I found a bug, if you have at least 1 piece of scrap inside the mass fab and a slow grinder adjacent to it, you can put in any material in the rightmost slot and the mass fab will fill up with scrap.
So if I put cobblestone it will convert to scrap if there is at least 1 piece of scrap in the mass fab.
I'm abusing the sh!t out of this bug
Ya i test it my self and it seems if you pot in the slow grinder only one items (cobblestone or any other material ) in one of the 4 vertical slots it will give you 1 stake of scrap in the mass fabricator like SteveofDoom said of course the you will have to pot a piece of scrap in the mass fabricator.
One more bug with the scrap thx for that bug i really did not test the slow grider next to a mass fabricator