Oblong circle junction

Hello, I have a design problem. I am currently making an entry screw allowing me to obtain a pitch as below:

As you can see, to catch up, I had to create an oblong sweep at the end of the screw. However in this view it seems clear, but from behind I have a junction problem and I can't solve it which creates this for me:

Do you have a solution to join an oblong with a circle on a sweeping material removal?

Thank you.

Hello

There you have your arrival and your departure. You have to create an extrusion (canister part) and then subtract this part to get to your final part...