Hey capner! I remember your original post and thought it very cool!
My credentials? I graduated college in 1985 with Associate Degrees in Industrial Technology (lathes, mills, surface grinders, welding, sand-casting, etc), Metalurgy and Drafting - major in Gunsmithing. But the gov wanted to keep track of my underwear size if it changed so, I turned to my other love. Bikes.
I've often had to resort to fabricating my own solution to a problem on older bicycle restorations but it is often cost-prohibitive for the customer. I sometimes do it just for the Love of it. I've had my share of failures, buddy. What you are attempting is admirable but, if you'll curtail your enthusiasm for a moment - I may save you some heart-ache. I know that I sound like a know-it-all. Not.
The first problem that I see? No problem with your choice of AL to use for the crankarms. The problem is that they need to be cold-forged. To machine them from billet for your own personal experiment - it's Your calf, not mine! Please don't go there if you intend to actually go beyond proto stage.
Even then, with both 6 or 7000 alloys, I would anneal them as both tend to be brittle under 'sudden impact' lateral loads such as a rock or curb strike. As you probably know, annealing aluminum and brass makes them 'tougher' as opposed to 'softer' as would apply to iron alloys such as steel. The 3 metallurgical charachteristics are Toughness, Hardness and ?? You can do it!
Foregoing any treatment, I am inclined to agree with xerxes on the thickness of the crankarms. I am totally in-tune with shaving grams but...
A 40 T ring with no spider. Oh boy. I'll have to sleep on that one but, you're not building a BMX racer for a 90 lb kid, either. Hmmmm. I would probably lose all of the cutouts and opt for a solid disc.
Curiosity question - What are the (4) .26" dia holes on the 4" dia plane? That would appear to be the logical place for bolting to a spider.
PS - Engineers drive trains. Why would you want thier thoughts??
Rob
Wheelies don't pop themselves. (from a QBP fortune cookie)