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Question

Solid modelling Shell command not working in AutoCAD (any version)?

 

Answer

The Shell command was introduced way back in AutoCAD 2000. It basically permits the user to specify a shell 'thickness' to modelled solid objects. Other products have similar features including Rhino and 3ds max Design and work to varying degrees of reliability.

Over the years I have found the Shell function to be a bit temperamental (massive understatement!) in function by seeming to work in some drawings, but not others. When the Shell command fails you get an unhelpful change in topology detected message. However in some files it works and you tend to assume that the latest service pack/hotfix has tightened up a few functions but this is not the case! It is more down to luck or position of geometry than anything else! At the time of writing this blog (March 2012) the Shell command is still a bit problematic. However, after doing some tests it seems to make a BIG difference upon where you are in relation to the World Coordinate System Origin position and once you know this a workaround can be achieved.

The system I am working on is 'Vanilla AutoCAD 2012' SP1, on my Windows 7 Pro Laptop, but the problem occurs on other AutoCAD flavours and Windows platforms as well.

To test the functionality; draw say, a box on the origin point with 1000 dimensional units for length, width and height.

Draw an identical object 50 to 100 metres away (50000 units if using millimetres)

Now try and shell both objects, the one on or very close to the origin will succeed whereas the one further away (with the same geometry) will fail. If this object is located close to the other box the shell command will mysteriously work!!

I put this down to a limitation on the computations working in millimetre formats (we in the UK tend to work with bigger numbers than the Americans) and as a result I don't think their testing of this function used such large figures in their 'beta' testing phase. When I get a moment I will raise this with Autodesk and see if we have any timetable for a fix. However in the meantime if we are at least aware of the 'move to origin' workaround, creating Shelled components is slightly less tedious process.