In the original Oz books (for which the movies were loosely based on), the origins of the Tin Woodsman are rather gruesome. Originally an ordinary man by the name of Nick Chopper (the name first appearing in The Marvelous Land of Oz), the Tin Woodman used to make his living chopping down trees in the forests of Oz, as his father had before him. The Wicked Witch of the East enchanted his axe to prevent him from marrying the girl that he loved. The enchanted axe chopped off his limbs, one by one. Each time he lost a limb, Nick Chopper replaced it with a prosthetic limb made of tin. Finally, nothing was left of him but tin.
Essentially Nick Chopper was a man who became a machine.
(Wicked World of Oz...)
With the death of both Wicked Witches, and the fleeing of the Wizard from the Emerald City, a power vacuum has fallen over Oz. The Scarecrow, despite being hand-picked by the Wizard as his predecessor, was viewed by many in the Emerald City to have no claim to the throne of Oz. Rumors have swirled of Ozma, the original king of Oz's daughter, still being alive and captive somewhere in Oz. The witch Mombi claims to know of her location, but demands the title "Witch of the West" before she will let it be known. Dorothy Gale, with fond memories of her time in Oz, is excited when she awakens back in the Wonderful World of Oz, only to be devastated when she realizes the state of the world...the Golden Road is in ruins, a cloud of war seems to cover the land...Oz is now far more Wicked than she ever remembered.
The Tin Woodsman, her old friend, who had been dispatched to rule the Winkies in lieu of the loss of the Wicked Witch of the West, finds himself cast from his throne as Mombi forces herself into his position. Without a peaceful rule of Oz there is little hope of the world's beauty returning. The Tin Woodsman, after discovering Dorothy has returned to Oz, seeks her out in hopes that between the two of them they can find the outcast Scarecrow, and develop a plan to find and rescue Ozma and see Oz return to its splendor.