Machining coves on the tablesaw

[murrmac]

When I worked in the States years ago, one of the routine jobs we had to do was to run coves (for cornices, or "crown moldings") on the tablesaw by clamping an auxiliary fence at an angle to the blade, and using multiple passes, raising the blade a fraction after every pass.

Accuracy of radius wasn't really an issue, as long as it was eye sweet that was all that mattered, but there are actually almost an infinite number of different curves which can be machined this way by combining various angles of fence with various tilts of blade.

Now I do realize that mathematically, all these curves are technically sections of an ellipse, rather than arcs of a circle, ( with the single exception of the curve generated by passing the workpiece over the blade at 90 degrees, and the fence at 90- degrees to the blade) but I just wondered if there was any mathematical formula whereby you could get a very close approximation to a required radius by presetting the blade tilt, and the angle of the fence, without messing around by trial and error. I am sure that there must be a formula, but my maths is definitely not up to it.

In particular, I am interested in being able to machine coves of 12" radius, 14" radius, 16" radius and 20" radius, using a 10" sawblade.

All input from mathematically inclined machinists welcomed ...

 

[Travis]

Is this any help?

http://woodgears.ca/cove/calculate.html

Travis

 

[Frank S]

Thanks Travis,
you have saved me a lot of brain ache as I need to do some of this in the new year.
Frank.

 

[murrmac]

Yes, thanks Travis, that is exactly what I was looking for.

I do visit the woodgears site from time to time but I had never seen that article before.

Murray.