An age-old method of creating a part is the use of casting.  This is the process where you fill a cavity that contains a negative image of the part that you want to create.  I am sure you have seen these videos on YouTube where an everyday household item is taken and sunk into a sandbox, and then taken out and replaced with molten hot liquid.

Now how would you get the same effect on Inventor, Your 3D CAD package?

You would use the derive command which is found on the Manage tab.

To create the scenario of the sandbox, you would need to open the part in an assembly and then model a larger box around the part where you will create your negative.

Once this is done save your assembly and open a part file.  Go to the Manage tab and select derive and navigate to the assembly.  Make sure that the part is not derived/referenced into the part.  If selecting the bubble next to the name of the part does not turn red (signifying that the part will be subtracted) make sure that on the derive style line, you select the furthest left option.

Once this is done select OK and you should have your negative.

The great part about this workflow is that if the original parts are modified the derived component is changed accordingly.  If you do not want any more changes because you are using the original as a master skeleton part, then you can break the link of the derived component from the master skeleton part.