How to make an inverted stl???

[JohnAdcox]

I've successfully created a 3D stl file which was then used to carve the object into a piece of wood using a CNC machine.  Now what I would like to do is use the same object and create an "inverted" stl file to CNC into the back of a piece of acrylic.  I'm going to light the acrylic from the bottom and the carved object will show up in the reflections.  The software I use to carve on the CNC machine will partially due what I'm trying to do as it will "push" the object down into the acrylic (using a negative shape height); however, some of the features aren't fully inverted. 

I may not be phrasing the question very well...and I apologize if I'm not.  The object I'm working with is a a relief of a head wearing a hat.  Only the front half of the head is showing in the 3D view.  In order for this same view to be carved into the back of the acrylic the stl has to be inverted. 

If this software won't do the inversion, please offer other suggestions for accomplishing this.  Thanks in advance!!

[SAV]

Hi JohnAdcox,

You should be able to do that with Autodesk 123D Design (free).
Build a cube, then load your 3D model (STL file) and place it 'inside the cube'. Then select both and choose COMBINE>SUBTRACT. That should do the job.

All the best.

Regards
SAV

[JohnAdcox]

I'll do that & let you know how it turns out.  Thank you!

[chrisd]

Depending on the side from which the relief is viewed, mirroring might be needed as well. Although, if there is no text or other feature that would look odd if it were reversed, maybe you don't have to worry about that.

[JohnAdcox]

Will definitely have to mirror it also.  Fortunately, the program for which I'm using the stl does this well.  Thx for the reminder!