They’re used for whatever you need them to do. There are some really
weird ones out there that have pretty specific uses, but those are
very rare, and if you had one, you’d know it, and know why you went
to the trouble of finding or making it.
Facetiousness aside, hammers come in two basic forms, or peens.
Wedge peen, where the working surface is a wedge. The most common
type is the ‘cross peen’ where the wedge is set such that the face
is at right angles to (or ‘cross’) the handle. There are ‘straight
peen’ and ‘diagonal peen’ hammers where the wedge is set either in
line (or 'straight) with the handle, or at 45 degrees to it,
respectively, but both these types are very rare, and used mainly by
Ball peen: a hammer where the working face is a sphere of some
radius. Sometimes small enough to look like a ball, sometimes the
radius is so large it’s easily mistaken for flat, but generally
there is some spherical radius to most ‘flat’ hammer faces.
2B) special cases: yes, planishing and chasing hammers really do
have totally flat faces, but they’re special purpose hammers. In the
case of a flat-fronted planisher, the flat face is intended for use
exclusively on convexly curved surfaces like the outside of a bowl,
while chasing hammers never hit the metal directly at all.
That’s it, there are really only two types of face on a hammer: a
wedge, or a ball. Everything else is just a question of orientation
or scale. Once you realize that, it becomes easier to evaluate your
hammer faces based on what they’re going to do to the metal, and
whether or not that effect is what you want.
As a side note, ball peens are generally symmetrical spheres, but
they don’t have to be. Equally, wedge peens are frequently ground as
straight ‘bars’ of front surface, but they really should be curved
back a bit on the edges. (keeps the corner from digging in if you
come in at an angle other than a perfect 90.) In the case of
anticlastic forming hammers, they can curve back quite a bit, so in
both cases it is important to look at the face of the hammer, and
evaluate in your mind what effect it will have on the metal.
What I tell my students about that is to imagine you have a lump of
clay on top of the anvil. Now form your hand into something
resembling the shape of the hammer face, and karate chop the clay.
Watch what happens. A ball peen is pretty much just a fist. Make a
fist, and slam it down on top of the clay. Where does the clay go?
It spreads out evenly in all directions from the impact point of
your hand. Which is what a ball peen does: spread the metal evenly
in all directions.
A wedge peen is pretty much like the side of your hand if you were
going to do a classic “karate chop”. So imagine the clay on the
anvil, and then ‘chop’ it. Where does the clay go? Mostly to either
side of the hand. No real motion in the two directions in-line with
the hand. All the motion is at 90 degrees to the hand. Which is what
a wedge peen does: move the metal at right angles to the orientation
of the face.
Wedge peen hammers with the face rounded back are a mix of both
forces, and the ratio of side-to-side motion in relation to
cross-face motion is a function of how curved back they are. The
more they curve back, the more side motion you get.
Equally, non-round ballpeens are a sort of a hybrid ‘fat wedge
peen’. So the proportion of metal motion in each direction is a
function of how curved that particular axis is. The non-round
ballpeens will eventually meet the curved wedge peens in the middle,
and the two types will fade into each other at a certain ratio of
curves in both axes.
So, look at your hammers again. Nevermind what numbers they’ve got,
or who made them. What do the faces look like? What are they going
to do to your metal? How heavy are they? (More weight equals more
impact force, which may (or may not) be what you need.) Personally,
I have very few hammers with their faces in the same shape in which
they were made. I always regrind and re-polish the faces to suite my
hammering style, and my needs at the time. So the fact that a given
hammer started life as a Fretz #105 has little bearing on what its
face looks like now, and that’s what matters.
In similar light, rather than looking for an 8oz raising hammer with
a 2mm radiused edge, look at what you’re trying to do to the metal:
what type of forming, or the type of toolmarks you’re trying to
leave behind, and then go find a hammer with a face that will do
what you need. Begin with the metal, and let that lead you to the
For whatever that’s worth.