Router Speed Control
for Large Router Bits

Some form of router speed control is a must have when spinning large router bits on the router table. It saves wear and tear on both the router and the bit, while at the same time, lessens the chance of burning, tearout and accidents.

While many routers have this feature built-in, either as variable speed or a speed selection switch, many new and especially older models don't. For these latter routers, an aftermarket router speed control is required to lower the rpm to one that's compatible with the diameter of the bit being used.

What to look for

Router speed control. This router speed control is rated at 20 amps and will work with routers up to 3 1/4 hp.

Most of the speed controllers available today are rated as standard duty (15 amp) or heavy duty (20 amp) and are basically high amperage light dimmers. A three position rocker switch will control the mode; variable speed, off, and full speed. The variable speed is controlled by rotating a knob.

If your router draws 13 amps or less, you can probably get away with using the standard duty unit, but anything over 13 amps the heavy duty model should be used. This will give you a margin of safety, especially if you're using an older router that draws more amperage than when it was new.

Because a router speed control will work with any replacable brush type motor, it can also be used to control other power tools such as drills, die grinders and jig saws. I use one on a portable table saw to cut lexan for table tops. The slower rpm prevents the edges from melting and leaves a nice clean cut.

Most if not all manufacturers state that their router speed control won't work with soft start routers or routers that already have variable speed.

How fast should a bit turn?

Most woodworking routers run with a top speed of well over 20,000 rpm at the shaft. Small diameter router bits actually work well at this speed, but as the diameter of the bit increases, the outside tip of the bit is traveling faster in relation to the router's shaft.

An important part of learning router basics is to know what speed (rpm) a given size bit should spin. Turning a large 3 1/2" raised panel router bit at full rpm is downright dangerous. Most router bits of this size have a recommended rpm in the 10,000 - 12,000 range, and exceeding this speed is likely to cause extreme vibration that could lead to all kinds of problems.

Most quality router bits come with the manufacturer's recommended maximum rpm. If you can't find it, check their website or give them a call. Frankly, if they can't tell you the maximum rpm their bits should turn, buy your bits somewhere else.

Router bit diameter x max. rpm
Bit Diameter Maximum RPM
1.5 - 25mm (Under 1") 24,000 rpm
25 - 50mm (1" to 2") 18,000 rpm
50 - 65mm (2" to 2.5") 16,000 rpm
65 - 89mm (2.5" to 3.5") 12,000 rpm

The table above gives the maximum rpm for Blue Tornado carbide tipped router bits. Most other manufacturers are similar, but you should always follow their recommended maximum rpm.

The maximum speed rating doesn't mean you have to run the bit at that speed. The shaker door bits in the photo below work best for me at around 16,000 rpm in most hardwoods, even though they are rated at a maximum of 18,000. Sometimes, less is more.

Router speed control - shaker door router bits. These shaker door router bits are 1 5/8" in diameter and have a recommended "maximum" speed of 18,000 rpm.

Of course, other factors such as depth of cut and rate of feed have to be adjusted to match the router's speed and horsepower. Several passes are usually required, removing a bit of material with each pass.

So what's the answer to finding the perfect combination of rpm, depth of cut and feed rate? In a word, practice. Here's a few router tips that work for me.

Before you start gnawing away at that expensive hardwood you've just sized for your project, grab some scrap from the same wood and play with the rpm, depth of cut and rate of feed.

Adjust the router speed control so that there is no vibration at all and set the fence for a moderate depth of cut. I determine my feed rate by feel. I try to feed at a rate that is just under where the router starts to slow. Too much rpm and too slow a feed will result in burning of the wood and overheating the bit.

Once you find the best balance of all factors, write it on the router bit box for future reference. It will make a good starting point for your next project.

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