CNC Spindle Contruction Help

Hello everyone. I'm new in this forum and maybe my question is not for this but I don't know where to ask for this. I have searched on internet regarding this but I haven't find something like I want.

Right now I'm trying to make a CNC machine, mainly for PCB drill/mill. My question is where I can find information related to spindle construction, I mean, DIY spindle construction.

If someone can answer the following question I will appreciate much his/her help. Thanks in advance.

The questions are:

Why is needed an spindle?
Why not connect the chuck to hold the drill bit directly to the motor shaft?

I'm planning to use an RC brushless DC motor (A2212/13T KV1000), for this motor:

Can it be operated with the rotor reversed? (See the attached picture)

I have seen some DIY spindles, they use atleast two ball bearings and a shaft. Can I use something similar in the following scheme?

Motor_Shaft [3.17mm] + Coupler [3.17 --> 3.17mm] + Shaft [3.17mm] + Chuck [with adapter for 3.17mm]

Thanks in advance for your help.

The problem with small spindle motors is poor bearing vibration and "runout" or wobble.

The trick is to find a dummy spindle motor with a 0.5 or 0.75" shaft and cascade this spindle driven by the smaller motor to reduce the runout.

Ok, thank you very much for your help. Also, if possible, can you answer my other questions? Thanks in advance for your help.