I want to DIY.. some questions please!

[JAZZCNC]

Ooookay, I will use supported :)

What is the best way to drive the 150cm long X axis? Should I use a central ball-screw or two ball-screws, one on each side? If the latter, what is the best way to power them, i.e. 2 motors connected to 1 controller?

Eh Eh sorry if I shouted a bit load but can't stress enough what a bad idea unsupported rails are.!!

RE Single or twin screws on X. If Y axis much wider than say 600mm then personally I'd go for twin screws.

Powering them you can either have 2 motors each with it's own drive, 2 motors on 1 drive will work but not correct way to do it and could give problems. . Another don't do it.!!
You could use 1 slightly larger motor and connect the screws together with a timing belt, this is my personal favorite and how i've built my machine. It works perfect so don't be put off by timing belts.

Both work ok but you have to be aware of the downsides to each and choose which you prefer to live with.!
Using 2 motors works ok but comes with the issue if one motor stalls the other keeps going which can do major damage to the machine if happens at high feeds.
It can also be a problem keeping the screws in sync if both motors are not tuned perfectly in sync with each other and one runs ahead or behind the other, this will result in slowly twisting the gantry thou can be corrected by regular homing and squaring, but still wrong and not wanted.
The way to avoid these issues is to not stress the motors so you effectively need to run them below there full potential because you can't afford any missed steps or threat of stalling from lack of torque at high feed rates or rapid moves. The quality of build regards ballscrew aliagnment as to be spot on, which it should be on any machine, but even more so with twin screws/motors as no sticking or binding can be tolerated for fear of stalling motors at high feeds where torque is lower.
Mostly this is not a problem if your aware of it before building because you can choose higher lead ballscrews or build in gearing again via timing belts to the motors to achieve the speeds & feeds you require and keep the motors operating well below the point they start running out of torque.
The sync problem is just a slow setup and tuning job which can either take 2mins or 2 weeks depending how fussy you are.!! Warn you thou it can lead to premature baldness.!!. . I'm down to my last few strands. . :naughty:

The 1 motor pritty much eliminates all this hassle with perfect sync and also means you can run the motors closer to there max. It's also very easy to change pulleys to gear the system up or down.
The only downside is it's not as easy to build because you have build in the belt system and it's tensioning/tracking system and also have the ballscrews machined different from standard machining to add extra bearing support for the pulleys which will put greater side ways force on the ballscrews effectively trying to bend them when the belts are tensioned.

In my experience this is all worth the effort because when done and settled down it never loses position or sync and ounce setup thats it forget about it and never needs doing again with absolutly no fear of racking the gantry unless the belt snaps. (which I've only snapped 1 in over 3 yrs and that was because I left the pulley lose)
If good quality belts are used then belt stretch is minimal and with correct tension and good tensioning system never an issue and very very accurate.
It's no cheaper(well slightly) than 2 motors because you have the belts and extra bearing supports to consider.

Like I say both work ok if your aware of there limits and tendency's it's just I personnaly favor belt system.

Oh and at 150cm your in a funny area regards pitch and screw dia.???? I'm pritty sure you'll want 10mm pitch which often (well from china anyway's) only comes in either 16mm or 25mm dia they don't do 20mm dia only in 5mm pitch.
At 150cm 16mm is on the very edge of being to thin and will give big potential for whip at high speeds. 25mm would be better but comes with the cost that it requires more torque and power to over come the extra mass and inertia of the screw/nut when accellerating and deaccellerating. This means bigger motors which require larger power supply's and drives that can handle the power so it basicly cost's more money.
You can over come this by either taking the route like jonathan did and use rotating ballnut on 16mm screws, but again this will come with expense and i'm sure jonathan will tell you exactly how much and all it's bennifits. . . OR . . bite the bullet and pay for the larger screws and use with nema 34's and correct drives, psu etc. . . . OR . . .use 20mm 5mm pitch and gear 1:2 to give an effective pitch of 10mm at the cost of slightly less torque. ( My machine use's this system thou with nema 34 motors) This gives a slight advantage as it's easy to gear back down and double the resolution of the machine without comprimising the area where the motor works good. It can be done the other way round and gear 10mm pitch to give 5mm resolution but the motors work in an area which is not ideal.!
It's horse's for course's really so take your pick.!!!!!

Hope this helps.!