How do I connect an existing 3 wire aluminum #4 line from my main panel to a sub panel that I will use to supply a 40 amp electric oven and a 20 amp circuit for a microwave. Do I connect the bare wire to the ground bar or common and do I a connect the nutral bar to the ground bar. I know how I would connect a 4 wire cable(3 insulaed conductors and one bare conductor) from the main panel to the sub panel but since I want to use an exiting 50 amp 3 wire cable(2 insulated conductors and one bare conductor) I am not sure how to make the connections to the sub panel.
Thanks for help,
Sal

You won't like this answer, but what you propose is not allowed, if your located in the USA.

A four wire feeder is required for a feeding a subpanel.

If the panel is in a detached building , ground rods are also required.