Router Guide Bushings: How to Use Them for Accurate Cuts
One of the reasons a router is such a versatile woodworking tool is that a relatively small assortment of bits can handle a surprising number of tasks. From cutting grooves and dadoes to creating decorative details and complex shapes, a router earns its place in just about any workshop.
But there's a simple accessory that can make those bits even more useful: router guide bushings. These inexpensive metal collars attach to the router's baseplate and allow you to guide the router along a template or straightedge with accuracy and control.
Most woodworkers are familiar with guide bushings because they're commonly used with half-blind dovetail jigs. But they're useful for much more than cutting dovetails.
With the right setup, you can use them to rout straight grooves, dadoes, curved openings, inlays, and other details that require precise positioning.
What Is a Router Guide Bushing?
A router guide bushing is a metal collar that attaches to the router's baseplate and extends slightly below it.
The bushing surrounds the router bit, creating a smooth guide surface that follows the edge of a template or jig.
Guide bushings are available in a variety of diameters and lengths. Some are designed for general-purpose routing, while others are intended for specialized tasks such as sign making, inlays, and dovetail joinery.
The principle is simple: The bushing follows the template while the router bit cuts the wood. Unlike a bearing-guided router bit, where the bearing is attached directly to the bit, a guide bushing is mounted to the router itself.
This means you can use a straight bit with a template to make cuts that begin and end within a workpiece.
How Router Guide Bushings Work
A guide bushing works much like the bearing on a pattern or flush-trim bit.
The difference is where the guide is located.
With a bearing-guided bit, the bearing rides directly against the template. With a guide bushing, the collar mounted to the router base follows the template while the cutting bit rotates inside it.
Because the guide bushing is independent of the bit, you can use it with different cutting diameters, provided the bit fits safely inside the bushing.
This opens up several possibilities.
You can use a straightedge to guide a groove or dado, follow a curved template to create a shaped opening, or use a specialized jig to cut precise joinery.
However, there's one important detail that affects every guide-bushing setup.
The cutting edge of the router bit doesn't line up with the outside edge of the bushing.
That difference is called the offset.
Understanding Router Guide Bushing Offset
The offset is the distance between the outside of the guide bushing and the cutting edge of the router bit.
Because the bushing is larger than the bit, the router bit cuts a short distance away from the template.
If you don't account for that distance when positioning your guide, the finished cut won't end up exactly where you intended.
Fortunately, calculating the offset is straightforward.
For a straight groove or dado, this means positioning the guiding edge of the template 3/16" away from the desired edge of the cut.
Once you understand this relationship, setting up straight cuts becomes much easier.
Using Guide Bushings for Grooves and Dadoes
Guide bushings are particularly useful when you need to rout a groove or dado in a precise location.
Start by marking the desired position of the cut on your workpiece. Next, determine the offset for your guide bushing and router bit combination.
Position the straightedge or template so the bushing can follow it while the router bit cuts along the layout line.
For example, with a 3/16" offset, the guiding edge of the template must be positioned 3/16" from the intended edge of the groove.
Secure the template firmly before routing.
As you move the router along the guide, maintain steady contact between the bushing and template.
The result is a straight, accurately positioned cut without relying on freehand control.
Woodsmith Tip: Make a Test Cut. Before routing your project, make a test cut in scrap using the same bit, bushing, and template arrangement.
Measure the resulting cut and verify its position. This is also a good way to catch any small error caused by a guide bushing that isn't perfectly centered around the router bit.
How Guide Bushing Offset Affects Curves
Straight cuts are relatively simple because you only need to position the template a fixed distance from the layout line.
Curved cuts require a little more planning.
The offset changes the radius of the curve on the template compared with the radius of the finished cut.
The adjustment depends on whether you're routing an inside or outside curve.
Inside Curves: Increase the Template Radius
When routing an inside curve, the template radius needs to be larger than the finished radius by the amount of the offset.
For example, if you want a finished inside radius of 1" and your guide-bushing offset is 3/16".
This allows the router bit to produce the desired inside curve while the bushing follows the larger template radius.
Outside Curves: Reduce the Template Radius
For an outside curve, the template radius needs to be smaller than the finished radius by the offset.
These relationships assume the template is guiding the router from the corresponding side of the profile. Always confirm the layout with a test cut before routing the finished workpiece.
Common Uses for Router Guide Bushings
Once you understand how to account for the offset, guide bushings become useful for a wide range of routing tasks.
Tips for More Accurate Guide Bushing Routing
A few setup details can make guide bushings easier to use and improve the accuracy of your cuts.
First, make sure the guide bushing is properly seated and secured to the router base. If your router uses an adapter or mounting plate, check that everything fits correctly.
Next, verify that the bushing is centered around the router bit. A centering cone or alignment tool can help with this adjustment on compatible router bases.
Use a template that's thick enough to guide the bushing securely, but not so thick that it interferes with the bit's cutting depth.
Finally, remember that the bushing extends below the router base. Make sure it has adequate clearance and won't bottom out against the workpiece or another surface during the cut.
Get More Precision from Your Router
Guide bushings are one of the simplest accessories you can add to your router, but they open up a surprising number of possibilities.
They allow ordinary straight bits to follow templates, make precise cuts away from the edge of a board, and reproduce shapes consistently.
The key is understanding the offset between the bushing and the bit.
Once you know how to calculate that distance and account for it in your template layout, guide bushings become a dependable way to add accuracy and control to your routing.
Master Template Routing
Guide bushings are just one way to use templates with a router. Learn how to make accurate master templates and use bearing-guided router bits to create smooth, matching furniture parts and other woodworking components.
LEARN TEMPLATE ROUTING → Make Accurate, Identical Woodworking Parts
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