I have a Martelli rotary cutter and had lost the spring. My LQS was able to contact her supplier and he (gladly) sent me two new springs. He also told her to tell me that the Martelli brand blades have a notch in them so they have a nice tight fit; and if I'm having problems with the cutter, I should stick to the blades made for it. I do notice when I cut through thick fabric, my cutter wants to 'wobble' and doesn't always cut through all the layers. So I recommend that for regular cotton fabric, use any blade you have, and for the really thick fabric, use the Martelli blade in the Martelli cutter.
cd