Don, first off I am going to assume that you are wet scraping vs dry scraping. Pappy dry scrapes and a different tool is used, Most fleshing knives are slightly curved and have a sharp side and a dull side. The sharp side is used for shaving gristle around the head of coon and also for heavy gristle on beaver, as soon as you get past gristle dull side is used to push fat and bits of meat from hides. For dehairing and graining wet hides you need and edge that is dull but must have an edge, you will pushing into your swelled hide hard but not cutting or shaving, this done on a fleshing beam. My personal set up for graining hide is a pc of 6 inch pvc slid over my fleshing beam. The pvc pipe has a much steeper radius than my fleshing beam so when I am graining I am taking off narrower strips of hair and grain. Graining is much harder than fleshing so I am pushing much less surface. Now to really confuse you, some people set there fleshing beam up so they are pulling instead of pushing.. Next every fleshing knife that have ever seen is a single bevel edge some are straight and some are curved blades. For a home made fleshing knife I would get a flat pc of steel about 12" X 1-1/2 and put a single bevel on it and put some rubber garden hose for handles. Rig your beam up level or down hill angle , rubber apron and hold hide with stomach against beam and push your grain and hair off, Do some experimenting for most comfortable way. Hope this helps ya some, Also see if you can get a copy of Deerskins into Buckskins by Matt Richards, lots of pics of what I am trying to explain to you and easy reading for steps on braintanning and making rawhide. Don hope some of this helps ya and shortens the learning curve. To answer your orginal question Yes a flat lawnmower blade should work. Bob