Have a look at
https://groups.yahoo.com/neo/groups/EDMHomeBuilders/info
There are design/construction books at HSM/Home Shop Machinist (Village Press) and Lindsey Publications (website gone, Abe Books?).
Also try a search on HSM's forum.
If you can access them, the British magazine Model Engineer's Workshop had a few to construct over the years.
Many designs on YouTube
There was a very crude design (that worked...sort of...maybe...) many years ago in Popular Science. I think the Google reprints deleted it
but it may exist somewhere on the 'net.