I think you are going to need to stiffen that up. I would add a gusset from the front right corner of your extension bracket to the opposing diagonal face. Maybe you could even make the split line in the bracket be the center of that bent face, and "sandwich" the gusset in between a couple of tabs bent in to the ends?
I would also look for a way to get more support to the front of the bracket/slide. Maybe a square tubing strut down to the column - it could bolt or weld to the right hand face of the extension bracket to give it a bunch of stiffness...
Overall it all looks fantastic. I hope you release the design - I would love to make one!
I think I recall you having a question about controls? It looks like a great use for a PIC, Arduino or Propeller or even a BASIC Stamp processor with a few hooks in to MACH through I/O points... Biggest challenge would be defining what tool slots to use, but it would only take 5 I/O to use a binary coded decimal that would support 16 tool positions... A total of 8 or 10 I/O points should be more than enough. Could get away with much less if you got "tricky" with the programming and didn't need graceful recovery if something goes wrong...
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