Hi Jag - A modern CNC VMC has a stiffness in the order of 150um/N. This is loaded at the end of the toolholder and restrained at the bed somehow. Unfortunately there is not a standard method for the test. Small benchtop mills are <10um/N. Something like a taig is <2um/N. The stiffest quoted machine I have been able to find is 650um/N. Pick a number and see how you go.... Many people restrain the machine at its bed feet but this over constrains the machine (results in it being stiffer then practice) and does not represent the actual load loop or load path. Model a vice for instance in the centre of the bed and hold something in it like in practice or at least some dummy geometry. Whichever way you do it be consistent. Plus you need to figure how your going to deal with bearings and bolted connections., Does your FE model bolts? From me doing FE on many machines you can "bond" all the connections and assume the model is about twice as stiff as it will be in reality. So say you aim at a machine stiffness of 50um/N then build the model at 100um/N... Peter