For many years I dropped a .030 valve spring shim meant for stock spring seats on before I cut for larger diameter springs. When the cutter got down to the shim, the shim would spin and keep the cutter from digging farther down. That was mostly to keep me from hitting water. I normally used a drill press. When assembling the new springs, a standard .030 small diameter shim went down on the bottom to make for a flat surface.