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Be in the know about when to go slow
QI’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
ABy 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 diameter 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 right 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.
With both bits spinning at 25,000 rpm, the edge of one travels much faster than theother. Like a larger wheel that covers more ground with each revolution compared with a small wheel, the tips of large router bits move much faster than the tips of small bits.