Stell frame cnc 1500X1000. Adjustable table your opinion please

[Jonathan]

Since it's already been said that most of the frame designs posted here would be more than adequate, I'll just highlight some general points which may be useful.

I think the frame design silyavski posted in post #30 is well thought out since, among other things, it makes relatively efficient use of material. I'd be inclined to swap the two 100x100mm beams and 80mm spacers with a single piece, perhaps 200x100mm possibly with a higher wall thickness than the rest (or scaled down accordingly if making the rest of the frame from smaller sections). This will obtain a similar stiffness to the original, but reduces the number of pieces to cut by 12 and the number of welded joints is reduced by 20. This would save a lot of time and reduce residual stresses due to there being less welding. Clearly it may not be as cost effective as making the whole frame from the same size material.

Something else to bear in mind is that increasing the size of the steel you use can sometimes make the structure strong enough, with a lot less work than adding lots of small supporting pieces and without necessarily increasing the cost. For example, if you have a piece of 60mm box section with 3mm wall thickness supported at both ends, then in general increasing it to a 80mm beam of the same wall thickness will reduce the deflection to less than 2.4 times the original.

Also, it's better to increase the size of the beam (within reason), than to increase the wall thickness. Continuing with the previous example, the 80mm beam will cost about 33% (assuming it's priced by mass) more than the 60mm beam, so suppose you instead invested that 33% extra in getting a 60mm box section with 3mm thickness to match the weight of the 80mm beam. Both sections will cost about the same, however the 60mm beam is still only just over half as strong as the 80mm, as you've only made it about 33% stronger (not 240% as above). The reason for this is that increasing the wall thickness only gets a linear gain (y=k*x) in strength, compared to a quartic (y=k*x^4) relationship from increasing the size. So in general, you only use a greater wall thickness when you haven't got space to fit a bigger section.

With regards to resonance, MDF is actually a great material to use for the machine bed as it has such good damping properties. I had a lot less problems with resonance on my machine with the MDF bed than I do now with aluminium. If I'm machining an aluminium part for which the finish is more important than the accuracy, I'll sometimes put a piece of 18mm MDF between the part and the aluminium bed as the MDF damps the vibrations, making the 'sweet spot' to get a good finish a bigger spot.