GWizard uses a K Factor of 18.8, both HSMAdvisor and NYCCNC use 10 and, based on measurements, the SFPF Workbook uses 20 cubic inches per minute per horsepower (cu-in/min/HP) for acrylic.
GWizard uses 4.7, HSMAdvisor uses 3.77 and the SFPF Workbook currently uses 3.34 cu-in/min/HP based on NYCCNC’s K Factor for 6061 T6 Aluminum. The other speeds and feeds calculators I’ve tried use values within this range for 6061-T6 aluminum.
Although K Factors for metals seem to be fairly well established for metals (which is the primary purpose/focus of almost all speeds and feeds calculators), that doesn’t appear to be the case for other materials. IMO and experience, a K Factor of about 20 cu-in/min/HP is reasonable for acrylic. So it likely takes from 4-6 times more power to cut 6061-T6 aluminum than acrylic at the same material removal rates (MRRs). If both the MRR and endmill diameters are the same, milling aluminum would require 4-6 times more cutting force than acrylic. 