This week, the first of a two-part
series on sharpening chisels.
How often have you put off sharpening a chisel because you're in the middle of
a big project and you didn't want to take the time? I'll admit I've done that
more than once. I usually end up grabbing a sharp tool that's either too wide
or too narrow. Then I'm usually sorry I didn't take the time to sharpen the
right tool. But it doesn't have to take a long time. There's no reason you
can't have a razor-sharp, long-lasting edge in less time than it takes to read
about how to do it. This is a two-step process. This week we'll look at
grinding the bevel. Next week, honing the edge on a sharpening stone.
The first step is to grind the bevel. I use an electric bench grinder for this,
so I get a hollow ground bevel -- the bevel reflects the curve of the grinding
wheel. First, it's easier to make a uniform hollow ground bevel. And second,
honing is easier because very little metal needs to be removed to hone the edge
razor sharp.
I recommend using a 60-grit white aluminum oxide wheel for grinding. It cuts
fast. And just as important, the binder or "glue" used on a white
aluminum oxide wheel allows the particles to break away faster that the
particles in a general purpose wheel. This is good for two reasons: The wheel
cuts faster because the cutting surface isn't clogged up with bits of cut-off
metal. And since it's not clogged up, the chisel doesn't get as hot while
grinding.
I think heat is one of the biggest problems most people have when sharpening.
If a chisel or plane iron starts to turn blue when you're sharpening, it's too
hot. There's no quicker way to ruin a tool. What happens is the tool heats to a
temperature that causes it to lose its temper. (Tempering is a heat treatment
that makes the metal tougher so it will hold an edge longer.) If the metal
loses its temper, it won't stay sharp.
Grinding a bevel is easy. The challenge is grinding a uniform bevel -- one
that's the same width across the end of the tool. And one with a cutting edge
that's 90 degrees to the side of the tool.
There are a couple of simple tricks for doing this. First, adjust the tool rest
on the grinder so the chisel (or plane iron) can lie flat on the surface of the
rest, not just against the upper or lower edge. This way you have more control.
The bevel on most chisels and plane irons is 25 degrees. So set the angle of
the tool rest to grind the same bevel on your tool.
Next, I clamp a small, rectangular block of wood to the blade so the long side
of the block butts against the lower edge of the tool rest during sharpening.
This provides a positive reference point (a stop) so you can lift the tool off
the wheel to inspect the bevel or dip it in water. Then the tool can be
returned to precisely the same spot on the wheel.
Some chisels are too short to attach a stop block. In that case, you can use a
small C-clamp as a stop. But if there's room, I use a stop block because it has
the added advantage of holding the tool perpendicular to the edge of the tool
rest. This makes it easier to grind a bevel that's 90 degree to the side of the
tool.
With the tool rest in position and the block clamped to the tool, you're ready
to grind the bevel. To do this, turn on the grinder and place the block against
the tool rest and move the bevel gently across the wheel. After a couple of
light side to side passes, check to see if the bevel is even, and if the
cutting edge is square to the side.
Note: If the cutting edge isn't quite square, increase or decrease the angle
between the side of the chisel and the top edge of the stop block. Then grind a
little more off the bevel until the edge is square to the side. When the angle
is right finish grinding the bevel.
Next week: Honing the edge. |
