That`s a very nice looking machine!
I am a little confused as to the comment you make. I assume it is the same comment as the first one from Wizard, and then the 'problematic' axis is what I thought people normally refer to as the X-axis (the two rails that sit on the gantry itself), not the Y-axis. Is that what you meant?
For now, that is at least what I have modified.
I have modified the design to something that is indeed much simpler. I originally chose the design I used to minimize the distance from the spindle centre to the bearings on the Y-axis to minimize the torsional moment on the gantry. With this new design, this distance is a little larger, but it is a much simpler construction. I do have to look into a way to fasten the bolts for the rail carriages, because they are now interfering. Since it is only a rough design, I will look into it when making the final design.
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On the flowdrilling: it is indeed a very nice proces, but I now realized I cannot use it here. I will be leveling the rails on a slab of self-leveling epoxy, and that will melt / burn of when flowdrilling afterwards. I think it will be a bad idea to flowdrill the holes before laying the epoxy.
I agree with you all that 3 mm is actually too thin to tap nice M6 threads, but I do not want to change to larger wall thicknesses of the main beams (my supplier sells 100 * 100 * 5 also, but it is more than 4 times the price of the 100 * 100 * 3....) Instead, I think I will weld in pieces of steel bar of e.g. 20 mm dia and 10 mm length at the bolt locations, see example below.
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