Hi synd00d,
You need to go to Daniel Gentiles home page and down load the 18 page Krusty spring helve hammer plans for free!
http://www.ferrum.cc/en/plans-drawin...-plans-17.html
It looks pretty good, and is a complete plan by the way.
iforgeiron.com has a two page hand draw design also, lillhmrplan
There are several websites with gobs of different designs. The most interesting design I've seen of late is a fellow from Pennsylvania that has made a spring helve guilded way hammer that is actuated with a squat under slung air cylinder. Great idea for those of us with low ceilings.
Most of these are considered 'forging' hammers. metalmeet and allshops have the rapper type helve hammer forums used for body shaping.
Spent to much time looking for a means to get around purchasing a new compressor to get good hammer control for sheetmetal forming. Best to use air if you want control, plus the mechanical components add up to bucks too.
Alan