You can proceed in different ways,

The easiest way is to observe and record the conditions as well as the outcome (both for the produced items and for the consumables), depending on intrinsic properties like material grades, speed, refrigeration flow, etc.

A more sophisticated or scientific approach is based on design of experiments (DoE), where the previous approach becomes standardized. You can have a look onto an interesting paper here.

The most sophisticated approach can be implemented by using IIoT technology, where you can collect data making it possible to build data driven models. This solution is much more complex and requires higher level of technological engagement. For sure, the capabilities brought are much wider. You could consider the energy used per item as an overall indicator of performance, and also, as relevant for LCI analysis.

Hope this helps

http://www.uni-obuda.hu/users/mikob/2013dmt.pdf

There are several way to optimize the machining process. however before starting the optimization you should know what you are going to optimized. for example for improving Material Removal Rate (MRR) you can increase the depth of cut or even the feed rate. each every single parameter has its specific influence on machining outputs and it repents of not only the machining process but also the material and tool that you are going to use.

one of the best way to understand the effect of machining parameters on the out put is using DOE and ANOVA analysis and you can easily get the optimum parameters. 

or even reading the papers is a useful way to choose the parameters which are suitable for machining.

on the other hand, the tool manufacturing companies suggest you the optimal parameters for each cutting tool in their brochure.

Regards,

Due to the   structure of the material some material is  difficult to machine. such type of  material improve their machining process using different mechanisms, but the action is depend on the material type.

you can  optimized by change feed rate, cutting speed, depth of cut, apply  cutting oil or fluid, tool Material, tool angles etc but this not always true, it depend on the working condition(machining process).

 For instance, in conventional machining process , Nike is poorly machine and manufactured. Improve Machinability of Nick using Special machining processes(EDM, ECM,USM). Also optimize by controlling cutting forces, cutting torque, acoustic emission from the tool, work piece and the interface between the, tool and work piece displacements, cutting temperature, cutting speed, tool wear.

Since, you should know the working environments(Machine, cutting tool, cutting material etc)