Dramatic curves from your table saw
Wide, sweeping coves such as those found in crown molding and raised panels are a hallmark of good woodworking. But how do you achieve them without a large-scale molding machine? On the table saw, of course.
Cutting coves on a machine used primarily for ripping, crosscutting, and joinery is unorthodox. But if done correctly, it’s a safe and efficient method. Essentially, you run a workpiece along a fence and diagonally over the blade in a series of shallow passes, to take advantage of the blade’s curvature.
Adjusting the height of the blade and the feed angle of the piece varies the depth and width. While the resulting cut requires a lot of sanding to remove the saw marks, the technique offers more versatility than stock profiles on cutters you’d use in a router or shaper.
While there is no magic in setting up to cut coves, a parallelogram jig simplifies the process. In this article, I’ll demonstrate how to set up and cut coves—and half-coves—first using typical layout tools, then with the jig.
To see cove cutting in action, visit www.woodcraftmagazine.com/onlineextras
Cove terminology and layout
A cove cut is defined by its width and depth. With wide, shallow coves, the curve is close to being a true arc, but with narrow, deep coves, the shape is more elliptical. The other key dimension is the offset, or distance from the edge of the board to the edge of the cove.
Blades for Coving
For best results, cut coves with clean and sharp full-thickness 40-tooth ATB or 50-tooth combination blades. Adding a stiffener (or two) can reduce vibration. Thin kerf blades don’t stand up well to excessive sideways pressure. If you do a lot of coving, consider investing in a designated coving blade. Several manufacturers offer these chunky 7" (+/-) diameter blades with rounded carbide teeth that leave a much smoother surface than a regular sawblade. Be aware that the smaller diameter does somewhat limit the size of the coves you can make, and (as of this writing) the blades are not compatible with SawStop’s brake technology.
Rough cut. The profile to the right was cut with a 40-tooth ATB blade, the one to the left was cut with a coving blade. Both require sanding, but the marks from the ATB blade are significantly rougher.
The basic set up
Once you have determined a cove’s size, use those dimensions to place your primary fence. Since the force from the blade will push the workpiece into this fence, make it from a stout length of scrap (I keep a length of 2 × 2 on hand). Your secondary fence can be lighter.
Adjusting the blade height to equal the cove depth, then determine where the blade’s teeth enter and exit the saw table. Drop the blade completely, align the fence as shown, and mark the feed angle on the saw table. Shift the fence away from your marks a distance equal to your desired cove offset, and secure the fence at each end to complete the setup. This can be tricky—you may need to make spacers to clamp to your saw’s fence rail.
Once the primary fence is in place, add a secondary fence to make the set up more secure. Use your workpiece as a spacer to position it.
Cutting the cove
Making the cove is a straightforward though repetitive task. (It’s also dusty, so use dust collection and wear a mask!) Due to the forces inherent in feeding stock over the blade diagonally—something the blade isn’t designed to accommodate—make multiple shallow passes. Raise the blade so its highest point is about 1/16" above the table, and feed your workpiece along the fence and over the blade to make the first pass. Use push sticks to keep your fingers safe. Raise the blade another 1/16", and repeat. Continue like this until the cove reaches its full depth. Taking very light cuts (<1/32") for the last two or three passes can reduce the amount of sanding needed. Scrape and sand away the saw marks to finish up.
Lotsa clean up. The only drawback to cove cutting is the sanding required. A curved scraper and a convex sanding block will ease some of the task. To make the block match the cove’s curvature, bandsaw it close to shape then finish up by taping coarse sandpaper inside the cove and lapping the block to final shape.
Cutting a half-cove
When making a raised panel, you need to cut half a cove. The setup is similar to making a full cove, except the fence will be clamped atop the blade instead of in front of it.
Set the blade height first, then determine the feed angle for a cut twice the width of your half-cove. For example, if you want a panel with a 2"-wide bevel, determine the feed angle for a 4" wide full cove, by pivoting the fence away from the exit point (p. 29).
Once you have the angle established and the blade lowered, mark the offset, in this case 2", towards the blade, and clamp your fence there. Now as you raise the blade, its front portion will cut partially into the fence, leaving the rear portion to make the cut. Guide your piece along the fence in several shallow passes.
The parallelogram jig setup
If you find yourself cutting coves frequently, it’s worth the time to make a parallelogram jig. The jig makes determining the feed angle a snap. It consists of two long straightedges connected by two arms. The exact part sizes aren’t critical, just make sure the pivot holes are placed consistently so the straightedges remain parallel as you adjust the space in between. To use the jig, set the distance between the straightedges to match the width of the cove you wish to cut. Set the blade height to match the cove depth and place the jig on the saw table straddling the blade. Pivot the jig so the straightedge in front of the blade intersects the entry point, and the straightedge behind the blade intersects the exit point. This establishes the feed angle. Then set up your primary fence to accommodate the offset as described on p. 29. Remember to locate the primary fence in front of the blade to resist the cutting force.