most of my machines work fine with just about any kind of thread. one of them, however, has developed a bad case of "uppity" and won't produce a decent result anymore unless i use 100% cotton. thank goodness i discovered threadart.com. 40 wt 3-ply 100% cotton for about half what i pay for polycotton blends at wally world. less than half if i don't count shipping. :-)