Quite an Unusual one

[JAZZCNC]

But i don't understand in fact- what will i gain with the 4 rails on the gantry and 2 Z axis ? If i go with heavy duty preloaded roller guides i could use them at the same time for CNC and the impact hammer. if they f"""k i will just change them. or you mean to be able to do 2 things without swapping spindle with hammer? if just for avoiding swapping, then an over-sized Y could do the same job and park one of the Z out of the way.

Yes was just to avoid swapping but if you have room for extra wide Y axis that would work OK

So Dean, my Big questions are:

Will Hiwin RGH 30 HA stand up to the task?

Yes easily. 30mm bearings are massive things and can handle large loads.

Will ballscrews with 2 nuts each stand up to the task and especially on Z, where additional vibrational creep has to be dealt with? Z with 2 ballscrews and 4 nuts should stand , yes? Ok, not forever, but at least for a time.

What size ballscrews on each axis? the normal speedy ones? 1605 and 1610 respectively?[/QUOTE]

Don't think 2 nuts would matter or help in this instance has reducing backlash isn't required.
Regards Z axis then Obviously two screws will take more punishment than one but single screw would just means changing sooner and less complicated.!

Yes standard screw pitch and either 16 or 20mm should be fine.