territorialmillworks Posted May 4, 2012 Share Posted May 4, 2012 I could have figured this out 30 years ago but my higher math skills are non-existant today. So I need to put three legs on a 20" circle. I know that I could draw a circle and use a divider and get there by trial and error but I'd like to use a formula for future projects. thanks, Keith Quote Link to comment Share on other sites More sharing options...
Glenn Posted May 4, 2012 Share Posted May 4, 2012 Reference:Whitesmith Use a large hex nut or bolt. Hit every other point Calculate the circumference and divide by 3. 360* divided by 3 = 120* or two 60* added together. Think of a 30, 60, 90 triangle here. Quote Link to comment Share on other sites More sharing options...
jeremy k Posted May 4, 2012 Share Posted May 4, 2012 360º divided by 3 = 120º add half the width of material to each side of the lines after you made your pie shaped drawing (120º incriments). Quote Link to comment Share on other sites More sharing options...
Jeff Seelye Posted May 4, 2012 Share Posted May 4, 2012 Got a protractor? 360 degrees in a circle divided by 3 is 120 degrees. If you do very many, it's worth the time to take 3 pieces of flat stock and weld them together at 120 degree intervals. Then you can clamp the legs (or scrolls) to the flat stock. The pieces will be 120 degrees on center AND vertical. Quote Link to comment Share on other sites More sharing options...
doc Posted May 4, 2012 Share Posted May 4, 2012 20"x 3.14 = 62.8/3 = 20.93 lay it out with your tape measure Quote Link to comment Share on other sites More sharing options...
Glenn Posted May 4, 2012 Share Posted May 4, 2012 Choose a center and draw the circle. Place a square's (carpenter's square) right angle at the center of the circle and draw along the legs to the circumference. Draw a cord (line) between the two points where the leg lines cross the circumference. Measure the length of that cord (line) between the two cross points and add 1/3 more (90*+30* or 120* total). Choose one of the cross points and strike an arc the length of the cord + 1/3 both left and right. You now have your 3 leg locations. Quote Link to comment Share on other sites More sharing options...
Chris Jones Posted May 4, 2012 Share Posted May 4, 2012 Assuming that is a 20" diameter circle then use a bit of straight rod of 17 1/3" long, place it on the circle so the ends touch the circle, mark both ends, keep one end on the circle and move the other so that it touches the circle in a different location (there will only be one place it can) mark this, check the original mark by putting the rod between mark 1 and 3. If the ends don't all meet on the circle then I've done my maths wrong. Or as other have said measure 20.93" around the circle and mark it, repeat and you should have 3 marks - i just find measuring round a circle more difficult than straight lines. If you want the maths for the straight line version for different circles then give me a shout. Chris Quote Link to comment Share on other sites More sharing options...
Dick L. Posted May 4, 2012 Share Posted May 4, 2012 http://www.metalwebnews.com/formulas-tables/coordinates.html Hope this helps Dick Quote Link to comment Share on other sites More sharing options...
John B Posted May 4, 2012 Share Posted May 4, 2012 I could have figured this out 30 years ago but my higher math skills are non-existant today. So I need to put three legs on a 20" circle. I know that I could draw a circle and use a divider and get there by trial and error but I'd like to use a formula for future projects. thanks, Keith If you have a 20" ring you put the legs on that diameter. If you are looking for a formula to determine the length of the spaces between the three legs, then simple trig will be required Formula works out to be 0.866 (the natural Sine of 60 degrees) Multiplied by the radius of the desired circle, and double the result. Quote Link to comment Share on other sites More sharing options...
JNewman Posted May 4, 2012 Share Posted May 4, 2012 This is a REALLY easy one. Scribe your diameter, then without changing your compass/trammels use the them to divide the circle into segments. The radius of the circle will divide the circle exactly into 6 even segments. You then use every other division. As long as you are carefull to set your points accurately on the divisions this is the most accurate way to lay out a large circle. To make it even a little more accurate run a center line through the center of your circle and then layout from each side that way any error does not accumulate. Quote Link to comment Share on other sites More sharing options...
kurtclampit Posted May 4, 2012 Share Posted May 4, 2012 eyeball lol lol just kiddin sri Quote Link to comment Share on other sites More sharing options...
Glenn Posted May 4, 2012 Share Posted May 4, 2012 Ask a blacksmith a question and 10 ways to do the same thing? Quote Link to comment Share on other sites More sharing options...
Glenn Posted May 4, 2012 Share Posted May 4, 2012 If you are looking for a formula to determine the length of the spaces between the three legs. 0.866 (the natural Sine of 60 degrees) Multiplied by the radius of the desired circle, and double the result. Is this measurement for the cord or the arc between the two points? Quote Link to comment Share on other sites More sharing options...
John B Posted May 4, 2012 Share Posted May 4, 2012 Is this measurement for the cord or the arc between the two points? Mark out the initial calculation result as a line, and swing an arc of the same length from each end of the line, and this gives you the three (equispaced) points for the legs centrelines To find the centre for the circle from these points, Join these three points up with a straight line for each side Mark out the middle of each of these and draw a line to opposing apex Where they cross is the centre of the circle Quote Link to comment Share on other sites More sharing options...
Glenn Posted May 5, 2012 Share Posted May 5, 2012 I get it now. You are forming a triangle, each leg the resulting length of the calculation, 0.866 (the natural Sine of 60 degrees) Multiplied by the radius of the desired circle, and double the result. Quote Link to comment Share on other sites More sharing options...
Frosty Posted May 5, 2012 Share Posted May 5, 2012 Wrap a single piece of string around the circumference and mark where the end crosses. Fold the string three times and mark the crease. Now when you wrap the string back around the piece transfer (ONLY) THREE of the marks on the string to the work. To make a leg, draw a line between the marks on the circumference and another from one mark to the center of the opposing line, this is the center line of your leg/arm/whatever. Oh yeah, to find the center of a line cut a piece of string that length, fold in half and use it as your scale. Who needs rithmatic, it worked for the Egyptians and earlier. Frosty The Lucky. Quote Link to comment Share on other sites More sharing options...
John B Posted May 5, 2012 Share Posted May 5, 2012 Yes Frosty, quite right and that's the easy and practical way, the original question was a request for a formula to use in future Quote Link to comment Share on other sites More sharing options...
rthibeau Posted May 5, 2012 Share Posted May 5, 2012 I'm with Kurt.....eyeball it and you will be well within tolerances for balance Quote Link to comment Share on other sites More sharing options...
Frosty Posted May 7, 2012 Share Posted May 7, 2012 Yes Frosty, quite right and that's the easy and practical way, the original question was a request for a formula to use in future That's true of course John but I don't do math unless I have to. I'm an easy way first kind of guy, so long as the quality is there. Frosty The Lucky. Quote Link to comment Share on other sites More sharing options...
John B Posted May 7, 2012 Share Posted May 7, 2012 That's true of course John but I don't do math unless I have to. I'm an easy way first kind of guy, so long as the quality is there. Frosty The Lucky. Me too, but the question was asked, I started another thread for marking out trivets, for the simple non math answers from folks, thought it would be useful for others like us, be interesting to hear how many variations are put forward, I can think of a few ways I use, but wondered how others do it. Quote Link to comment Share on other sites More sharing options...
Francis Trez Cole Posted May 7, 2012 Share Posted May 7, 2012 some times the right numbers do not look pleasing to the eye I would get a feel of it. Quote Link to comment Share on other sites More sharing options...
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