This thread has helped me so much so far. I'm building a large format shapeoko and have had a lot of problems with X axis flex. I'm in the process of reinforcing the way you describe above. However I had a quick question as to how you fastened the 2 makerslides to the extrusion. It's hard to tell from the pictures, but did you fasten them exclusively by drilling clearance holes through the uppermost groove of the slides, or did you use the bottom grooves as well?
Also, great looking machine. Do you have any videos of it in action? I'd love to see them.
I'm glad it has helped.
On to your question. I fastened them on both the top and bottom, though I did the bottom on the other side of the x-axis so you can't see it. I originally wanted to have all of the button head cap screws to be on the back side of the x-axis to give the front side a clean look, but the top screws were interfering with the bottom screw of the x-axis stepper motor so that's why the top screws are in the front and you don't see any bottom screws. Ten 45 mm screws are the minimum I'd recommend for fastening the makerslides and extrusion together but 16-20 would be better if you have access to more.
Since we're on the topic of the x-axis I actually plan on rebuilding mine this summer to accommodate a 13 pound 2.2kw spindle and beefier stepper motors in the 380-415 oz range. This will of course require custom motor mount plates among other changes. Samc99us and I will actually be collaborating on a possible Shapeoko 2 upgrade kit designed for more heavy duty applications for users who want or need significantly more rigidity and strength to accommodate high performance spindles and stepper motors. My personal upgrades to my current machine will probably be heavily based on this upgrade kit, if not identical. That's all I have to say for now, I'm not sure if Samc99us wants to officially be talking about it right now, but I'm sure when he does we'll be making updates about it from time to time.
And yeah sure I'll see if I can make and post a video of my machine sometime, maybe tomorrow.
All the best,