# How to get center of mass for curves in Rhino?

Discussion in 'Software' started by poubj, Mar 22, 2012.

1. Joined: Dec 2009
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Location: Hood River, OR

### poubjNew Member

Hi naval designers,

Yes I know curves have no mass.

This is for the weigth estimation (mass analysis) and cost estimation for a framed metal boat modeled with Rhino.

I am using curves to represent lengths of various metal extrusions for which I know the weight and cost per length.

I can select all the curves of a given extrusion profile and ask Rhino for the total length, multiply with weight/ft or cost/ft and I have my weight/cost estimate.

Now I just need the center of gravity for those curves (for surfaces I use the area centroid).

Is there an easy way to get this?

Or is my method for doing weight estimation unsuitable and there is another better way?

Thanks,
poubj

2. Joined: Oct 2009
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### DCockeySenior Member

No direct method that I know of.

An alternative would to create a "pipe" around each curve with a diameter small relative to the length of each curve, and then use the volume analysis tools on the pipe. Normalize the results by the cross-sectional area of the pipe.

Another alternative would be to create a 3D model of each extrusion using the sweep commands. I set a construction plane perpendicular to the end of the curve in one view, create the cross section of the extrusion on that construction plane, and then use Sweep1 (not Extrude) to create the extrusion.

A twisted extrusion complicates matters.

3. Joined: Dec 2009
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Location: Hood River, OR

### poubjNew Member

Thanks David. I thought of using pipes and other surfaces, but then I loose the ability to determine the total length (purchasing of materials is done with lengths). So I need to have those objects modeled as both surfaces and curves (surfaces for determining the horizontal and vertical CG, and curves for total length. It gets messys and cumbersome.

Is there a way to get lengths of pipes or extruded/swept profiles?

4. Joined: Oct 2009
Posts: 5,030
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Location: Midcoast Maine

### DCockeySenior Member

Start with a curve. Length of the curve can be determined using Length

Use Pipe to form a pipe around the curve. Use a radius which is small compared to the length and easy to calculate the area of. For instance if the length is 5 use a radius of 0.1 which will make the cross-sectional area .01 * Pi.

You might want to spend some time going through the Rhino tutorials and exploring and trying out the various commands.

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