I want to DIY.. some questions please!

[Tenson]

Hmm.. I think I'll go for unsupported rails on the Y axis to keep the gantry weight down, plus build simplicity.

[JAZZCNC]

Hmm.. I think I'll go for unsupported rails on the Y axis to keep the gantry weight down, plus build simplicity.

BIG BIG BIG MISTAKE if you do this unsupported rails are a complete waste of money.

Don't be affraid of weight, Mass really helps when it comes to cutting, It absorbs resonance and gives far better finish.
If your trying to cut weight to gain speed then you are sizing components wrong. The feeds you require for the machines intended purpose should be got from choosing components to allow it.
Ballscrew pitch and motor selection along with drives and correct voltage all play a part combined with build quality and component quality. Get anyone wrong and you comprimise the machines abilty in some way.

In your case unsupported rails with a light weight gantry of any resonable width would be a disaster for cutting any material denser than softwood, and even then with lower DOC.

DONT DO IT. . . . FORGET UNSUPPORTED RAILS.!!!!!!

[JamesMoore]

A ball bearing screw is just that: a screw which runs on ball bearings. The screw and nut have matching helical grooves or races, and the ball bearings recirculate in these races. There is no physical contact between the screw and the nut. As the screw or nut rotates, and the rolling balls reach the trailing end of the nut, they are deflected or guided from this "pitch" contact by means of a return tube and returned to the leading end of the circuit.