Hi Gregg,
In the 1910 Henry Co., Kawanee, IL census, there is a Robert Corkill, born 1875, Illinois, a blacksmith, with wife, Tillie, and children, Rebe, 13, Ruth, 13, and son, John R., 1 11/12. Could your grandfather's name "Jack" have been a nickmame for "John"? If you think that this may be your line, e-mail me off the board and I can send the census image to you.
Regards,
Lionel--my e-mail is Li2cot@aol.com