I decided to do a quick test. I did a model of finger joints. The plywood used here is 3/4" thick, I used a 1/4" endmill, the depth of cut is 6.3 mm for each pass, Total depth of the fingers is 19 mm so 3 passes for each ‘slot’. There is some breakout because no backing was used. The router used is a full sized router and the Altmill spindle is still mounted (can’t see it behind the router). All cutting parameters were the same as if I had used the spindle on a horizontally mounted work piece.
The cuts were a bit noisy, probably because the stock was pretty high up. The bottom of the fingers should be just a tad above the spoil board.
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