I need some trig help. I need to create a bending form but I need the diameter (or radius, it doesn't matter) based on the following
The diameter of a circle formed by an arc with a 1/8" apex over a 1 5/16" base length. Yeah, I know some of the terms are wrong, but I'm a long way removed from Mr. Cole's high school trig class.
No real way for ASCII art for the circle, so I'll try just the algebra from a verbal description. Consider the triangle of the radius perpendicular to the chord and another radius to one end of the chord. That's a right triangle whose hypotenuse is R, one leg is your desired L/2 and the other side is the radius R-1/8.
So, letting x = L/2, r = R and h the projection desired, Pythagorus says
x^2 + (r-h)^2 = r^2
x^2 + r^2 - 2rh + h^2 = r^2
x^2 - 2rh + h^2 = 0
r = (x^2 - h^2)/2h
Substituting in your values I get r = 6.828 --> 6-53/64", approx.
Yeah, I saw I didn't transpose the one sign when I did the copy and paste from one line to the next after I too briefly looked at it earlier...dang bifocals! (Got to blame something, can't be I just made a misteak! :) )
"Joe" wrote in news:RVtOi.38972$ snipped-for-privacy@newsreading01.news.tds.net:
It's been a while for me too, but I think a diameter of 3 9/16 is close to what you want. It seems small to me, but I did it twice and checked, so like I said it should be close (I got 3.57 inches for the diameter, which is just a hair over 3 9/16.)
The setup is to draw the chord and radii from each endpoint and one from the center of the chord - that gives four right triangles, two big ones with a radius as hypotenuse and two little ones. Solve the small triangle (you know the two sides, 1/8 and 21/32), then figure out how the little triangle is related to the larger one with which it shares a side. I figured out that the central angle of the larger right triangle is just twice the smaller angle in the little triangle. I got r sin 2(arctan {(1/8)/(21/32)}) = 21/32 and solved for r. You can check me on this, but I think it's right.
You got the answer (and I didn't until somebody pointed out the algebraic error in sign I made, so take this for what that's worth :) ), but in this particular case you can solve for the radius directly from one of the large (right) triangles as two sides are known in terms of one unknown (the radius) without even having to solve the quadratic as the quadratic terms end up canceling...
Joe wrote: | I need some trig help. I need to create a bending form but I need | the diameter (or radius, it doesn't matter) based on the following | | The diameter of a circle formed by an arc with a 1/8" apex over a 1 | 5/16" base length. Yeah, I know some of the terms are wrong, but | I'm a long way removed from Mr. Cole's high school trig class.
Sorry to have arrived so late. This gets asked enough that I put up a web page with diagram and step-by-step algebra at the link below.
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