I tried to make difference between the two concentric cylinders. The boolean operation works well for the straight ones. But after I added the curve modifier to both cylinders, and did the difference again. There were some redundant faces left for the large cylinders.
Why did this happen? Could someone give some hint or advice? Thanks!
My file can be downloaded here: https://www.dropbox.com/sh/l1vw12idg72vccn/AACewT_XSUhBMdP1CqA8L5zsa?dl=0