I plan to install an undermount sink in a new butch block counter top that is 1 1/8" thick. In order to cut the sink hole, could I simply attach the undermount sink to the counter in the correct position, then use a router with a bottom bearing flush trim bit (1 1/4" carbide) to cut the sink, with the bearing running along the inside of the attached sink? I have read about the method of creating a template out of plywood, but this method seems like it would be easier and more accurate. Is there something I am not thinking about?
Putting a bearing against a sink surface seems to be looking for trouble. The slightest resistance in the bearing could tear up the finish of the sink, whether it is porcelain, fireclay, steel or resin.
I think it's worth the trouble to make a plywood template, unless you are in the sink business and have spare units to burn.