Sub-sampling a 3D Geometry

[Anonymous]

I think this is simple and I'm just being stupid, I thought one of the solid tools would be able to do this, but perhaps I'm just using them wrong.

Context: I'm working on a project which involves 'sampling' a distinct area by moving a block across it in my CAD designs. These blocks are in 3D so I don't think I can use the trim option. There are no simple boundaries, so I can't simply delete the bits I don't want to sample in general.

I can give a simple example of what I'm aiming to achieve using two blocks:

The 3D red block represents my sample space, whilst the 3D green block is the larger region which will be sampled.

The aim is to keep the whole 3D red box, contents included, as well as the overlapping section of the green box. The red block in this example would then be moved along the X axis until it reaches the end of the green box.  So, when it is in the centre, the cut would remove two sections of the green block.

Hopefully this makes sense and someone can direct me, I think poor search terminology is the only reason I can't find a useful post to solve this problem. I have tried Union, Subtract and Intersect, but was unable to achieve the desired outcome (again this is probably poor setup on my part).

[leeminardi]

What did using the Boolean operations Union, Subtract and Intersect, not do for you?  What results did you get and how did they not meet your needs?

[Anonymous]

For the example:

Union combines the objects giving me a larger object which isn't the intention.
Subtract deletes all the stuff I want to keep and leaves behind the stuff I want to remove.

Intersect keeps the overlapping pieces, but not the upper part of the red box in the example given.

In general:
Intersect often gives me 'null solid created - deleted' instead of anything useful.

The boolean commands often remove all the components inside the boxes which isn't helpful either.

I hope that helps to clarify the issue, any further questions would be appreciated.

[leeminardi]

1. Copy 0,0,0 the red box.

2. Union one of the red boxes and the green box.

3.  Intersect the result from step 2 with the red box that remains.

[Anonymous]

This works well for the example but doesn't seem to generalise so well to boxes which contain objects. I'm looking at a physical model of a 'black-box' simulation, so the embedded objects make sense physically and the distinct geometric objects are important in subsequent finite-element modelling.

Geometries:

Essentially, the green box now has 3 distinct layers, the upper half of the red box contains some stuff and the lower half has a box and a sphere which both intersect all 3 of the layers inside the green box. The 'red' and 'green' box in each case is constructed by placing objects at the correct positions inside the larger box, this is mostly done to ensure distinct geometrical entities when this is output as a .STEP file.

Step 1: Copy the 'red' box and place in the same location giving 2x that geometry. Step 2, the union creates:

 

Which loses some of the layers in the lower-box. I think I can fix this if I copy the new 'green' box into place in the same way maybe?

The bigger issue is that the intersect command doesn't combine the new objects, giving:

Any further ideas would be much appreciated.

[leeminardi]

 Can you post a file and, if possible, her solution you would like to achieve.

[Anonymous]

A file is attached. The intention at the current location would be:

 Essentially just trimming off stuff which is outside of the cyan box, whilst keeping everything inside the box unchanged. The next file(s) translate the cyan box (and its contents) along the X axis, so it would trim some excess off at both ends for simple box boundaries. Inside the region being trimmed, there maybe complicated objects - say an air-plane part - which would need to be cut exactly along the boundary, with my hope being to consider cuts along more complicated surfaces in general, but boxes will be good for the short term.

Thanks for the help!

[leeminardi · Accepted solution]

"Essentially just trimming off stuff which is outside of the cyan box..."

You can use Boolean subtract or intersect to do the trimming.  

When either of these operations is used on several solids at the same time the individual objects that would be left over are in effect, unioned together. To ensure that you maintain separate objects it may be necessary to make several copies of your objects.

Please note that you should not group objects together.  This will complicating the Boolean process so ungroup the solids.  Consider using layers to aid in associating related objects.

Assuming we are going to use the subtract option create a box that is large enough to cover all the objects that need to be trimmed and position it so one of its surfaces is adjacent to the trimming plane as shown below.

It looks like 3 objects will be affected by the subtract operation so you will need to make 3 copies of the yellow box to use in the three subtract operations of the yellow box from the brown, gray, and green box.  Perform three separate subtract operations.

In this example a box is used but you can make a more complex solid to create more complex trimmed results. 

[Anonymous]

I actually thought I was making life easier with the groups, I guess this gets more involved as the number of objects inside the boxes increases. Brilliant solution and its nice and easy to follow, thank you!