Copied from elsewhere:

Jack,
The specialty software you mentioned in the earlier post is mostly referred to as 4th axis software. Art at turningaround.org has links to some very neat wizards that let you do decorative carving on columns and such without the need for "other" CAM software other than the wizards. If you want to do high relief carving (like a statue or totumpole) then you will need other software on the 3d modeling and CAM engine side of the computer.

Popular choices are:
RhinoCAM with 4th axis module.
(requires Rhino modeling software and the CAM plug in)
ArtCam by Delcam
Carve3d by Vectric.
- there are others, but these I have used recently with success.

Each of these has a price point and features that show the burden of the cost.

Personal note:
(I use RhinoCAM for continuous rotary 4th axis tool pathing and have learned to use it...albeit a fairly steep learning curve)
For simple 4th axis like machining (rotation every 90 degrees's or 4 face machining) I use Carve3d.....really simple...really fast toolpathing....you can learn it in a day! (but it has it's limitations)

Mechanical
Adding a 4th axis is quite simple on the mechanical side.

Making or having made the 3d geometry on the software side requires a bit of talent - or money.


PS.
Many have added 4th axis modules and you can find good examples of what others are doing here on the MM forum and Shopbot forum.

I might add that a true rotary axis is not necessary for some high level 3d carving - just a taller gantry height for the part your cutting. Options, all can be achieved with the Mechmate with a little prior proper planning.

Good luck.
Sean