Centreing 3D Objects Using Constraints

Scott McKenzie

Hello:

Recently I began working on a 3D model of a component for a project that I have been working on for a while. I have two objects, one is essentially a chunk of square HSS and above it is a piece of symmetrical formed plate that has no vertical edges, but it does have a horizontal top face and a horizontal bottom face with a nice radius on the corners.  To simplify things, and because I cannot send any screenshots or drawing files around because of an agreement, consider that you have two cubes, both of different unknown sizes and would like to stack the small cube on top of the large cube, and centre the small cube on the large cube, is this possible using 3D constraints?  I was thinking of using four distance contraints to centre the cubes, assuming the cubes are stacked along the Z axis, two distance constraints on the X axis and two on the Y.  Since we do not know the dimensions of the cubes, we can't just do a bit of math to determine half the distance to inset the cubes and insert one dimension constraint for each axis, rather, we have to use two equal dimensions for each axis.  The reason I say that we can't just do the math ourselves is that in my real situation it will be difficult to measure the distances, and then if something changes I need to recalculate.  Any ideas?

Joe Dunfee

 I have little experience with BricsCAD's 3D constraints.  If I were to guess about an approach something like this, I would probably make each of the components a block.  Then, on those blocks I would create some sort of place holder or marker.  It might be a diagonal line across a rectangular shape, and snapping to the center of that line would give me the geometric center of the rectangle.  Another approach is to just put a point, and manually keep it where you want your mate to be. Those 2D objects would be what I try to mate objects against.  You might put these markers on a special marker layer, but I can see it getting difficult to distinguish between the markers on two separate blocks.

-Joe

Scott McKenzie

 Thanks for the suggestion Joe, I will have to give this a go...

What would make this really easy is if the three world coordinate system planes were shown for each of the entities, all we would have to do is build our cubes, centreing them on 0,0,0 so that when we view them in our assembly the xy, yz and zx planes run through the centre of the cubes, we could then grab the planes for each object and use them along with the mates like parallel, distance and coicident.  

Scott McKenzie

As a work around I have created a small block, which when inserted at 0,0,0 or at a predefined spot allows a you to emulate the XY, XZ and YZ planes and the origin of the part so that you can easily create mates to align the origins or axis of the components.  I just place the block on a non plotting layer and freeze the layer after inserting the part so that it doesn't show up in the generated views using the VIEWBASE command.

Planes.dwg

Richard Gardner

Hi there

I use the BricsCAD purely for 3d modeling. To achieve what you are achieving just use the constraints option. ie. to move the bottom box, select it, select origin as anywhere along the bottom, then for "Enter second point", type in ".z" (return), and then select the top edge of the large box. Make sure that ORTHO is active.

I hope this helps, and this is what you are looking for?

Thanks

Richard

Ralph Massie

Surely the answer is to create a 'base point' for the block to be inserted at its reference point - then insert this at the required point of the main drawing. Because both entities could have different coordinate systems you might still need to manipulate rotation around the x,y + z axes, but, hey . . .

Hans De Backer

Please have a look at the attached screenshot and drawing: is it something like this what you're looking for or did I misunderstand?

cubes.dwg

cubes.png

Scott McKenzie

 Hi Hans, yup, that's what I am after, thanks!

Joe Dunfee

1st note that I am on v14.  I am now having to deal with the issue of centering objects using 3D constraints as well.  I generally do not keep the Mechanical Browser objects as mechanical object.  This seem to create issues when I want to make a block of some mechanical components, and non mechanical components.

I just thought of another way.  If the two objects are solid, you can create a small hole at the center of each surface that will be touching.  Then you can use the co-centric constraint to keep them centered.  You will also need to add a coincident constraint to the two faces, and a parallel constraint to a surface on one side of each, to prevent it from rotating.

However, I just realized that this won't work in my case, since I must keep some legs made of square tube, centered on its foot pad.  In this case, I would need to use some sort of equation on dimensional constraints.

-joe

Joe Dunfee

@Joe Dunfee:

However, I just realized that this won't work in my case, since I must keep some legs made of square tube, centered on its foot pad.  In this case, I would need to use some sort of equation on dimensional constraints.

I just realized that I can't do this because I can't add constraint or reference dimensions to ACIS solids, so that I can refer to those lengths to create a formula.

Also, Hans De Backer's model does not work reliably for me.  Sometimes the cubes will re-center, and other times they will only recenter along one axis.

Any other ideas?

-Joe

Jason Bourhill

3D constraints creation requires BricsCAD Platinum
http://www.bricsys.com/en_INTL/bricscad/comparison/

BricsCAD Professional can only solve 3D constraints.


Regards, Jason Bourhill CAD Concepts 

Joe Dunfee

 I have v14 Plantinum.  But, it seems that 3reference dimensions cannot be added to ACIS solids. Am I correct?

-Joe