Over at cardmodelers.org in the April ezine in Matthias Harbers' review of a Fokker DR1 model is a simple circle centre finder. It's simply a set of concentric cricles in 1mm radius steps printed on transparency. Made one. tried it out and it works pretty well. As usual - the more care you take the better the result. If anyone wants to try this but doesn't want the boredom of doing 40+ circles in 1mm increments, e-mail me and I'll send the file. Regards, Charlie

Charlie Thanks for your email and the tool! I ran it through corel draw and then Acrobat to convert it to pdf for anyone not capable of opening .doc files. I've placed it in the 'parts bin' for anyone who needs it. I for one will be running it off on transparency asap as I'm forever trying to find the centre points or establishing the arcs for cones. Thanks again! Ron

using a compass (a simple compass= a pin with a piece of string and a pencil on the other end of the string) : 1) put the point on the circle and draw an arc touching two points on the circle. 2) make a line connecting those two points. 3) mark the midpoint of that line and draw a line connecting that midpoint to the point on circle you used to define the arc. 4) repeat 1-3) at a different location. 5) the point where the lines drawn in step 3 meet is the center.

As an extra added bonus, print on white paper and hold it at arms length, move your arms clockwise to see the propeller spinning

I agree you can can find circle centres with geometry but often the number of circular parts in a model will make this a daunting prospect. I did a quick count of a fairly simple AFV model - came up with about 200 circular parts or parts with arcs to find the centres. I think I'll take Mattias Harber's idea. Regards, Charlie

I know the concentric circles on clear plastic it the easiest and most usefull. I just love what euclid could do with just a straight edge, a stick and a piece of string. 3000 years ago.

Jon In fact 2,300 years ago. One of Euclid's greatest contributions was to ensure the application of rigour, logic and precision to the pursuit of mathematical arguement. The tradition having been followed for more than two millenea it is perhaps a little sad to see it so lightly discarded in a single generation. BTW did you know that he even knew what was defined by the set of points equally distant between two points ? Maurice

I'm concluding that the answer is a plane, (the set of points equidistant between two points, in 3d space, a line in 2d space.) I would not be surprised that Euclid came to the conclusion about 2d space, but I am unaware of any 3d work by Euclid.