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Get your router rate right.

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.

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