Hi Paul O,
I've seen a jig online that can help calculate cove width etc. Of
course, I've hunted but can't find it now. So I drew up a crude drawing
on MSPaint as a gif image here
I've never used the jig but here's my theory. Anybody...feel free to
add or correct if needed.
What you do is take 4 sticks (2 blue and 2 green) and attach them so
they fully pivot. A recessed nut and bolt should do the trick. Stick
dimension can be 1/2x1/2 or 3/4x3/4, doesn't matter. I would use
1/2x1/2 for mine. I'll explain why later. You'll probably not make a
single cove molding any wider than maybe 6 inches so the max width
between the two blue sticks will be about 6 inches. So the green sticks
will be about 7 inches, giving extra room for attaching the green to
the blue. You get the idea.
For your example of a 1/4 inch deep by 4-1/2 inch wide cove, first set
the balde height to 1/4 inch. Then open up the jig and position it on
the tablesaw as shown in the picture. Bring the blue sticks to the
cutting tips of the blade. Next rotate the jig clockwise until the
distance between the two blue sticks is 4-1/2 inches. This is the angle
you will use. Then proceed with the normal start of making coves on the
I would use blue sticks that are 1/2 in width. Once I have my angle
figured out, I'd clamp two straightedges on either side of the jig,
then remove the jig. The distance between the two straighedges is 5-1/2
inches. I'll now pass a 5-1/2 inch board to make my cove molding. If
5-1/2 is too wide for the install, no big deal. I can always trim it up
to the specs I need after I'm done.
Oh yes, cut more than you need...just in case. :-)
Hope that helps.