Quote Originally Posted by dsc View Post
I'm pretty sure four M6 bolts can handle 1kg, so it's basically a matter of machining the parts correctly to get 90degs everywhere.
Four M6 bolts would probably handle several hundered times that before shearing! As mark said, you don't need the bolt holes to be deep - bear in mind you'll struggle to tap M6 more than about 25mm deep anyway. The strength of the joint depends on the friction between the surfaces, and thus the clamping force and surface finish. Increasing the length of the bolts beyond 20mm of thread wont make much difference to the force you can apply.

If you don't mind me asking, what is the part for? It can help a lot to know since we might be over examining this, or worse vice versa.