I don't believe I can give you answers with sufficient reliability. I am going to test it some more and pay way more attention to the fine details. Can I get back to you on this?
I'm not 100% that I'm following you. It sounds like you are suggesting something like a single part cut in an X shape that fastens to the tops and bottoms of each leg on a side. I also envision metal running from the top of leg A to the bottom of leg and from the top of leg B to the bottom of leg A, forming an X shape. In that configuration, you would need to have a fixing fastener where the two cross pieces intersect. It occurred to me that you might be considering a single diagonal, but wouldn't that leave one leg on each side without additional support. Is this what you had in mind?
--(Refers to adding the spoilboard as a possible fix). I wouldn't argue against you on this. My base is much like Avid CNC's for the Pro line, except I used double sized, heavier weight extrusions, and used extrusions for angle bracing instead of their steel version. I wondered about the amount of leg left unsupported, but concluded if what Avid used worked okay, mine should work better. Bigger and heavier is usually better (but not a substitute for good design). It's looking like a bad conclusion on my part.
Not a thing wrong with spoiling your spoilboard. Pretty much everyone does it. It's just that it takes virtually no additional effort for me to keep it pristine. It's certainly not about the money. In the scheme of things, the MDF cost is nothing. However, time and personal preference is another matter. I glue my MDF to the base layers. If I want to replace it, I have to glue a large piece back on and then cut the slots for the t-track, or I have to cut several strips and glue them on individually. It's just one of the things I prefer avoiding, if I reasonably can. And in this case, I reasonably can.
Nope. I don't cut into my spoilboard when making profile cuts (cutting out a piece). I zero my Z axis to the spoilboard surface. Then, I move Z up the nominal thickness of the material I'm cutting (e.g., 3/4" for 3/4" plywood, even though the plywood is thinner than 3/4"). Then I reset work zero to 3/4" from the bed surface. When I run the profile cut, it cuts though the material, but doesn't touch the spoilboard (well, it has to touch it, but does not cut into it). When you see it happen, it's almost like a miracle. Every time I take a piece off and look at the spoilboard, I have have to touch the spoilboard to assure myself that my eyes aren't deceiving me. Works every time, though. I use a touch plate. What is critical is setting the offset exactly. If set properly, the endmill cuts all the way through the material, but does not cut into the spoilboard. It only takes a few seconds more time to do it this way. The extra step involves moving Z up by the nominal thickness of the material, and hitting the axis zero button. That's it.
Don't get me wrong. I'm not the least bit critical of folks choosing to spoil their spoilboards. I just pass on a technique I find interesting, and for me, useful. Folks are free to use it or not. I have strong feeling about a lot of things. This isn't one of them.
Thanks again, David,
Gary