Understanding Guide Bushings
Guide bushings may be the most misunderstood of all router accessories. Their uses include cutting dovetails with a jig, lettering, inlay work, even reproducing furniture parts. Here's the basic concept: A guide bushing mounts to the router's subbase with a tube that protrudes below. A straight router bit extends through the tube. The outer surface of the tube rides against an edge guide or template, keeping the bit a set distance (offset) from the edge guide or template.
In the marketplace you'll find two bushing styles. The Porter-Cable two-piece (shown below) uses a screw-on locking ring to hold the tube part to the router base. This style fits a wide range of models. The other style of guide bushing clicks into place, but only fits Bosch routers. If your router doesn't accept these common styles of guide bushings, consider buying adapters and subbases . Guide bushings are sold individually or in sets. Individual bushings cost $6-$8 each, while sets range from $30-$45.
To use a guide bushing, you need an edge guide or template to follow. You'll want to size the template slightly larger or smaller than the workpiece. Templates can be made of just about any material, but we prefer 1⁄4 " hardboard.
To make a template, simply affix your drawing or pattern to the template material with spray adhesive, and then bandsaw or scrollsaw it to shape. After completing the cut, file or sand-smooth any irregularities or rough spots on the template edges.
Attach the template to your workpiece with a few drops of hot-melt glue or double-faced tape. Use a router pad to hold your workpiece in place on your bench while you rout. If you are going to rout completely through the workpiece, attach a backer to protect the router pad and benchtop.
Using a straight bit that fits through the guide bushing, rout out the waste area surrounding the template. Remember to rout counterclockwise when cutting around the outer edge of a template, and clockwise when routing the inside. Once you're finished routing, gently separate the template from the workpiece with a chisel.
Figuring template offset
Offset is the distance from the outside of the guide bushing tube to the cutting edge of the router bit. This dimension determines where the bit cuts the workpiece.
To figure the offset, measure the outside diameter (OD) of the guide bushing tube and subtract the diameter of the bit. Next, divide this figure by 2 to determine the offset. In the example at right, subtract the 1⁄2 " bit diameter from the bushing's 3⁄4 " OD. You get 1⁄4 ". Now divide this number by two and you arrive at a 1⁄8 " offset.
Know your alternatives
There are router bits capable of template routing without guide bushings. See the chart on the next page for comparison information.
Adapters and subbases are available from many mail-order sources, including the following:
You can skip using a guide bushing altogether if you use a pattern bit or flush-trim bit. Both types have a bearing that rides against the template, just as a bushing does. But because the bit cuts flush with the bearing, you don't have to calculate offset.