Straight & Spiral Router Bits

Straight & Spiral Router Bits Buying Guide

There are many decisions to make in straight router bit selection. The most popular choice is a two-flute carbide-tipped router bit. They will have less vibration and a smooth cut. A single flute is used when cut speed is more important than the finish. Fewer flutes mean improved chip clearance and faster speed. A straight router bit with shear has angled carbide flutes for a slicing rather than a chopping cut which produces a cleaner edge. If a plunge cut with a flat bottom is desired, look for the plunging straight router bits. For the ultimate in finishing, nothing beats a solid carbide spiral bit, but they can be delicate and costly.

Spiral vs. straight bits

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 1⁄4 "-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.

Why Spiral Router Bits Are Better For Plunge Cuts

Because a spiral bit is designed much like a drill bit, it makes plunge cuts easily. The cutters of a straight bit do not overlap, so if you plunge straight down deeper than 3/32 in., you might burn away the wood in the middle, but you won’t cut it.

The increasingly popular spiral router bits borrow technology from the metalworking industry. Spiral bits look like drill bits and are most often made of solid carbide, so they are super sharp and leave a superior cut on wood. Two flutes ground around the body of a spiral bit smooth vibration by spreading the cutting action over a longer edge. With their drill-like point, spiral bits are also better for plunge-cutting. All of these advantages also mean less wear and tear on the router, but don’t throw out all of your old straight bits just yet.

The new solid-carbide spiral bits come with some disadvantages. The first is that the cutters are expensive. A typical solid-carbide spiral bit is likely to cost at least $50. A similarly sized straight bit with carbide-tipped cutters will run somewhere in the range of $7 to $23.With a cost differential that large, you will want to know what you are going to do with this bit and that you will use it often enough to get your money’s worth. To highlight other differences, let’s compare the qualities of spiral bits and straight bits.

Both Spiral and Straight Bits Have "Plunge-Ability"

You can plunge with both types of bits, so they’ll both work for, say cutting mortises. But because most spiral bits are ground on the tip end of the flute, somewhat like a drill, you can plunge straight down as far as you like, without stopping.You can’t really plunge any deeper than about 1/8 in. with a typical straight bit. Inspect the end, and you’ll see why (see the photo at left). On most straight bits there is a space above the web, between the cutters, where no cutting takes place during a straight plunge because there is no cutter overlap.

Chuck a straight bit into your drill press and plunge it into a piece of wood. After about 3/32 in., the middle of the bit bottoms out. To go any farther, the bit has to abrade the wood away in this middle area.

This doesn’t mean you can’t cut mortises or plunge with a straight bit. You just have to sweep the router while you are plunging. You should probably cut mortises in passes not much deeper than 1/8 in. anyway, but with a straight bit, such shallow passes are just about a must.