Need some brain power better than mine

[dcyrilc]

What the ****?? That just made me feel a tad dumber than when I started:confused2: lol
So you are saying is.... wrap the tape around the outside of the circle and mark every 5 and 21/32" inch and all 5 bars will be equally spaced???

Correct. Brad & I both came up with the same measurement. LOL

 

[dcyrilc]

if you don't have a flexible tape handy to wrap around the circle, another option is to take one point at the outside of the circle, and measure from that point to another point along the outside that is about 5 1/4~5 5/16 away. repeat this from each new point and you should find yourself back at the start point. same result, just a different way of measuring. the math behind it is as follows:

chord length = 2*R*sin(A/2) = 2*4.5*sin(72/2) = 9*sin(36) = 5.29 inches

where:
A is the angle (360 degrees in a circle, divided into 5 equal 72 degree segments)
R is the radius (9" diameter = 4.5" radius)
chord length is the straight line distance between two points along the outside of the circle

It's been too many years to remember that formula. I took the easy way out.:laughing:

 

[jjmarotz]

WOW!!!!!! Guess a bit more attention in math classes 20 yrs ago would have paid dividends!!!

This is where I get lost... but I think I am following what you fellas are saying. Thanks!!!!!

 

[Brad_Blazer]

Here's another option. If you inscribe a pentagon inside a 9" circle, the length of each side is 9 x sin(36) or 5.29". That's close to 5-9/32. If it was me I would abuse my digital calipers by setting them on 5.29" and using them as dividers to scribe my way around the circle.

 

[dcyrilc]

WOW!!!!!! Guess a bit more attention in math classes 20 yrs ago would have paid dividends!!!

This is where I get lost... but I think I am following what you fellas are saying. Thanks!!!!!

Brad & I gave you measurements around the outside edge, lostcause gave you the straight line distance across the disk to the same point. Try it out and see how it works.:thumbsup:

 

[Brad_Blazer]

You guys are too fast!

 

[lostcause]

It's been too many years to remember that formula. I took the easy way out.:laughing:

to quote sean connery in one of the raiders of the lost ark movies.... i wrote it down so i wouldn't have to remember. actually, i just remembered what book it was in:

american institute of steel construction - allowable stress design manual, ninth edition, 1989. pg 6-16, properties of the circle.

 

[dcyrilc]

to quote sean connery in one of the raiders of the lost ark movies.... i wrote it down so i wouldn't have to remember. actually, i just remembered what book it was in:

american institute of steel construction - allowable stress design manual, ninth edition, 1989. pg 6-16, properties of the circle.

Cheater:laughing:

 

[DJ54]

This is where an angle gauge would come in handy if you have one.

I would pick one of you marks, any one of them, and mark with an "X" so as not to loose your starting point. Set the gauge at a 90 degree angle, then close the angle 18 degrees, to 72 degrees. Set the gauge on you starting line/mark, then mark at the 72 degree angle mark. Maybe mark that one with an "X" also, so you know it is a 72 degree angle.
Leave the gauge set where it is, and move the gauge to the line you just made. Your gauge is set for the next 72 degree angle to be marked.. Just follow on around, on those new marks.
Basically, you are dividing a 360 degree circle by 5, making 5 even spaces of 72 degrees, or a pentagon.

Been there, done that... An angle gauge, and construction calculator, sure help getting through stuff like this...

 

[Brad_Blazer]

What the ****?? That just made me feel a tad dumber than when I started:confused2: lol
So you are saying is.... wrap the tape around the outside of the circle and mark every 5 and 21/32" inch and all 5 bars will be equally spaced???

That's right. With either method when you measure from the last mark to the start point if it is the same you are golden.