OK, bit more about the close spacing of the X axis bearings.

If you run through the 'force_and_moments4' spreadsheet (post#1), you'll see the maths to support it. But there are design conditions:

1. You need to put the C of G of the gantry in the middle of the X axis bearings, i.e. a lean back style of gantry. This removes the moment on the bearings due to an offset C of G.

2. You need to arrange for the ballscrews (or whatever) to be placed at a set distance away in Z away from the side bearings, relative to the distance in Z to the tool from the side bearings. See formula at the bottom of tab3. This ends up being at a similar height to the tool, the slight difference being due to taking account of the friction in the bearings. In practice this would probably mean dual X axis ballscrews so you can run one down each side of the gantry. A single centre drive would get in the way of the cutter!
With this arrangement, at a fixed cutting height, there is no moment on the bearings during cutting.

3. You need to tend to cut at a similar height, i.e. thin materials. Once you cut at other heights, e.g. 3D, then the cutter is not always at the same distance from the bearing in Z, but the ballscrew is. This then starts to add a moment. You could get around this by raising and lowering the workpiece, which I have seen done, but adds complication.

4. You need to use modest accelerations, and modest gantry weights. In my example I used a 20kg gantry, and up to 2000mm/min2 acceleration. The gantry inertia was very small, and hardly added any moment. The main factors were the cutting force moment and the ballscrew force moment which basically cancelled to leave no moment.


With these conditions met, and no resulting moment, you are then left with the X axis bearings only supporting the weight of the gantry. They can then be close together or far apart and react the same load, so you can choose to mount them close together to gain travel. The danger of racking (made worse with closer bearings) is gone because you almost certainly need to drive the gantry with twin ballscrews to meet condition 2.

Hope this helps . . .