I am planning to buy CNC mechanical skeleton without motors, spindle and controller. I will be using the CNC mainly for aluminium milling. Are there any specifications for minimum torque requirement for stepper motors and spindle to perform aluminium milling ?
Here's the important thing to realize:
While many want to size motors based on cutting forces, that approach will almost always leave you with a motor that's too weak for good performance.
Cutting force is not the limiting factor the motor needs to overcome. Rather, the limiting factor will be acceleration. Your motors must produce enough torque to accelerate the axis quickly enough to reach your machine's target speeds in a reasonably short distance.
I've written some detailed articles about how to figure all this out in Ultimate Benchtop CNC Mini Mill Part 4: Motion Performance.
There are many calculators online for determining this sort of thing, and there are many factors involved. Here is one such calculator.