Spiral bits plow through material the way business jets slice through the sky; but neither of these high-tech tools fit everyone's budget.

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Bit going around curve

Spiral bits plow through material the way business jets slice through the sky; but neither of these high-tech tools fit everyone's budget. Like a coach-class airline ticket, a standard straight bit gets you where you need to be, and for a lot less money.

So why pay for the upgrade? All spiral bits share one advantage over straight bits: Their angled cutting edges slice, instead of chop at, the wood. This leaves a cleaner cut because a portion of the cutting edge constantly touches the wood. And unlike straight bits with carbide cutters brazed to a steel body, spiral bits are all carbide. That lets bit-makers use wear-resistant carbide formulas that stand up better to abrasive glues and resins in plywood, particleboard, and medium-density fiberboard. Most spiral bits use one of three flue patterns descibed on the next slide.

Flue patterns

Up-cut. (Picture the left-to right rise of the spiral.) Like a twist drill bit, these bits quickly evacuate chips from deep cuts. They lift the veneers on sheet goods when making plunge cuts, but push down on veneer when used in a table-mounted router with the "good" side up on the workpiece.

Down-cut. These spiral bits press veneer down as you make plunging cuts for dadoes, rabbets, and grooves. They don't clear chips as well as the others, so use multiple 14 "-deep passes, or cut relief kerfs with your tablesaw. Be careful: The downward force can lift a handheld router off your workpiece.

Combination. Up- and down-cut flutes meet in the middle of the bit (below) to compress veneer on both faces of a panel tightly against the substrate.

These big three have been joined by such specialty spirals as bearing-guided up-cut and down-cut flush-trim bits for template work and combination bits with short up-cuts and longer down-cuts for chip-free mortises.

Photo of spiral bit cutting plywood
On a combination bit, the down-cutand up-cut flutes both press veneersagainst the particleboard substrate.

When to turn to spirals

Spiral bits, with their shearing action, make sense for hard-to-machine woods with complicated grain patterns, such as curly maple, and for veneered sheet goods. That speed and cut quality doesn't come cheap, but you can justify their prices a lot more easily when you reduce tear-out waste in an $80 sheet of quartersawn oak-veneer plywood.

For example, one Internet source sells a 12 x114 " straight bit for $18, but an up-cut spiral bit the same size costs $47. A 12 x112 " bottom-bearing straight flush-trim bit costs $21 versus $111 for a comparable down-cut flush-trim bit.

In addition to their lower price, straight bits work best on wide cuts. While the all-carbide construction of spiral bits limits their width to 12 ", straight bits can cut a path up to 112 " wide for single-pass cleanup of tablesaw dadoes and rabbets.

So hold onto your wallet, and push your straight bits to their limits. Then use our chart below to choose the best bits for your job.

Sources
The following companies offer a variety of spiral router bits:
Amana Tool, 800-445-0077; amanatool.com.
CMT, 888-268-2487; cmtusa.com.
Eagle America, 800-872-2511; eagleamerica.com.
Freud, 800-334-4107; freudtools.com.
Lee Valley, 800-871-8158; leevalley.com.
MLCS, 800-533-9298; mlcswoodworking.com.
Rockler, 800-279-4441; rockler.com.
Whiteside, 800-225-3982; whitesiderouterbits.com.
Woodline USA, 800-472-6950; woodline.com.
Woodcraft, 800-225-1153; woodcraft.com.

Bit chart