Hi,

I need to calculate the center of gravity for an object made out of
two materials. And would appreciate any tips regarding to solve
this task ... thx

http://moi3d.com/forum/index.php?webtag=MOI&msg=7421.71 for have a center of gravity of one volume

So for 2 Volumes with different weight A & B
Draw a line between the 2 center of gravity
M somewhere on this line : you must have A * M = M * B

But maybe Max plugins makes that automatically (?) : I don't verified!

In a uniform gravitational field:
Calculate the center of gravity for each material.
Calculate the mass (weight) of each material.
The mass of material one will act as if it is all located at cg1.
The mass of material two will act as if it is all located at cg2.
So now it is a teeter totter... (Now look up teeter totter physics on wikipedia.) (The mass of the plank is zero.)
From memory, I think that the distances from the fulcrum times the corresponding mass will be a constant. (more math.)
d1 x mass1 = d2 x mass2.
d1 + d2 = dTotal = distance between cg1 and cg2.
The fulcrum point is the combined center of mass.
- Brian

d1 = D*m2/(m1+m2) (?)

https://www.onlinemathlearning.com/lever-problems.html

It has been very hard to find good references on google :-)

===SeeSaw

Thank you both for pointing me towards the solution.
As I need to place the overall gravity center at a specific location,
it may take lots of iterations to get there.

Specific location with respect to what?
One of the masses?

- Brian

Hi Brian,

I am adding a sketch.

Given are 2 volumes with different materials / densities.
They should be optimized (shaped in such way) that their
common center of gravity matches the desired position x/y/z.

view 3D is missing for presume if that can be possible and density estimated for a such result :)

Hi Mr Frenchy, I am sorry your comment is to poetic for my understanding - waht did you
mean exactly ? :-O

Thanks Tom,
I am not an engineer, but it seems that it is necessary to define a bunch of specifications for these 3D bodies.
As Pilou stated, the densities, and 3D depth are two parameters.

It is pretty easy to get the centerpoint of a uniform "cuboid".
What is meant by "Shape"? Are the "cuboids" to be bent?, have portions removed?
Touch each other or be separated?
The combined center of gravity would be located back in the 3rd dimension.
If mass A is fixed in relation to the combined center of gravity, mass B looks to be a bit low.
It seems possible that with too many constraints, the combined center of gravity will be elsewhere than where desired...
- Brian

Pretty sure that the three centers of gravity need to be in a straight line...

Just that ;)
I doubt that these 2 groups have the same center of gravity you show, and more if density of any of them are very different...
Maybe I am wrong but...

Thanks for the interest in the topic !
Soecifics in the attachment. Of course it woul be great just to
let a software spit out the CofG as many itertions will be needed
ti get the CofG where it should be ...

back in the day, i used to use autodesk inventor for this sort of thing. it was really good at calculating inertial properties and showing the cg. there are some autodesk fusion users on here. they might know if the free version of that can calculate inertial properties and show the cg. i've never found a low cost or free program that could do this type of thing justice.

anthony

Great tip Anthony, thank you !

The blade taper should help a lot at "raising" the cg. And there is symmetry in the "thickness."
There is likely a tang to consider also? And pommel weight. And "rivvits" holding on the handle.
The blade smith making show on TV just passed 100 episodes.
- Brian

So far I see there is no solution within MOI.

I will experiment if Cinema 4D Dynamics works exactly
enough to handlethe problem.

Out of curiosity, I did some more google searches, and learned more.

CVolume2 by Max calculates the volume of 3D objects, and their center of mass, assuming uniform density.
(I'll assume that the calculation is correct, but this should be verified for critical situations.)
So CVolume2 would be used once for material1, and again for material2.
Given materia1 has density 1 unit, the relative density for material2 would be needed, to adjust the volume of material2 up or down by the mass ratio.

material2 can then be replaced by a solid, say a cube of the adjusted volume, centered on its cg.
material1 can stay the same, or be replaced by a cube of the same initial volume1.
Then run CVolume2 on the two new cubes, to find the combined cg.
----------
Centroids can be calculated by geometric decomposition.
A hole or void or missing 3D triangle, say of the edge taper, is considered to have negative mass.
Choose x-axis, y-axis, and z-axis, so the origin of the frame may as well be located at the desired cg.
The cg formula can be decomposed into x, y, and z formulas. Vectors can be used for the distances, and each distance could be either positive or negative.
The goal is to have the distance R from the desired cg be zero.
0 = R = (1/(total mass)) * Sum from 1 to n of (mass(i) * rDistance(i))
If R does not equal zero, grind off mass, or add mass, until R = 0.

A list of some good references:
http://www.splung.com/content/sid/2/page/centre_of_mass

https://en.wikipedia.org/wiki/Center_of_mass

https://en.wikipedia.org/wiki/Centroid

https://en.wikipedia.org/wiki/List_of_centroids

http://www.simscrane.com/how-determine-center-gravity-any-load/

http://ruina.tam.cornell.edu/Book/COMRuinaPratap.pdf

http://ims.co.il/download/How%20to%20calculate%20the%20Center%20of%20Gravity.pdf

Throwing also yields some interesting information.

Maybe CVolume2 could be changed to adjusting the mass of different objects...

- Brian

Hi Brian.

Thats great info you are comming up with.
Of course it would be much more convenient to have a software
doing the job than doing it by hand ;-)