The
process of Rockwell testing was patented back in
the mid 20's by a couple of guys named Hugh and
Stanley Rockwell to determine quality control on
the bearing races that they manufactured. They needed
to know (like me) if they were producing a good
and consistent product. Hugh and Stan developed
it for themselves but found a commercial use for
it and later patented the machine.
Every
so often I take my woodcarving knives and hunting
knives to a local manufacturing plant and have the
Rockwell hardness tested. I do this because I want
to know if my hardening methods are still effective
and
consistent. It really is a neat process and I will
attempt to explain in simple language.
How
is it done?
Well, it really is pretty simple. I give the testing
person a flat heat treated (hardened and tempered)
sample. It must be flat or the test is inaccurate.
He then puts it on the bed of a machine that looks
a lot like a desktop drill press. He sets it up
by pushing some buttons to program the test. The
machine starts by pushing down a small cylinder
with a point that looks like a pencil. It first
places minor pressure into what would be the blade
edge and then takes a second pressure test in the
same exact spot with a more major load. 150kgf,
(150 kilograms of force) or if you're like me and
haven't learned the metric system, approximately
331lbs of force on a tip of a diamond point. That's
like having your brother-in-law standing on your
knife blade. Yikes! This leaves a nice little dent
in the blade. The depth of this dent (Shallow =
hard, Deep = Soft) gives us a measurement of hardness.
The difference between the two resistance numbers
(major load and minor load) is the Rockwell hardness.
This company does it on two places on the same blade
to give me an average for that blade.
Most
of the time, I bring 4 blades that I have pulled
randomly from my inventory, have them each tested
and take the average. If my blades fall too far
(soft) or way too high (Hard), I have to take a
look at what may
have changed in my heat treating process. It could
be that my quench oil is too old and isn't hardening
anymore or my heat treating oven isn't holding temperature
or... well, let's just say that it begins a whole
new
investigation.
Now
what?
Great, you've got a Rockwell hardness number now
what does this mean? Well, I shoot for a hardness
between 60-64 because I like the characteristics
of a blade that hard. It sharpens fairly easy and
stays sharp a little longer and has some toughness.
I don't want to constantly sharpen while carving.
Most knives fall into a range of 50RC up to 65RC.
Now potentially steel can be hardened to 70 but
it really is not useful for wood carving, because
a fine edge would chip out on a nice piece of winter-cut
basswood. Some carving knives hold a Rockwell number
of 55 and they sharpen easily, but you sharpen often.
Rockwell
testing isn't limited to steel you can test plastics,
ceramics, brass, copper, sheet metal etc. The letter
behind "Rockwell" but before the number
determines what kind of point or "indenter"
was used. Different indenters for different materials.
So for example in my case, Rockwell 62C or 62RC.
When testing my knives a diamond cone is used. If
a different indenter was used, the letter C is different.
Well there you go. At your next Carving Club meeting,
you can astound your friends with you knowledge
of Rockwell hardness testing.
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