In my world, a 10hp would pretty much be minimum. As part of my employment as a senior manufacturing engineer I buy and install machines in a metal cutting industry.
If I think about a large cabinet shop where time is money and the machine needed to be feed several cabinet builders, and keep a production line running, I would highly recommend the 10 hp. All things said by GER21 standing - fast feedrates, fast cutting, time is essential. Full 3/4 cut with 1/2 cutter.
If I think about a one man shop where cabinet making is really important, but I am the only guy building and money in critical, but I cannot build any faster, then 5 hp is a really really nice option.
Currently, my personal shop - not my source of employment - I have a 4hp water cooled spindle. I do NOT push it to max, not by any stretch of the imagination. I can cut at 250 IPM with ease. I would take on cabinet building, if I liked to do that, which I do not. I do it on a very very small scale just for the money, but not as a source of income.
In a commercial shop - my 4hp would be a minimum, and considered a small machine.
I do use small machining centers to cut metal with a 5 hp spindle, and they function well. I have mill-turn machine with 5hp milling spindles and they function well removing significant amounts of metal (A2, O1 tool steels). But routers are a little different than metal cutting machines.
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