You are here

Straight Talk on Straight Bits

6900.jpg
by Jim Heavey
Woodworkers love routers for their ability to create fancy edge profiles that draw the eye. Though profile bits get all the glory, the humble straight bit serves as the true workshop hero. Its work, often unseen, creates dadoes for shelving, rabbets in carcases, mortises, duplicate parts, and so much more. Let’s take a close look at the three main types of straight bits.

The basic straight bit

Use a straight bit for cutting grooves, dadoes [Photo A], and rabbets [Photo B]. They come in one-, two-, and three-flute variations ranging in diameter from 116 " to 1". (Small bits have room for only one flute.) Larger bits with two or three flutes provide smoother cuts, but at a slower feed rate. You’ll find these bits in a variety of cutter lengths from 14 " to 112 ". Choose longer bits to cut deep grooves, and when routing with a guide bushing and template [Photo C].

7179.jpg
Rout a groove or dado the width of the bit by making a single pass. Move the guide, even just slightly, and make additional passes to widen the slot.

7144.jpg
An edge guide adds stability when routing rabbets, as more than half of the router may extend over the workpiece edge. Adjust the edge guide position to plow wider or narrower rabbets.

1003972.jpg
When using a guide bushing that runs against a template, a long bit reaches past the template to the desired cutting depth.

Upcut and downcut spiral bits


When routing deep grooves or dadoes, trapped sawdust can cause the bit and router to work harder, overheating the bit, and burning the walls of the cut. An upcut spiral bit pulls out waste, similar to the way a standard drill bit evacuates shavings from a hole [Photo D].

263i05.jpg
As an upcut bit spins, it pulls debris up and out of the cut. A downcut spiral bit cuts from the top down, shearing wood cleanly. A compression bit cuts from both ends toward the middle.

Sometimes a clean edge takes precedence over clearing waste. In these cases, choose a downcut spiral bit. As it spins, it begins cutting on the top surface of the workpiece and shears downward, preventing chip-out. Because it tends to drive waste into the cut, avoid burn marks by making a series of shallow passes to reach final depth.

A compression bit combines the best of upcut and downcut bits. At its tip, it upcuts material while the balance of the length shears downward. Use this bit when trimming edges of veneered plywood or melamine. Because each end of the bit cuts toward the center of the panel, both faces emerge chip-out free.

Pattern and flush-trim bits

A straight bit with a bearing the same diameter as the cutters can follow and create parts the same size and shape as a template. Secure the template to a workpiece slightly larger than the template, then set the bit height so the bearing rides on the template. 

Which bit you choose depends on the job, and whether you prefer or need the template above or below the workpiece. A pattern bit, with its bearing above the cutters, can rout a dado following a template. It also requires exposing the full cutter so the bearing contacts the template [Photo E]; a flush-trim bit allows you to expose only as much bit as is needed [Photo F]. With a pattern bit, you can rout workpieces thicker than the bit length [Photos G, H]. You can even use the two bits in tandem [Photo I]. 

1011354.jpg
A pattern bit places the bearing between the cutters and shank. The router rides on the template.

1012331.jpg
A flush-trim bit bearing sits at the end of the bit. The router rides on the workpiece.

7185.jpg
After routing almost the full depth of the bit with the template in place, remove the template and ride the bearing against the freshly routed surface.

7189.jpg
On this 2"-thick workpiece, the pattern bit reached just over half of the thickness. To complete the cut, flip the workpiece over, mount a flush-trim bit, and rout with the bearing riding on the previously routed edge.

Use large-diameter bits whenever possible, as they prove more stable and less prone to breakage. Choose a small-diameter bit to reach into tight concave curves.

Now that you know a “bit” more about them, make sure your collection includes a variety of these perhaps underappreciated, but ready to perform, bits.

Read more about

Tip of the Day

At-a-glance grinder gauge

Here’s a simple way to reset your grinder’s tool rest to the correct angle. After adjusting the... read more