Yesterday, I received a Yashica J-P camera, fitted with the desirable Auto Yashinon-DX f2 50mm lens. The lens filter ring is dented.
I have a simple jig I use, to straighten these - a piece of wood with a row of different sized holes (made by drilling a row of holes with a hole saw, and then ripping down the plank). Sit the ring in the appropriate hole, on a piece of paper, and tap on a dowel to knock out the dent.
I've done something similar myself, and using improvised wooden 'tools' is a better alternative to some of the purpose-built modern metal tools sold to do this single task. There's a similar discussion in this thread about the process, and I remember seeing something long ago about using several different layers of tooling leather strapping placed as 'shims' inside a jig similar to yours. Apparently the idea was to have a firm but pliable shock absorber that doubles as a resizing modifier so you don't need every filter diameter imaginable... if it needs to be smaller, add another 1mm or 0.5mm strip into the jig to decrease the diameter by the proper amount.
Fortunately, at least on the lenses I've dealt with, the aluminum is usually stiff enough to avoid a BIG dent, yet soft enough to gently, slowly move it back into shape.
I have one of those metal expander tools. They are easy to use, but I've found that while they may push out the dent on one side, they end up CREATING a outward bulge on the OPPOSITE side. That's not usually a big deal, since it's pretty easy to bang the bulge back into place.
Once I get it straightened out as best I can, I add a UV filter -- with good front threads -- to be kept in place permanently.