If you work with curves, you need a marking gauge that can deal with curves.
Me, I make chairs. So I need a gauge that can follow the curve of a seat so I can delineate the lines for the scooped-out saddle, the spindles and the “gutter” – a shallow and decorative channel on traditional chairs. I also need a gauge such as this for marking out the armbow and the crest rail.
These curves on a chair are convex and generally represent one radius. So the easiest solution is to use a marking gauge that has two points (instead of a flat face) that will follow a curve.
This type of gauge was once quite common. And you can still buy them new if you look hard enough.
A simple solution is to modify your existing marking gauge so it can follow curves. Most of my traditional marking gauges have been modified like this. One face is for curves and the other is for straight edges.
I do this by adding some small bits of dowel to one face. It’s a 15-minute modification you can do with a short bit of dowel stock, some nails and a block plane.
Flatten That Dowel
You don’t absolutely have to use a dowel for this, but it makes it simple. Find a dowel that is 1/4” in diameter and about 4” long. Secure it in a vise and plane a flat on the dowel that is consistent and about 3/16” wide.
With the dowel still in your vise, drill some pilot holes for headless brads – these pilots will prevent the dowel from splitting when nailed to the face of the gauge.
Crosscut the dowel into two 2”-long bits. Using small headless brads, nail the dowels to one face of your marking gauge on either side of the beam. Note that the dowels have to be located well below the beam for the tool to work.
You are ready to go. Press the two dowels against the edge of your work to mark your desired line with a pin, knife or pencil. If you have to follow a lot of inside curves, however, consider making a gauge with one dowel nailed directly below the beam. This arrangement will allow you to follow an inside curve (with practice), even if the radius of the curve changes.
— Christopher Schwarz