OK, that looks and sounds a whole lot less scary, so I think you're getting much closer to the realistic performance envelope of your mill.
You can start to jack up either the stepover or the feedrate until it gets wobbly looking, and then you will know how hard you can push in this material.
Back off a bit from there and you will be in your sweet spot.
With respect to why your cutters are still getting dull too quickly for your taste. there are a number of potential causes:
1) If you sand cast the blank, there will be sand in the casting and it will beat the shit out of your cutters.
2) When you recut the chips it will have a similar effect.
3) If the mill is still deflecting rather than forcing the cutting edges of the cutter to bite in and shear a chip, the cutter will start rubbing.
A rubbing cutter will hurt you in two ways:
a) The cutting edges are loaded in shear instead of compression, so the carbide cutting edges begin to crumble.
b) The rubbing can work harden certain materials to the point you can't force a cutting edge into them without a huge amount of force...I don't know if silicon bronze is one of those, but I would not be surprised.
But what I see in your latest video is so much better than what I saw in your first video, that I'm greatly encouraged.
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