volume calculation of a cross-section by rotating around the central axis

[Brian]

Hey everyone, I'm fairly new with using AutoCAD mechanical 2008. I've been using CADkey '97 for years now, so this new technology is so ridicuoulsly simple to use once I figure out how to use it (that's the hard part).

My question is: Say I have a circular 3D figure, in which the cross-section is not rectangular. I'm trying to figure out the volume of the figure by drawing it's cross-section and revolving it around it's central axis. From there, I would like to find the area of the cross section, and the x and y location of the centroid.

What I would do for CADkey was first move the cross-sectional drawing to [0,0,0], with the base-point being the bottom, right-hand side of the shape. I would then click "data>verify>area/centroid>type 0.01". This would give me the area and the centroid of the object. I would then use the following calculation for volume:

V = 2*pi*A*(centroid + x-distance from basepoint to central axis).

That would give me the volume of the 3-D shape.

If anyone can help me out with this it would be greatly appreciated. Thank you all very much in advance.

[GE13579]

I think if you're after any details you might want to try the List command, if I'm understanding your requirement?- You have a 2D section of which you need the area and from there you have a calculation for the volume?? :?

[Brian]

I think if you're after any details you might want to try the List command, if I'm understanding your requirement?- You have a 2D section of which you need the area and from there you have a calculation for the volume?? :?

Yes, exactly. Say you have a cross-section of a 3-D circular object. In my case it is a metal ring. The metal ring has an OD, small id, large id, and a height. Therefore, if you look at the cross-sectional area of the ring you will be left with a non-rectangular shape. I need to find the area of that non-rectangular shape, and the centroid of it as well. Using those two values I can calculate the volume of the ring. I just don't know how to do it using AutoCAD mech. '08.

[GE13579]

Am I right to think that if it's a ring you have 2 mirrored sections? If so one way around getting the mid point could be to draw a line from the outer diameter midpoint to the opposing outer diameter midpoint (you do mean that by "OD"?).

Have you tried LIST to get the area?

[GE13579]

I should add that I'm a 2D guy but I'm sure the 3D lot will just be able to draw the ring and have ACad give the volume, but for now this is the only way I can think of. :)

[riga]

If you create a polyline that suite your cross-section you have the area (just by selecting and look at the properties) then you have to convert that polyline into a region and with the massprop command will give you the coordinate of the centroid.
Well, I can not be sure because I can not check right now, but if you haven't solved let me know and I will give it a try

[Brian]

Am I right to think that if it's a ring you have 2 mirrored sections? If so one way around getting the mid point could be to draw a line from the outer diameter midpoint to the opposing outer diameter midpoint (you do mean that by "OD"?).

Have you tried LIST to get the area?

Yes that's what I mean by the OD. I have tried "list", and that does give me the correct area of the cross-section (I think it's the correct area, I compared it to the same drawing I made in CADkey, and it gave me a similar, but not exact result. But it doesn't give me the centroid.

I'll try and simplify things a bit. Instead of thinking in terms of what I'm actually doing, let's just say I have a shape that I drew on autoCAD. I'll draw a triangle, but it's not an isosceles triangle (meaning that there's no simple calculation for where the center of gravity is). I just need to find out the area and the centroid of that triangle. I do believe that the "list" function you gave me gives me the correct area, but I still need the x and y values of the location of the centroid.

I appreciate your help though. Thank you very much.

[riga]

Ok, I try to be more clear.
You have your triangle:
Command line > _REGION > ENTER
now you select all the lines composing your triangle
> ENTER
now that you have created a region give the command > _MASSPROP > ENTER
Select the region
> ENTER
A window with area and centroid coordinates will pop up

[GE13579]

If it's a cross section of a ring won't you have the object mirrored? I'm a bit confused... can you post a screen shot?

[Brian]

Ok, I try to be more clear.
You have your triangle:
Command line > _REGION > ENTER
now you select all the lines composing your triangle
> ENTER
now that you have created a region give the command > _MASSPROP > ENTER
Select the region
> ENTER
A window with area and centroid coordinates will pop up

AHA...EXACTLY!!!! Thank you very much for your help.

I have A LOT to learn about this program. I didn't know about the "region" command first of all, which was part of my problem, and then I didn't know about the "massprop" command either. Thanks again.

[Brian]

If it's a cross section of a ring won't you have the object mirrored? I'm a bit confused... can you post a screen shot?

I'm not sure how to post a SS, but I'll see if I can figure it out as well. Thank you very much for your help as well. It's much appreciated.

[GE13579]

No problem, I thought someone from the 3D side would have a solution, thanks Riga! :D

[Brian]

Just for future references. How do would I post a screen shot?

[GE13579]

Shift + PrtSc, then paste into paint or similar, save as, then click the attach paperclip and browse where you saved it. There are file size restrictions so if it's a large file you might need to zip it. Also save it as .jpg I think as some file types can be hard to see.

[Brian]

3077

ok here it is....Do you happen to know how to change to colors of the dimension? (I know I'm a complete noob with this program)


Basically, what I was trying to do was calculate the area and centroid for the shape on the left (which is mirrored around the central axis. I first had to make the shape into a polyline, then I had to copy and paste the object at the origin with the bottom right corner of the object as the base point. Then from there I used the Massprop command. It's basically the same procedure that I had to do with CADkey.

[GE13579]

Type "D" and enter, that should bring up the dimension style dialogue window, although I only have 2004 at home and it doesn't work, but I'm sure that's the command for 2006 on... failing that type DIMSTYLE.

Otherwise try Assist tab, Options, AMStandards, double click Dimension in the right hand box.

[Brian]

thanks again bud.

[riga]

Ok, happy that you solved your problem.
I am a 2D guy too, that's why I know about the region command ;)
If you have a solid, the massprop command will give you a lot af intersting data as volume, centroid and so on. Bur to have the centroid of a 2D surface the solution is to create a region as I suggested before, because it seems that autocad handles regions as a 3D entity... Mmmmh, now I am maybe going beyond my competences and it's better I quit before I say something too stupid :D

[Brian]

This new version that I'm using is blowing my mind. There's so many new features then there was in CADkey '97, it's ridiculous!!! I have to just play with the program for a while I guess until I figure out the "philosophy" of this software.

Thanks again.