Routers come with variable speeds. This is important because certain bits require certain speeds, not only to give you the best possible cut, but also for the sake of safety. Most routers have a range of 8,000 rpm to 24,000 rpm [source: Spielman].
To help give a better understanding of what that means, let's talk about rpm. RPM stands for revolutions per minute -- it's important to keep in mind that this isn't actually a measure of speed. With regards to routers, it is simply a measure of how many times the bit spins around in one minute. The laws of physics tell us that the outside edge of a bit with a large diameter is actually moving much faster than a smaller bit at the same shaft speed. Since this is true, the speed of a router should decrease in proportion to the diameter of the bit being used. In other words, the bigger the bit, the slower the rpm. This prevents vibration created by bits with more mass.
While most bits will come with information providing a maximum free-running speed, there are speeds commonly used according to the size of the bit. Bits 1 inch or less can be run at the max speed of 24,000 rpm. Bits 1 to 2 inches should be run somewhere around 18,000 rpm, while 2- to 2 ½-inch bits operate around 16,000 rpm, and bits 3 inches or bigger at 12,000 rpm or below [source: Router Workshop]. Sticking to these guidelines will help ensure smooth cuts.
Now, the size of the bit isn't the only factor in determining speed. Other elements that come into play include the router's horsepower, the condition and quality of the bit, the material being used, and, very importantly, the feed speed of that material. If the feed speed is too fast you'll get bad vibrations, and if it's too slow you'll burn your material. The speed of the router and the feed speed should be tweaked to make the best possible cuts. This can be accomplished by practicing on a piece of scrap material before making your final cuts.
Read on to find out more about quality router bits.