Instrument maker

[JAZZCNC]

at least 70kg gantry with all mounted

Too many variables to say that. What size machine.? How deep bowl.? What material.? What cutter size/length.?
Smaller machine well designed can still cut deep bowls with good MRR and doesn't need to be weight of small tank.

3D work by it's nature doesn't have large step over so the cutting forces are low when doing the finishing. The bulk is removed by roughing so finish isn't important only tool life becomes a factor. So again well designed machine can handle this with good tool life but without being massively built.
Mass only really comes into play when cutting deep and requiring respectable finish without using finish pass. For any decent work that requires quality finish then machine weighing 20ton will still need light finish passes, infact several semi finish passes for some work.
The Massive weight is then wasted and infact acts like stone around it's neck. To move this stone is costing money.

So what I'm saying is while Mass is good sometimes it's not always benifical or required for certain types of work. IMO 3D being one of them.
You want fast moving machine that can take light cuts which doesn't require heavy machine and all the extra expense that goes with it.!

Strong doesn't have to mean heavy or like you seem to think and preach must be STEEL.!! Good design and Aluminium will work just as well.

I'll make this point Clear by refereing to 5 Axis JOBS Machine (costing £250,000 20yrs ago) which I nearly bought few years ago which originaly came from Mclaren F1 and used for machining the Steel moulds for there Composite parts. The WHOLE Z Axis which was over 1Mtr long was made from ALUMINIUM.!!!