Hi Ard - You need to build a dummy spindle and tool so the applied load is at at typical tool load point. make the tool very stiff so that is out of the equation for now. I have a "rigid" setting in my materials. Its an infinitely stiff material and I set the tool to that. The spindle can be steel as it is steel.... some images would help. I apply the load to the bottom of the tool surface. Peter

The dark blue parts are "rigid" the yellow arrow is where I'm applying the load usually 1000N. The post and cone are there so when loads are applied you can see what is happening at the tool.

As a general design point - The static stiffness of a machine is not what happens in practice. This is called the dynamic stiffness. The tool and the job and the cutting point is intimately connected. If the tool moves so does the job either by deflection or by material removal. If the tool is doing its job it chomps away happily at the material and it must be a small oscillating motion. If its skipping and missing cuts then its chattering. Plus the force of operation is not at the tool. The tool as we model it is a reaction not an action. The force is applied by the motion system and has to find its way to the cut site. So in summary the static stiffness physical test is an approximation that is easy to do to establish the apparent stiffness of the machine. We know that machine stiffness and performance are connected but we maybe testing it the wrong way, but its what we have at the moment...