It's Thrashing Machine. This version is from the Yetties (here).

Thrashing Machine

I knew a farmer, I knew him well,
he had a daughter, her name was Nell
Although she was only sweet sixteen,
I decided to show her my thrashing machine.

The barn door was open, so in we did go,
into the corn to have a good go.
She worked the throttle and I worked the steam,
as I showed her the works of my thrashing machine.

Six months later all was not well,
something was wrong with our little Nell.
And under her apron could clearly be seen,
the works of my dirty old thrashing machine.

Three months later all was not well,
a baby boy was born to our Nell.
And under his nappy could clearly be seen,
a brand new two cylinder thrashing machine.

There's another version in the forum here.

Cheers!
Ian