I was recently helping a friend with an Avalon 3D app and he was very confused as to why his models seemed to vanish with certain transforms.  He had committed a common newbie mistake: his Transform3DGroup did not follow the recommended ordering: scale, rotate and then translate.  You can really confuse yourself if you break from this ordering.  Way back in 2004, Dan Lehenbauer had a nice post that explained this if you want to understand why this ordering typically does what you expect and why a different order causes strange things to happen. But just remember: SRT! 

Kind of like Indigo: Secure, Reliable, Transacted.