Using a router to create a nicely finished edge on wood is similar to painting a drab room. You can improve the look of boring edges in a fraction of the time and with minimal effort. There are plenty of ways to go wrong that can be found with just a little woodworking experience. Many of the nuances of routing involve setting the correct router bit speed. It also involves the interaction of this speed with drill diameter, stock hardness, feed rate and drill sharpness. Bit speed information is not always readily available, although more and more manufacturers are printing suggested bit RPMs on their packaging. We've created a handy router bit speed chart for you to use if you wish.
Why You Need to Set Your Router Bit Speed
As a general rule of thumb, router bit speed is related to what we call edge speed . Take a few milling cutters with very different diameters to see how important rim speed is. They can come from larger plug-in routers like the Triton TRA001 or smaller trim routers like the Milwaukee M18 FUEL router. It really doesn't matter.
Some basic geometry quickly illustrates the fact that 1.5" diameter and 3.5" diameter drills running at the same RPM have very different rim speeds at the edge. The rim speed difference for the 3.5" bit is more than twice that of the 1.5" bit at the same RPM!
It doesn't seem scary without the proper context. However, suffice it to say that this is an unsafe speed – for the wood and the user – in the bigger picture. It's the difference between moderate acceleration and Back to the Future DeLorean tire rubber on fire. For one thing, it can actually burn wood. It can also exacerbate the slightest imbalance in a drill or router and cause vibration or dangerous drill failure.
Of course, running the drill too slowly can also cause vibrations that can damage the stock.
Here is a general guide or a router bit speed chart to get you started. (Refer to the manufacturer's recommendations and remember that their recommended speeds are maximum) :
|diameter||Maximum speed +/- 10%|
|up to 1 inch||22,000|
|1 to 2 inches||20,000|
|2 to 2.5 inches||18,000|
|2.5 to 3.5 inches||10,000|
Typically, the rim speed is about 100 to 120 miles per hour. You can't time it, so use RPM as a proxy for edge MPH.
How fast is the bit rate of the router?
To understand how drill diameter affects speed, let me walk you through a few examples. The formula for calculating rim speed lets you find the circumference of the drill, calculate it as inches per minute, and finally convert it to miles per hour. Let's start with a few different circles:
Circumference = π x (router bit diameter)
Let's first calculate the girth of some simple bit size, for example:
1 inch diameter drill bit = 3.142 x 1 inch = 3.142 inches
2 inch diameter drill bit = 3.142 x 2 inches = 6.28 inches
3 inch diameter drill bit = 3.142 x 3 inches = 9.42 inches
Calculate rim speed
Now we can calculate the rim speed – or the speed at any point along the outer edge of the cutting mill – using the following equation:
Rim Speed = Circumference x Router Speed (RPM)
We get results expressed in inches per minute. For simplicity, let's assume a router speed of 20,000 rpm:
1 inch diameter milling cutter bit speed = 3.142 inches x 20,000 rpm = 62,830 inches per minute.
2 inch diameter milling cutter bit speed = 6.28 inches x 20,000 rpm = 125,660 inches per minute.
3 inch diameter milling bit speed = 9.42 inches x 20,000 rpm = 188,490 inches per minute.
If we convert these into the more (US) familiar miles per hour (mph), we get:
1 inch diameter milling cutter rim speed = 59.5 mph
2" diameter milling cutter rim speed = 119 mph
3" diameter milling cutter bit Rim speed = 178.5 mph
So, using the example above, a 1 inch mill spinning at 20,000 rpm would cut at 59.5 mph. This cutting rate occurs at the edge where the drill does most of the work. However, the same router drives a 3-inch drill bit at the cutting edge at 178.5 mph.
If you fail to adjust your router speed when moving from a smaller bit to a larger bit, you can end up cutting at 3x (or more) the speed of the smaller bit. This can lead to loss of control or worse.
Consider multiple inspections of the material
While we strongly recommend that you only go with the cuts you feel comfortable with, there are certain cuts you can easily make. It's best to remember that you don't have to remove all material in one go. When we created our round gray table, we used the router multiple passes to cut the curved outer edge. With each pass, we descended a little deeper until we were through 2+ inches thick material. It worked better than any jigsaw and gave us a nice straight edge to sand.
As the diameter of the router bit increases and/or the wood hardens, you can use the router more than once. In this way, you can maintain a safe and efficient rim speed without putting any undue stress on the tool or yourself. Let the tool do the work and "feel" the tool. If you feel like you're pushing too hard with the tool, or if you need to take real pressure, you need to adjust.
Check your router bits are clear
In addition to following our router bit speed chart, you'll want to check the condition of the bits themselves. Only want to use bits that are in good condition. We recommend wearing safety glasses even if you're just walking through the store (how often do you say, oh, I'm just going to do this or that soon?). However, it is very important to do so when operating high speed rotating tools.
It's unsettling (but easy) to imagine the damage that metal flying at 120 mph in any direction could do. Whenever you use a router, first inspect the bit for signs of damage. If you see even the slightest sign of damage, replace it with a new one. This also applies to grinders and grinding wheels. Bits can collect sap and wood material, so be sure to wipe them off after each use to keep them in good, sharp working condition.
Using Variable Speed Routers vs. Fixed Speed Routers
We do not recommend using large diameter drills on fixed speed routers. Fixed-speed routers typically have speeds near the high end of the RPM range. This means they are just the right and safe or smallest diameter drill bits. Don't try a bigger drill or use any aftermarket equipment designed to lower the RPM.
Routers are one of our favorite workshop tools. You can turn ordinary dimensional lumber into a beautiful piece of furniture in a fraction of the time. To ensure the best results, start by setting the optimal router bit speed.
If you're a pro and have some tips on setting your router's bit speed, add them in the comments below — or share them with everyone on Facebook, Twitter, and Instagram.