What your wanting is called a Stitching Groover, it has an L shaped blade that is adjustable for however far from the edge you want to put the stitches. Tandy normally carries these in two types, one that is adjustable via a small screwdriver and the other is adjustable via a knurled wheel. The latter is a larger tool that makes a larger groove, its called the Saddlers Groover. I have both, and like the Saddlers Groover a little better as its easily adjusted by hand.