Hi Erik
Before you go much further can I suggest that you set out the basic spec for the machine you have in mind - you may have done this. A CAD drawing is not necessary just a sketch on a scrap of paper. Having determined what the machine is for i.e cutting foam or heavy duty steel then decide on the accuracy with which the parts are to be made. Accuracy has two main facets in CNC. First is true dimensional and the second is repeatability from part to part.
Having got the peliminary design spec you can then set about trying various design ideas still at the sketch on scraps of paper. I guess you have done this but then got seduced by some nice looking bits which were too good to pass on. (My good wife says "do you really need that" every time I make an impulse buy!)
What you might have done was then to have looked at the requirements for the rails (or any other mechanism) and their support structure for their ability to support the load with the required accuracy. Unfortunately the load has two components the static loading due to the weight of the parts to be carried and the impulse load caused by the reaction betweent he cutter and the work. Rather than calculate the actual cutting reaction the torque of the motor at the maximum tool diameter is a safe approximation. The tool reaction can take place at any angle and as seen by the 'rails' has a plus an minus value and a direction.
This leads directly into the consideration of static deflection - you have calculated this at least for the vertical dead load. The real situation is that under this vertical load there will also be an implied cutter reaction force at right angles - assuming a vertical mill machine - the cutter reaction force will try and alternately push and pull on the rails. This will set up a vibration which leads to chatter marks on the work. The aim should be for maximum ridgidity i.e. no vibrations. What I am trying to convey is that machine ridgidity (the rails being but one component) is related to the material being cut and the spindle power. Many people are going to try for the maximum machine speed in terms of stock to swarf and this means that machine stiffnes / ridgidity is one of the deciding factors on the quality of the cut. To be absolutely fair it is normal practice to rough out with a high feed rate and then to take one or more light cuts to finish off.
There are other rail methods used that are less expensive. Suggest you might look to use scate bearings running on ground stock supported by a light section RSJ which is part of the frame. Here in the UK house walls are most often double skin brick and the wall above the window opening is suported by a rolled sheet steel section that might make a very good cheap bed for a wood working machine. I mention this as building trade outlets can turn up some nice bits if you have the eye to see out side the box.
Take hope in the fact that everyone who has built a machine from scratch has inevitably bought and collected bits that have not been used on the current build. As a result they have the makings of another smaller - bigger and better machine as well as a well stocked 'junk shelf'.
Onwards my friend.
Pat  |