what CAD package is that?
It was drawn on Solidworks
I am lucky in having use of Solidworks thru' the Company that I do work for
It is a brilliant package for designing CNC Tables, you can animate all the gantry movements etc
The picture I posted was created after I loaded the assembly into Photoview that creates a 'Rendered' view of the model
This is what the model looks like when actually designing it.

Click image for larger version. 

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The weldments tool is brilliant, I drawn the complete framework for the table in 20 minutes

Your machine is starting to look really good. A suggestion (from a non-plasma expert!) is to consider running a simple bearing from the back of the Z axis plate onto the top and bottom of the upper 40mm rail. This would give a bit more support to reduce the sag, and possibly help with the resonance (if that is due to the round unsupported rails vibration). Just a thought . . .
Not a bad idea
Is that a double travel arrangement that you have on the z axis, again not a plasma expert but it looks like its just adding extra weight
That is a vital part of a Plasma system
It is a 'Floating switch'
What happens is Z axis lifts up and moves to its pierce position, the Z axis then lowers until the Torch makes contact with the material.
The torch is fitted to the second linear slide, and just rests using gravity.
When the torch touches the material it trips a micro-switch, this then sends a signal to Mach3 and zeros the Z DRO
I will start a new build thread for my new machine, keep the suggestions coming!

Andy