When buying cheap rabbet (rebate) router bits, whith a bearing, you only get one bearing. Instead of buying more bearings to fit you can make your own "spacer" to adjust the distance cut by the bit. This makes the bit so much more useful. The bearing doesnt spin when engaged so theres no spinning action destroing the plastic. Safety is your own personal concern but I've never had one come loose. With this said you really should adjust and print theese to a really snug fit. I have to gently push and pry mine on and off the bearing. The source file is included and the sketches easily adjusted to get the cut width you want. https://duckduckgo.com/?q=rabbet+router+bit+interchangable+bearing&t=ffnt&iar=images&iax=images&ia=images