Thread: cove cutting question - basic geometry

Look at the blade side on, when its vertical it is a circle.
If you tilt it and look from the side it is an elipse.

If you are talking about a fingerboard 2 inches across cut with the top edge of the elipse you could approximate it to a larger circle (that you have a segment from), But it is still not a circle segment exactly.

If you are talking about 1.5 Inches its a better approximation of a circle segment.

Do you still want the maths done?

Peter

I find it easier when coving to use arc width and arc height inputs.
To your first question tilting the blade will affect the shape of the cove not the radius as such.
Back to the calculations. Once I have the arc width and the arc height which can easily be converted to a radius using one of the many calculators on the net I use Blocklayer.
With the arc height I take the radius of the blade that I will be using for me that is generally 127 mm and work out a new arc width for an arc height with the saw blade radius and arc height. This is used to work out the angle for passing the timber across the blade.
I treat the width of the cove I want to create as one side of a triangle and the saw blade arc width as the hypotenuse.
Just a word in general on coving take it about 3 or 4 times slower than you think you need to. For a cove 6 or 7 mm deep I you usually be making between 12 and 15 passes. It also puts quite a strain on your saws bearings I have had to replace those on my carbatec tsc10hbr.
I hope you can understand this waffle

Ben

apologies
i have a 10" diam saw that can only cut a 5" radius cove
been thinking about this for days and just suddenly realised my initial premise is completely wrong

Old picker I have never passed the timber over the blade at 90 degrees to the blade and thinking about it at 90 degrees the cove would be symmetrical even when tilting the saw blade this would enable you to have a bigger arc width for less arc height. I am not sure of the exact shape you would end up with but it wouldn't be far off

I just calculated that tilting the blade to 45 degrees and passing at 90 degrees to the saw blade would give a arc width 75 mm for arc height of a smidge under 4 mm if that helps at all.
Ben

the idea came from here
from the venerable oztradie master guitar builder who is a forum member
i thought that extending this out a little and using the bigger blade of the table saw would give more accurate sanding blocks

if i can't get a decent job out of it i am going to have to import specialty sanding beams from the states at around $300 for the pair inc freight and a couple other lil pressies