Im new, am a 3D designer. Need a New cnc machine and need advice and guidance

[routercnc]

.. How far should i keep the bearing carriages apart? ..

With a single ballscrew in the centre then racking becomes a problem whereby the gantry can rotate or twist if you are cutting anything towards the edge of the gantry. To minimise this you can space out the bearings further, but there is a compromise on travel.

With twin ballscrews all these problems go away, which is why it is highly recommended. The gantry cannot rack as a ballscrew holds it at each end. Your considerations then come down to placing the bearings so that the centre of gravity of the gantry (which will be somewhere behind the spindle centre line) and the cutting tool centre line both lie within the region between each bearing, as viewed from the side. If you need to deviate from these positions then don't worry as the profile linear bearings will take quite a load.

As I guide I would say between 100-200mm is about right, probably going closer the 200mm end of the range, giving you maximum travel.

[Jonathan]

Also bear in mind that the deflection of the axis, due to the linear bearings, for a given cutting force is inversely proportional to the square of the bearing spacing. A relatively small increase in the spacing you choose can therefore make a big difference, assuming the stiffness of the system isn't dominated by another weaker part. The reason for this is that the force on each bearing decreases with their spacing and the deflection of the tool also decreases for a given bearing deflection if their spacing is increased, so overall you get a quadratic relationship.

for a good machine what is recommended, A router or spindle? cant see any bvious advantage or disadvantages to either... Yet to see anyone give a preference on any of the build logs

Simple answer is get a proper spindle. But please read the build logs more - people have already linked to some good ones in this thread, yet all the questions you've asked so far are already answered in the logs.