building MK2 with rack and pinion

[universally]

thankyou very much :) some very usefull information there and its given me the info of what i need to be looking for, i have in the mean time found a relativly cheap supplier of ballscrews that was mentioned in one of the threads, and he can supply a 2.5m one for a little extra, so maybe ballscrew is still an option and less brain taxing for me :)

[JAZZCNC]

thankyou very much :) some very usefull information there and its given me the info of what i need to be looking for, i have in the mean time found a relativly cheap supplier of ballscrews that was mentioned in one of the threads, and he can supply a 2.5m one for a little extra, so maybe ballscrew is still an option and less brain taxing for me :)

Yes less taxing but still you need to approach with care.! Some of the same considerations apply.?

Because the screw length is long you'll get whip if spun fast. This can be lessened by using larger diameter screw but doing so brings it's own troubles.?
Large diameter screws have much higher inertia which severly affects performance and motors. So it's very important you don't over size the Diameter.
But Smaller Dia screws bring there own set of issues, namely Whip and resonance. These affect performance because they have to be spun slower to avoid problems.
There are ways to get around these problems like Rotating the ballnut but this gets more involved requiring either making your own or buying very expensive ballnut.
Another option is to buy screw pitch higher than is needed to reach the speeds your looking for. For instance 20mm pitch instead of 10mm. Then apply a ratio like 2:1.
This has the affect of halfing the screw speed but still giving the same travel speed and resolution has 10mm pitch. It also increases torque.
There's small down side in that mechanical torque the screw provides is less than that a 10mm pitch would give using same motors but it's nothing worth bothering about. The positives far out way the negative.

If your going to go with BS which I suggest you do. Then make sure you get them machined for FIXED BK bearings at each end and if you really want to stiffen up then have them machined for double Fixed bearings at each end. The end support really makes a difference to how long length screws perform so don't scrimp in this department.

If you use the above setup you'll get away with 20mm Dia screws. Don't go below 20mm or above 25mm.