On Tuesday, September 15, 2020 at 3:48:01 AM UTC-7, charliebrown wrote:
I'm thinking of making a tool to be slid up inside a bicycle frame down
tube that would expand and push out dents from the inside.

Instead of a press, why not use an internal hammer?
Just get a heavy rod, set it into the tube, and slam the whole frame
down onto a sturdy table; the rod's inertia will hammer the dent
out.

Another popular solution is to force a large hardened BB through the tube.