For such a straightforward procedure, cutting coves on a table saw offers amazing versatility. Simply running stock at an angle across an average table saw blade gives you the ability to cut a broad range of curves, each of which can be infinitely fine-tuned by making a few minor adjustments to your cove cutting set-up.
And as we pointed out a short while back on the blog, getting set up and cutting near-perfect coves can be an easy, frustration-free procedure - especially if you have a reliable cove cutting jig. But while the general concept and process of coving on a table saw is easy to get your mind around, what’s happening geometrically is a little more complex than you might first guess. So, to make matters more clear, here’s a little more on what’s actually happening when you cut coves on a table saw, and how set up for exactly what you want:
When you cut a cove on a table saw, it seems natural to assume that you are getting a perfectly “circular” cut. But on a moments reflection, you’ll understand why this is rarely the case. For a perfectly circular cut, the stock would have to cross the blade at a 90 degree angle. At any other angle (other than zero, of course) you are actually cutting a section of an ellipse. And as the angle of the coping jig’s fence the blade becomes more acute, the elliptical shape of the cove becomes pronounced - as you can see in the (slightly exaggerated) drawing at the left. Changing the saw blade’s bevel angle results in another change in the shape of the cove: It shifts the apex of the curve away from the center of the cove. As the bevel angel of the blade is increased, the asymmetry of the cove also increases.
Combined, these two properties actually offer a lot of possibilities. Along with the ability to cut standard cove moldings – including those in dimensions well out of range for a router – coving on a table saw opens the door to an endless variety “custom” shapes. This range of asymmetrical elliptical shapes is especially handy for anyone who needs to copy an existing, out of production length of molding. Used in combination with a few other router and hand shaping techniques, you’ll find that you can work out a set of steps to replicate just about any piece of decorative molding that comes along.
What do you need to take advantage of all of this versatility? You’ll need some sort of apparatus for guiding the stock through the cut, of course. And as we’ve mentioned, we think that a solid, easy to use coving jig is the best way to ensure that the entire process will come off smoothly and safely. There can be a degree of tweaking involved in the set up process, and a jig that is easy to adjust and move from position to position can be a major time-saver. Also, it is extremely important – for both safety and a good cut - that the jig provide secure, reliable support for the workpiece at all times.
Just as necessary is some method of getting the moving and non-moving parts in position. For a symmetrical cove, the main challenge is determining the correct angle of the jig’s fence in relation to the blade (since the only other consideration - the height of the blade – is simply equal to the desired depth of the cove).
Here, you have two possible approaches: one mechanical and one mathematical. Of the two, the mechanical method is easiest for most people. In essence, it’s accomplished by laying a template with an opening the same width as the desired width of the cove over the table saw blade. Adjusting the template’s position until just touches the saw blade teeth gives you the correct fence angle (see the photo on the right). The process is covered in detail in the Rockler Cove Cutting Table Saw Jig Instructions, and makes for a fast and reliable set up.
The ability to coves on a table saw can add significantly to your woodworking versatility. Done correctly, it’s a safe and fairly straightforward technique. But, as a final thought, we need to add that it’s not wise to jump in until you have a firm grasp on the procedure. Before you try it, make sure you understand the entire process, and as always, if you’re uncomfortable with any part of the procedure, take the time to study up, re-read your jig instructions, or ask some who’s got the technique down.