A loop cutter knife fixture for cutting vegetable products such as raw potatoes into spiral or helical loops. The knife fixture includes a circular blade holder adapted to be rotatably driven within a vegetable product flow path. The blade holder includes a central core cutter in combination with a spiral base plate defining a radial slot between axially spaced upstream and downstream ends thereof. Small slits are formed in this base plate for seated reception of preferably multiple U-shaped strip blades, which are in turn overlaid and clamped in place beneath a generally pie-shaped slab blade having a sharpened knife edge exposed at the base plate radial slot and held in place by preferably multiple clamp screws. The strip blades and/or the slab blade are formed from an inexpensive material, such as sheet metal or the like, whereby blade changeover is quick and economical.