At first i tried 30% grid infill, but the top part broke when using a normal wrench and made it unusable (first Try)

Then i took 100% concentric infill, so almost like just perimeters. This time i turned it with a nut. Like you can see in the other picture the wall of the part broke curling down from top to bottom, so it has no specific weak point. Note that i drilled a hole into the top because the axle of the wheel was sticking out too far (it was another bike then on the first try because i had no special tool for keeping the casette from turning and this one had another axle where you don't need something like that)...
Also there was a screw nut in the way, which meant that i couldn't push the part into the casette very far, which could be a reason for the failure, because the point where the force was applied was quite far away from the point where the tool grips in. Like you already said - need to somehow avoid stress concentration in the transition

The 3rd try was with all the same settings and stuff and it kind of broke in the same way, but then fell completely apart. So it definitely was not a one time printing issue.

I think when laying on the side it wouldn't fit anymore and its hard to deal with the probably needed supports...
Maybe it could work if the tool would fit further in by being a bit longer, so the force is spread around a larger area.
Also the idea of reducing the depth of the inner hole could work, but not in all cases, for example if there is a screw nut in the way. In this case you could extrude a bigger cylinder from the top and let this bottom out against the casette.

The big question is if the part in the cassette, which actually applies the force, can handle this or breaks anyway. If it does, idk how one could prevent it from breaking, because one is obviously limited by the space in there

Casting sounds like a good idea, but have you ever tried that with Aluminium? Doesn't the plastic melt away when casting? Or does the Aluminium cool down fast enough?

I like this kind of thinking and engineering btw... ;-)

Shimano casette removal tool

Shimano casette removal tool

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