The rotary blade will only cut the thread, odd as that seems, it works. Try it on some scrap seams and see. I have never cut fabric while using the cutter as a seam ripper. Just touch the blade to the thread as you pull the seam apart. I have a small Fiskars 28mm rotary cutter from long ago and use it for a seam ripper but I'll use any rotary cutter laying near me.