Straight Talk on Straight Bits
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 1⁄16 " 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 1⁄4 " to 11⁄2 ". Choose longer bits to cut deep grooves, and when routing with a guide bushing and template [Photo C].
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].
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].
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.