Stell frame cnc 1500X1000. Adjustable table your opinion please

[Boyan Silyavski]

#1 ( very important to know what I am trying for )
When you measure the flatness of a beam ( or epoxy surface) against a straight edge, what is the acceptable tolerance-gap ( so we assume the surface is straight) . I use light to understand whether my top beams attach the straight edge. The touching points can be easily seen. But at other points althought i can see that there is no touch ( light passes through) , the 0,02mm Gap Filler cant pass through the gap. So what is the critical gap ? the light passing test ( that is less than 0,02mm ) or the 0,02mm gap filler test?

I looked to see if it was tight fit. All theory evaporates when you are trying to check 3m rails, flashlight in your mouth , in a tight garage holding 14kg straight edge. Just dont be in a rush, call somebody to help you, prepare good flashlight and look every 15cm if all is ok. Hold the straight edge with not much pressure, scrape it around and try to feel if all is same or ok. At the end the sound and feel helps more than the eyes at that exercise

#2
When someone use the solution with the adjustable top beams ( without epoxy) or the shimming method how does he succeed to have the top rails in the same plane. How does he know when the two rails ( left and right ) are in the same plane? Is the thread method accurate ( the method that with the use of two threads you create the diagonals of the top rails like X. If the diagonals are in touch at the center that means that the rails are in the same plane.

At the end it seems they use eye and feel. That doesn't mean really its very precise or of real importance. In reality, measuring between 1800mm separated rails, i can lift one of the rails 1mm and it will still seem to you checking with the edge that all is ok and they are on the same plane. Cause the judge contact area is very small. That is what i discovered the other day. But on the other side if it looks straight and both rails look to be on the same plane checked against straight edge, thats enough good. Cause later when the bed is surfaced say you make a 20cm detail. If your table is wrong say 0.5mm, which is tremendous in my eyes, the 20cm detail will still be made to 0.05 or better. The other point being here that all long and large pieces made on a cnc really don't need such precision/wood, mdf/ or just the temperature difference /expansion overrules the actual machine precision.
However in the future i intend to do gantries, surfacing for small machines and similar stuff in aluminum, that's why i pushed my build for perfection and wasted so much money on epoxies. But that's my explanation of thing, it came to my mind the other day when i was trying to shim my right long rail cause there was some new problem with the epoxy that i have not seen.