made from a file the way i’ve done it that might make this little furrow unnecessary, at least from the safety perspective.

By sectioning the file into ‘blade’ and ‘the rest’ on the sides of the blades with a simple cut, there is left a big chunk of steel protecting the corners of the blade form stray fingers, etc.. Especially with the furrow discussed above, leaving this part of the file in place isn’t going to get in the way of sharpening (much). Nor should it get in the way when using the drawknife (except for very special circumstances). I may still fine-tune the shape of these surrounds. But generally i’m glad to get to include them in the shape of the knife.

Something i’m a little worried about is the appearance of fine lines that must be cracks in the steel between teeth on the upper and lower surface of the file. Hope they’re just skin deep. They’re the barely visible light orange (rust) lines.

-- Never is longer than forever.



2 comments so far

View jap's profile

jap

1231 posts in 721 days


#1 posted 03-09-2013 01:32 PM

very cool, seems to be coming along well

-- Joel

View RobynHoodridge's profile

RobynHoodridge

126 posts in 996 days


#2 posted 03-09-2013 03:27 PM

Thanks Jap.

-- Never is longer than forever.

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