index marks at 120 degrees...A DUH moment.

When stepping round a circle I've always found it virtually impossible to get it exact so I usually work both sides of the first point in an attempt to minimise the small discrepancy. I've only had it exact a few times in my life and still dont know why but I was pleasantly surprised when it happened.
I have to assume its the dividers or compass moving or flexing very slightly because it cant be me can it?:laughing:
 
I understand. Very elegant. That is a new technique to me and one I intend to try! It never occurred to me that 6 equilateral triangles fit in a circle making the length of the cord the same as the radius!
Robert
 
Robert -

Just for fun, take another look at the link that Randy provided:
https://www.engineeringtoolbox.com/chord-length-circumference-circle-segments-d_1870.html

Note on the table that as the number of segments gets large, the "Summarized length of Chords" keeps getting closer and closer to 6.2832, or 2*π (actually, 2π*R), which is the circumference of a "smooth" circle. π*D = 2π*R. A "smooth" circle is the mathematical limit of an infinite number of infinitely small chords. Calculus, here we come!

As for cutting the circle into 6 pieces by striking radius lengths around it, think of this as "taking shortcuts" across the inside of the circle. So the "Summarized length of Chords" is 6*R instead of 6.2832*R.
 
Yep. Very cool. The limit of the sum of the cords as N approaches infinity is 2π*R! I drew out a 6 bolt hole circle last night with a caliper and it was highly accurate. I would have used this to mount my lathe chuck if you guys had been more timely! Ha!
R
 
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I've used that method on large bolt circles as well, up to about 120", but worked from the starting point and marked half one direction (CW) and half the other (CCW), thinking that any error in my picking the new arc center would be cumulative and should be minimal that way. It is a little awkward on large circles using trammel points, but two people make it a simple task.
 
Funny thing about the six part division you advised above as we now remember reading something describing it recently here or on PM site recently and had forgotten about it.

We have a few higher grade dividers that we have picked up at estate sales for this function but the light was not lit well...

What happens when one is just making things as they go without any drawings...

The project started as a need to fix something and some sketches were drawn as general concept but only one part is close to sketch.

More fun this way sometimes...
 
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