When to Change the Router Bit Speed

One question that I get all the time is why variable speed and router is so important. And the answer to that has to do with the diameter of the router bits. The rule is that the larger the diameter, the slower the bit has to run and let me help you understand what that’s all about.

So here, we’ve got a 3½ inch diameter bit and a bit that’s about1 3/8th inch in diameter. Now, I’ve got some circles drawn on this board so 3 ½, 1 3/8 in diameter. Now, if we go back to some geometry, we can figure out that the circumference of this circle is nearly 11 inches. The circumference of this circle is just a little bit over for. So, imagine that that’s circle is a straight line and what we’re talking about is the time it takes to get from one end to line to the other. And on this one if it’s a straight line, the time it takes to get from one end to line to the other. So, that’s the distance or the speed that we’re going to get when that bit is spinning. Because this distance is greater than this one, what it means is that what’s called the RIM speed out here on the edge is greater than this bit when they’re running at the same RPM. So, here are the numbers.

If I run my router at 22,000 RPM and I run my 1 3/8 diameter cutter in there, the RIM speed right here around the edge is about a 125 miles an hour. And this 3 ½ inch diameter cutter, if I run it the same speed, if I run it 22,000 RPM, I’m going over 200 miles an hour, about 230 miles an hour, way too fast for the bit. So, although they’re at the same RPM, when the diameter is bigger, the circumference is bigger so the RIM speed then is also higher. That’s where it gets dangerous. If we slow this bit down to 12,000 RPM or so, that brings the RIM speed down to about a 125 miles an hour that gets it in the safer range. You know that still sounds real fast but that gets it in the safe range as a RIM speed for this cutter. So the larger the diameter, the bigger the circumference, the faster the RIM speed of it so we have to slow those bits down, big diameter, low RPM’s.