Be in the know about when to go slow

I've gotten by with a fixed-speed (25,000 rpm) router but am wondering if I can improve my results, and use a wider variety of bits, by buying a variable-speed router that runs as slow as 8,000 rpm. When and how might I benefit from owning a variable-speed router?
—Jack Purdy, Seattle

By varying the speed of a router bit, Jack, you can improve operating safety and comfort, ramp up cut quality and control, and possibly extend the life of a router and the bits used in it. Let's take a closer look at each of those benefits.

■ Safety and comfort. Slowing a router bit, regardless of its diameter, makes the router more comfortable to operate. And your ears will welcome the noise reduction.

For bits larger than 1¹⁄₂ ", consider variable speed a must-have feature. That's because as the diameter of a bit increases, the speed at its cutting edges increases dramatically. Take, for example, the two bits shown above. At 25,000 rpm, the panel raiser far exceeds the 100 mph "speed limit" considered safe. Its 260 mph tip speed combined with the considerable mass of the bit would cause a router to vibrate excessively (and should scare the bejeebers out of you). Fortunately, by dialing down the speed of a 3¹⁄₂ " bit to 10,000 rpm, its tips travel at 104 mph. That's more like it!

■ Cut quality and control. While a bit's dia-meter determines its maximum speed, several other factors affect a bit's ideal speed­—the sweet spot that yields the best cut quality. Key variables determining an ideal speed include: type of wood being routed, bit sharpness, feed rate, and cutting depth.

To find a bit's ideal speed, first consult the chart below for maximum speed. Some routers have a variable-speed dial with numbers on a scale (from 1 to 10, for example) that do not tell the router's actual rpm. To determine maximum router-bit speed for those routers, chuck a bit, start the router at a low speed, then slowly increase the speed until the router vibrates. Back off the speed until the vibration goes away, and jot that setting in the third column of the speed chart.

Now, set the router for the bit's maximum speed and make test cuts in scrap matching your project wood. You may want to keep a record detailing the ideal speed for your bits under various conditions. Make note of the wood used; some species, such as cherry and maple, require slower speeds to avoid burning. Dialing down the speed will also help minimize grain tearing in highly figured woods. Your notes will save time in the future.

■ Router and bit longevity. Spinning a router bit faster than the suggested maximum speed stresses the router's drive spindle and bearings. And even the stout ¹⁄₂ " shanks on large bits could become damaged when spinning too much steel and carbide at excessive speeds.

Today's routers and variable-speed control
Nearly all 3-hp routers have variable speed for accommodating panel raisers and other big bits. But variable speed is also finding its way onto small routers, even laminate trimmers. That's because delicate work, such as routing inlay grooves, benefits from the control achieved by slowing bit speed. And even small bits, such as ¹⁄₄ " round-overs, can burn some woods if not slowed down.