I bought an extra base for my DeWalt DW611 router (the base that came with the router does not fit bushings). With this centering cone, I was able to place my new base exactly in the middle, aligned with the milling cutter. I'm confident I can use my bushings now knowing I'm in line with the router. In my opinion, without using a centering cone, it would be impossible to correctly align the base of the router with the router bit. I believe a centering cone would be required in other cases as well - if you have a jig the bit may be farther from one side than the other if the bit is not centered. Conclusion: If you are replacing the base of the router, you should use a centering cone. this one works perfectly.