Hello Mark. I've just got my workshop set back up after a good few years of not using the machines. I designed and am making a simple steam engine to help me get back into the swing of things.
I am just about to post another thread to show people the engine so look at the thread above this in a few minutes time!
What you want to make is an oscillating engine or wobbler as they call them in America for your first one. My design isn't the simplest you'll find but you could amend it to make it simpler.