Bumper!

In about 1950, Elgin released what is generally acknowledged as the first American-designed and -built automatic wrist watches, carrying the 18j, 5/0 sized 607 movement.  These watches are called 'Bumper Automatics', because rather than a rotor, they are wound by the motion of an oscillating weight that swings between two buffer springs.

Here's mine, and a 1951 Elgin ad with the same model.

 I love that - 'Fresh, clean-cut masculine look'!

I was lucky enough to get one in excellent condition, so I have not even tried to remove the caseback.  I really wanted to see what made it tick, though.

A month or so ago, an uncased 607 movement came up on Ebay.  I bid and won it!  This, I decided, would be my victim!

 Under the dial, it's just like any Elgin of the era.  Nothing particularly special from this angle....

Viewed from the back, the 'Bumper' moniker becomes more obvious.

You can see the oscillating weight at the top of the picture, and at the bottom, the two buffer springs.  The weight has roughly 160 degrees of travel, from buffer to buffer.  When you're wearing it, you sometimes feel a very light 'thunk' when the weight hits the buffers.

The first step is to remove the winding assembly.  Undo the two screws, and off it comes.

 Here's how it works:

The pinion in the middle of the autowind bridge engages the crown wheel, so that turning it turns the crown wheel, which turns the ratchet wheel, and winds the watch.

Here's the assembly...disassembled.

The rest of the movement is pretty standard.  

The sweep second hand is driven directly, meaning that the 4th wheel runs concentrically with the center wheel.  It has a long post that runs down through the hollow center wheel arbor.

 Because of this, the movement is quite thick, even without the autowind works.  Look at the length of the set lever screw on the left hand edge of the movement.

The rest of the disassembly is just the same as any other watch.  Here it is in pieces, waiting for me to clean and reassemble it.

I'll update the post when I get to it!