Rick Christopherson said:
Router bit RPM is all about tip-speed, which is a function of diameter. It has been many years since I looked at those numbers, however, all but the smallest bits can operate down at the speed of a shaper and maintain adequate tip speed. More importantly, is that having too high of a tip speed is what causes burning, and most woodworkers can attest to the frequency they experience router bit burning. That's because the speed is set too high.
Rick, here are the numbers I come up with, if top speed of a shaper is 9,000rpm, and the top speed of a router is 22,000rpm, consider a ½” straight bit installed in both machines to be used to cut box joints. The tip speed of the bit in the router (at 22k rpm) will be around 48 feet per second (fps), this will result in a very clean cut for the joints. The tip speed of the bit in the shaper (at 9k rpm) will only be about 19 fps, that is significantly slower than the router. The result will most likely be a lower quality cut on the joints. There is more than a feed rate issue involved here, a small diameter router bit is specifically designed to cut material at a high rotational speed, that is where the best cutting action is obtained.
On the other hand, consider a 3 ½” diameter raised panel cutter run in a router set at 9,000rpm (large diameter bits MUST be run at the slowest speed on a router, they should NEVER be run at high speed!). The tip speed is about 137fps. The same bit can be run on a shaper at 9,000rpm and the cutting action will be the same because the cutter will be turning at the speed it was originally designed to operate at. Depending upon the motor size of the shaper the overall operation may be a little better on the shaper though due to a potential significant difference in available torque.
To give an additional example, consider using the ½” diameter straight bit in a router to make a rabbet. With the router set at 22,000rpm the tip speed is about 48fps. A 4” diameter shaper cutter running at 9,000rpm has a tip speed of 157fps, at 6,000rpm it would be 105fps. Because of the tip speed, torque, and the cutting action which takes place more tangential to the stock, a higher quality cut should be the result.
And sometimes there is the cost to consider. When I ordered my Felder slider (saw/shaper combo) I was at first considering the the router spindle option. Having a "router table" with a huge sliding table would have been great. The optional spindle came with collets for router bits and increased the speed from 10,000 to 15,000 rpm. But the added cost of the option was more than that of a well equipped router table that can run at higher speeds. I felt that I would have greater versatility in having both the shaper and router table.