Here's the deal. the square part does not rotate. it slides along the work and therefore cannot burn the surface finish as a spinning bearing can. the bearing is spinning inside of the square part. glue/contact cement can build up on the metal bearing surface as it rubs along the material being trimmed. there is some heat in the spinning bearing that actually melts the glue just a little bit and it builds up on the metal bearing surfaces. the square teflon part is stick-free and the contact between the square part and the material never gets hot.