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
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
Live and learn, and I hope this saves someone from making
the same mistake I did.
Visit the oldtools book list at
Since this article was published, we received a few important comments:
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.
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
Millrat in Riverside, CA.