Hi Barry.

Ok I get it now, you have just short circuited the calc and mulitplied the constant by 4. Is that from Roarks or an asumption?

You might have made the same mistake I did initially (or may be I am wrong !), which is to find out the stiffness of a beam with just one end restrained and the other far end twisted.
I got confused because the torsion constant is a function of its shape and not the loading condition (being a constant and all ) but thats the joy of engineering eh ? lots of ways to acheive the same result.

I didnt mean I wanted to overwrite your sheet just asking for permission to use elements of it in another form.

I've been working on another spreadsheet which analyses the principles involved in cnc router (or mill) design, and the effects of geometry and inertia. When I feel ready I'll post this as well . . .

Looking forward to seeing that.