I need to look into the math on the plenum before I start playing with the TR1 I have for my 340. Nice job. The cutoff disc obviously worked, but I think for jobs like this a slitting saw is the optimum tool to use. Good slitting saws get pretty expensive so I've been using various diameter (3"-7") carbide tipped trim saw blades. Even the cheap thin ones from HF are useful. You can buy cheap arbors but I just make them from scrap on the lathe. In aluminum they go like a hot knife through butter. Cut very well on steel too, but you need to pay attention to rpm and feed rate. Probably set the mill to minimum speed and plenty of lubricant to prevent smoking the teeth.
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