Err…what exactly, do you mean by a ‘spring hammer’?
There are two wildly different things you could be talking about.
(So far as I know. There may be more…) (A) a blacksmith’s ‘spring
hammer’ (or treadle hammer) which is a great huge thing designed to
fill in for the increasingly unobtainable power hammers.
The only reason I mention it is that with the proper tooling, they’d
do a great job of flattening trays and other such things. Big huge
thing, stands about 6 feet tall, and you kick down on a lever to
swing the hammer. Usually hand made by the blacksmith.
or, (B) a ‘spring faced hammer’, if you’re a Brit. (I’ve never seen
(or heard of) one in the states.) This is essentially a fore-runner
of the modern dead-blow hammer. It’s a normal 'mechanic’s size’
ballpeen hammer with a flat steel plate wired loosely in front of
the face. The corners of the plate are bent back to form hooks to
hold the binding wire, and the plate is hardened, and lapped flat.
The one I saw in school in London had a pad of many layers of very
heavily compressed newspaper folded up between the plate and the
hammer face. The idea being that the flat plate would hit the
workpiece first, and because it was wired onto the hammerhead
loosely, it could adjust itself to perfect flatness in the
microsecond before the full weight of the real hammer head slammed
into the back of it. The paper was there as a sort of spring to keep
it from bouncing around due to metal-to-metal contact with the hammer
and the plate, and to give it a little ‘give’ to let it align itself.
The only one I’ve ever seen was hand made, and I’ve never (ever)
seen one listed in a tool catalog. From what I understood, there was
something of an art to using them. The main intent was to ‘make flat
things…flat.’ So it would be used for trays/salvers & etc.
The actual hammer you use to make one doesn’t really matter, so long
as you can find a way to wire the plate onto the front of it,
although you want to make sure your plate isn’t much bigger than the
flat of the hammer face. The plate’s really what matters, as it’s the
thing that actually contacts the work.
Now that I’m thinking about it, I may make one, just 'cause.
Hope this helps,