Quote Originally Posted by cropwell View Post
I googled the problem and found this https://planetcalc.com/1421/ you have to scroll down to the 'Segment defined by chord and height' and that will give you the radius of that circle that passes through the centers of the smaller circle. So you just add on the radius of the smaller circle.

Cheers,

Rob-T
Rob - unfortunately, it's not quite that simple! I generally try to find some sort of sanity check when I'm doing sums like this - does it give the right answer for the extreme cases? Does the answer move the right way if I change one of the given values? In this case, if you increase the size of the small circle to be equal to the segment height, the answer should be "infinity" for the radius of the big circle as the circle becomes a straight line. With your approach (which will be fairly close if the size of the small circles is small). as the small circle size increases, R increases - but that means that it no longer touches the vertical line. What's needed is the chord length and height at the points of tangency with the small circles - which you don't know at this point.

If it's any consolation, I'm typing this rather than being in the workshop because I've just fouled up a job on the CNC router. I wanted to move the tool up and clear so that I could add an extra fixing screw towards the end of a 1.5hr machining operation. Due to a complete brain fade, I moved Z up using machine rather than work coordinates - which meant that it plunged down rather than going up before moving in X and dragging work and spoil board sideways, leaving a big gouge in the work. The only good thing was that at least it was a thick spoil board so that the cutter didn't reach the machine bed, and I didn't break the 2mm carbide cutter. We all have off days...