What exactly is tumbler gear mechanism?

Also called Norton's mechanism, is a method of changing feed rates.

You can find it on google.co.in/books?ei=OauuT-vvKdHQrQeO6rCYBA&id=_wWET38FZqsC&dq=machine+tool+design&q=meander#v=onepage&q&f=false">http://books.google.co.in/books?ei=OauuT-vvKdHQrQeO6rCYBA&id=_wWET38FZqsC&dq=machine+tool+design&q=meander#v=onepage&q&f=false

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