Showing CG of an assembly in a Drawing 1

[davidh729]

I want to display the center of gravity location of an assembly on a drawing and have that location move as dimensions are changed in the assembly. I have placed a assmebly datum point at the CG using the mass prop parameters so the datum point is always at the CG. Is there a way to have a real CG symbol move with that point? I'm using wildfire 2.

Thanks
Dave

 

[3dlogix]

From the Vault of PTC, try this, its a tried and true method.

Steve

Suggested Technique for Creating Datum Coordinate System at the Center of Gravity of a Part

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The following example will step through the creation of a coordinate system at the center of gravity for any part. A similar technique may also be applied to assemblies.

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Procedure

Create a default coordinate system on the part by selecting Datum, Coord Sys, Default, from the INSERT Drop Down Menu. Note: Any type of coordinate system may be created.


Calculate mass properties for the part by selecting Model Analysis from Analysis Drop Down menu and Model Mass Properties from the Type Input Panel. Accept the default relative accuracy of 1.0000e-05 and select the coordinate system created in step 1 above. For this example, the following information is displayed:


MASS PROPERTIES OF THE PART

VOLUME = 2.3775283e+01 INCH^3
SURFACE AREA = 4.9949963e+01 INCH^2
DENSITY = 1.0000000e+00 POUND / INCH^3
MASS = 2.3775283e+01 POUND

CENTER OF GRAVITY with respect to CS0 coordinate frame:
X Y Z 0.0000000e+00 0.0000000e+00 2.4544557e+00 INCH

....

ROTATION ANGLES from CS0 orientation to PRINCIPAL AXES (degrees):
angles about x y z -90.000 0.000 -90.000

RADII OF GYRATION with respect to PRINCIPAL AXES:
R1 R2 R3 8.8098137e-01 1.5325060e+00 1.5325066e+00 INCH




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Select Datum, Point, Offset Csys from the INSERT Drop Down Menu, With Dims, Select the coordinate system created in step 1. Specify the coordinate system type Cartesian, and select Enter Points from the POINT ARRAY menu. Enter some arbitrary x,y, and z translation dimensions.


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Create relations involving the datum point's translation dimensions, which will place the point at the center of gravity of the part.

Syntax:

$d#=mp_cg_x("path","coord_sys","path")
$d#=mp_cg_y("path","coord_sys","path")
$d#=mp_cg_z("path","coord_sys","path")
d# is the dimension number

The first path is the path to the component for which the value will be calculated. The second path is the path to the component which contains the coordinate system. Since this is a part, there is no path to specify, because a part has no components.

coord_sys is the coordinate system name that the mass properties of the part were calculated from.

The dimensions on the left side of the equations are preceded by dollar signs to allow for negative values. Without the dollar signs, the following error message will appear:

ERROR: Invalid attempt to assign negative value to 'd#'. Please re-enter:

For this example, the relations will be:

$D35 = mp_cg_x("","CS0","")
$D36 = mp_cg_y("","CS0","")
$D37 = mp_cg_z("","CS0","")

This will place the datum point at the center of gravity of the part.

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Next, place a coordinate system on the point with axes parallel to the corresponding axes on the coordinate system created in step 1 above. This will be used to measure the angular rotation of the center of gravity from the coordinate system.


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Next, recalling the rotational angles from Step 2, create a coordinate system at the point, rotated the same amount as indicated by the mass property information. For this example, the rotational angles are x=90, y=0, and z=90.

http://www.sprdesign.com

http://www.3dlogix.com

 

[mloew]

Dave,


The analysis feature can be used to directly create a point and/or coordinate system at the mass center. You can also use the analysis feature output parameters to manually create the datum feature. I use this functionality quite often.

Best regards,

Matthew Ian Loew


Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Hora]

Subscribe to Matthew's solution. It's elegant and easy.

-Hora

 

[davidh729]

I guess I wasn't really clear.
I already have a datum feature that moves with the CG. I want to put a real CG symbol in the drawing that moves with that datum point.

Thanks

Dave Hyman
iRobot Corp

http://www.irobot.com

 

[mloew]

Dave,

I just made a datum (curve) feature that looked like I wanted it too. You can create the filled in quadrants of the circle with surfaces.

Best regards,

Matthew Ian Loew


Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Hora]

Dave,

Create a point in your 3D model (offset with 0,0,0 from the CG coordinate sysytem or add it in the ananlysis feature).

In drawing mode, just snap your CG symbol to this point. From now on your symbol will follow the CG.

-Hora