It's time to up the ante in this battle of parentnoia. OK, here's the plan: Show your dad and grandpa that you're going to turn into a vegan hipster if they continue to take your knives and/or suppress your manly blue-collar desires.
Start by wearing skinny jeans. Quickly work up to wearing mismatched plaids, then tighten up the screws by sweeping your hair over one eye. Practice bad posture and adopt a holier-than-thou attitude about everything. They'll cave like the locking mechanism on a batoned folding knife that's virtually as strong as a fixed blade.
Here's a visual aid to help them rethink their position: