I'm going to lead off by saying I'm very pleased by the responses you
have all given - they have been helpful in getting me off to a good
start. I like the idea of the air muscles mentioned earlier. They
certainly look promising. But you have raised some very important
issues, Robin S. Here are some more details:

I'm probably not interested in spending any more than $2000USD on the
hand. It would be nice if it could crush rocks but not necessary. As
far as size and strength goes you might imagine one of these hands
attached to the arm of one of those big pieces of construction
equipment. I think the general idea is to get a hand working, and then
build an arm to attach it to, and then a shoulder, working all the way
until I have a giant ... "robot"?

I'm very much intrigued by the idea of an exoskeleton, only I think it
would be quite difficult to make one that is close to the size a human
- at least for a person working on it in their garage without some sort
of big financial backing. I think it would be easier from a
design/engineering standpoint to build a large-scale exoskeleton, like
a large humanoid vehicle. That is why I want to build this "hand". It
seems like a good starting point, I will have to work out the actual
construction of said hand as well as the system used to control it. As
much as possible I would like the operators hand movements to be
translated to the mechanical hand for the most part 1 to 1. So if the
operator does a thumbs up, the mechanical hand would do one too, in as
close to realtime as possible, meaning that the mechanical hand forms
the thumbs up as the operator forms the thumbs up and they both stop
moving at approximately the same time.

Some sort of force-feedback mechanism would come in handy here, because
it would allow the operator to control the gripping strength of the
mechanical hand just by interpreting the feedback from whatever
pressure/force sensors are installed on the hand. What may be required
is some sort of computer layer in between the operator and the hand, so
as to allow for the gripping of squishy or delicate objects like eggs
or people (Imagine a big hand pulling off the wall of a tall burning
building and extricating the people who may be unconscious inside)
without crushing the held object. I doubt the size of the mechanical
hand I have in mind would allow for the gripping of an object the size
of an egg. Perhaps the smallest object it could grasp would be a
softball or a mushball (about 6" in diameter, like a cabbage).

At any rate... I figure before I go off to build a 20 foot walking
mechanical robot suit thing I should start small - thus, the hand. At
the very least if I were to be able to build such a hand there might be
some market for it in construction or other areas if it were to advance
enough to really be useful.

How strong is the human hand? I envision my mechanical hand to be on
the order of 5 to 20 times stronger than that. Probably close to ten
times as strong. But I don't have any specific application for it as
of yet so it is hard to say. I just want to build one. I want it to
be articulate, and I want to be able to control it with my own hand.

I can't remember the name but someone mentioned Legos and the Technic
pneumatic pieces - that's a stunning idea. I'll try that out as far as
prototyping goes to get the structure/design/frame figured out.

A couple of questions that just came to mind - or just one:

If you have two identical hydraulic cyclinders next to each other,
operating on the same object and working off of the same base, is the
net force just the sum of the forces of the two cylinders?
In other words, if I have a platform that is lifted by 4 hydraulic
cylinders attached at each corner, and each cylinder can exert a force
of 25 newtons, is the total force 25 newtons or 100 newtons? I think
it would be 100 newtons.

Ok I'll let you guys comment now on how insane I must be to dream of
making something like this...