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  Chisel Geometry by Ken Greenberg    

I had a bit of a sharpening epiphany in the shop yesterday and it brought up a reasonably important point that I have not seen discussed here - nor do I address it in the sharpening classes I teach (yet, anyway).

I recently acquired a Buck Brothers square sided socket firmer chisel in one of the few sizes I don't have and decided it was a keeper, despite having a fairly bad handle. So I went through the usual steps in bringing it back. Made sure the tip was brought back to square, did a hollow grind to 25 degrees, worked the back all the way up through the grits, and started to work on the bevel.

Here is where the problem arose. I started out working freehand since it is easy enough to "click" a hollow ground edge into place on the stone, but something just didn't look right after a while.

When looking at the end of the chisel, the arris was tiny on one end but thicker on the other. So I switched to the VERITAS jig and set it up for the proper angle, and went to work on the coarse stone.  Instead of creating a flat (or a pair of flats) evenly on the bevel, the flat was proceeding diagonally across the bevel from lower right to upper left, still with one side of the arris much thicker. 

I examined the chisel to see if it was twisted in some way, but no.  So I stopped and applied a bit of thought to the problem and decided that the only reasonable cause of this behavior would be if the chisel was actually thicker on one side. It would have to be quite significant, and when measured this was indeed the case. One edge of the chisel was about 1 mm thicker than the other.

The reason this totally messes up the geometry of the arris is pretty obvious if you stop to think about it.  Clearly, when you flatten the back (which some would call the face), it is its own reference.  But many of the jigs we use - VERITAS, General, etc. - are referenced to the "opposite" side of the chisel.

 

That is, the surface opposite the back, where the logo is usually stamped.  Even the grinding jigs are often this way.  Since this article was originally published, a number of people have kindly pointed out that there are grinding setups on the market that would be able to sharpen tools with this defect properly.  While many grinding jigs reference the bevel-side of the tool, the VERITAS Mark II honing guide references the face or back side. (The VERITAS grinder tool rest references the bevel-side, however.)

Don Naples, who developed the Lap-Sharp, notes that the jig associated with that machine also properly references the back.  So my best advice is to carefully consider the design of the tool-holding mechanism for any sharpening setup, motorized or hand-powered.  Normally this is not a problem, since the two faces of the chisel are very close to parallel.  But it can be a problem for a poorly-made tool.

In this case, someone had ground the back flat but out of parallel so the chisel was not "square." The result of all of this was that the arris, instead of coming to zero thickness all the way across, became wedge-shaped.  It was really easy enough to see in the case of this chisel just using the naked (OK, with corrective lenses) eye.  The solution was simple.  I just reground the back to be not only flat but parallel with the help of some electrons.  So much for the mirror finish I had put on it, but it clearly was never going to work without this fix.

Sort of amazing that in all the dozens of chisels I have ever sharpened in my life, I have never run into this phenomenon before.  But when you get a bunch of old tools that have unknown origin and have clearly been abused as some of the other tools in the box were, you really need to check for this possibility or you will waste a good deal of time in rehabbing the tool. I used a micrometer (B & S #11) to let me know when I had things back in order and could restart the sharpening process.

Live and learn, and I hope this saves someone from making the same mistake I did.

Ken
April, 2006

Ken Greenberg
http://www.calast.com/personal/ken/wood.htm
Visit the oldtools book list at
http://www.calast.com/personal/ken/booklist.htm

Since this article was published, we received a few important comments:

Paul Womack

The article says :  "But all the jigs we use - VERITAS, General, etc. - are referenced to the "opposite" side of the chisel."  This is NOT true of the VERITAS Mk II.
BugBear

James D. Thompson

VERY interesting, and something I had never even considered. Now I am going to have go out to the 50 plus bench chisels I own and check every one of them with a micrometer.

I can easily understand, now that it has been pointed out by Ken, that forging is not a precise process and it would be easy for this error to creep in. The polishers would not even notice it unless it were a really glaring problem. I love this list!
Jim Thompson, the Old Millrat in Riverside, CA.

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