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Hi, there are a few factors here contributing to this.
The easy one is tool change time, personally I record this as a rapid move across the full travel of the machine plus tool change time. Have you adjusted the default to your machines tool change time?
The next one is more tricky. Its to do with acceleration and deceleration.
Take milling a square with 90degree corners.
The machine cannot start from 0mm/min to 1000mm/min, Fusion currently does not account for this.
Next is when we hit the first corner, the machine cannot go from 1000mm/min in one direction directly to another, you would shear something, or at least not do your ball screw much good. So machines have something called a "jerk" limit. Again we dont know what this is, as its different for all machines.
In addition to this if the machine had to slow down to 100mm/min to do the 90degree turn, the deceleration has to be taken into account, as well then as the acceleration back upto the cutting feed.
In short we simulate the perfect scenario but this is unfortunately not what actually happens. You can add a value in the "feed Scale" to adjust the calculation, I use 85% on our HAAS VF2 YSS as a guide but this decreases the more complex the part is.
I was involved in some really experimental work earlier this year where we were looing at this problem. The idea being you would air cut a series of imaginary parts, plumb the actual machine time back into Fusion and then we would know what your "feed scale" should be. But this was just a WHAT IF project and we haven't done much with it yet.
I know this is not a solution by any means, but I always think understanding why the values are given don't match at least helps.

Richard Stubley
Product Manager - Fusion Mechanical Design
Fusion