## arc center differences

### arc center differences

arc differences.skp
I have successfully installed Sketchups "point at center" tool. However, it produces a different result than I had anticipated and require. I am trying to array 4" dia. dowels over the top of an arched beam to form the latticed roof of a pergola.

Let me provide the measurements for the top arc of the arch to illustrate my problem. The top arc is 227.5" end point to end point and the height is 31.375" measured from the mid point of a straight line connecting the two end points and the mid point of the arc. Using the point-at-center tool produces a center point that is 53.875" shorter (nearer to the mid point of the arc along a radius line) than a radius point calculated using the formula to determine the radius of an arc segment. (the formula is Width squared/8H + H/2 {227.5 squared/8 x 31.375 + 31.375/2 = 221.88}

Using the rotate tool to place an array of the four inch dowels with the point-to-center tool mark causes the first and last dowels to rest nicely on the top of the arc, but those between to be varying heights above the arc. Using the rotate tool with the formula-derived center mark, creates and array with all of the dowels properly arrayed on top of the arc. It would be extremely helpful to have a tool that calculates the radius mark using the formula.

Am I using the point to center tool incorrectly or does it, in fact, determine a mark not the radius of the segment? Being a 70+ crumudgeon with zero programing skills, your always excellent, speedy and courteous help is most appreciated.
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Last edited by Cardinalgary on Sun Aug 19, 2012 5:35 pm, edited 4 times in total.

Cardinalgary

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### Re: arc center differences

Any image for explain the text ? (it's 40° at shadow here
Frenchy Pilou
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### Re: arc center differences

I set up a new arc with your dimensions and made the radial array without any trouble using the center point generated by Point at Center. I increased the number of segments for the arc to 48 just to make it appear smoother.

I don't know how you could be using the Point at Center tool incorrectly but perhaps you're using the Rotate tool wrong when you make the array or something else.

Although it doesn't look to be the problem in your model, if you use a low number of segments in your arc, it wouldn't be surprising if some of the dowels in the array are not actually touching the drawn arc. The Rotate tool will swing the copies through a real arc by your drawn arc only hits the radius at the vertices. If the dowel copies don't wind up on the vertices, they will appear to float.
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### Re: arc center differences

Your calculation of the radius of the described arc is correct. However, in your example model, the top and bottom of the beam is a "curve" and not an "arc". I suspect that is the problem with the "center point" location. Use the Arc Tool to create the arc by keying in the span and bulge after picking a start point.
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### Re: arc center differences

Your arch arc isn't on an axis - it's slightly off.
This is itself mightn't cause an issue, but if you pick points locked axially it could be a problem...
Try recreating your arch arcs parallel to the the green/red axes, find the center point and re radially array the 'dowels'...
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