Iam planning to build a 5 axis cnc for making 3d sculptures in wax or foam. I need a very very accurate machine that can get me very fine details and very smooth surface finish. Iam planning to make it 150cm X axis, 150cm Y axis and 125 cm Z axis. I ned to know which is better to get me the best results, ballscrew, precison rack pinion or belt drive.
It might be worth raising your X axis rail up, so half the Z movement is reaching below the X axis and half above; this should reduce the torque on the X axis slides at the extreme ends of travel.
I agree with CarveOne, even though the cutting forces are low, this will have to be quite a substantial machine if you wish to achieve a good finish.
All three drive types are equally well suited for this application. Belt drive may be the easiest to implement. But as others have stated, your machine structure will require the most attention. Long lever arms require large structural cross sections and wide bearing block spacing to prevent flex.
I would also consider a moving table in the x axis so the tall z could be rigidly mounted to the supporting structure of the machine and would allow for a very wide stance. Since you are going to need 125 cm/50" I would consider having the uprights either be very wide monolithic pieces of say aluminum or use braces that approach 45 degrees as this will give a very wide stance.
If you use 150 mm/60" as the base of the uprights, this will give a 33 degree angle which should be rigid enough.
The other consideration is what type of accuracy you are looking for?
+-.010 should be doable, +-.001 is going to be expensive and anything less will most likely be out of reach for the average do it yourself guy.
Warning: DIY CNC may cause extreme hair loss due to you pulling your hair out.
Also with this type of accuracy you will most likely need to use servos.
Zapp Automation Ltd