OK, as I said before you need to learn about thermal parameters. A TIP120 running at 6A has a saturation voltage of 4v, thats 24W dissipated. Each transistor needs to be mounted on a 2.5degC/W heatsink to stay inside its thermal parameters. Thats a lot of expensive heatsink. So a TIP120 is NOT a good choice for a stepper motor. Better choices are very low 'on resistance' MOSFETs such as IRFZ44 series, these will dissipate 0.2W at 6A and dont need any heatsink, but are more complex to drive and need more understanding of the dynamics.
Unless you have a good understanding of electronics and circuit parametric design, and access to good PCB manufacturing facilities, designing and building a good stepper driver from scratch is not possible, except for very tiny motors that are never going to be of any use for a CNC machine. You would be better spending money on a commercially available driver, it will be cheaper than building it in the long run.
If you insist on building your own, then look at pminmo.com where there are several tried and tested designs based on various chips as well as PCBs you can buy.
A final word of advice... design your CNC machine first, then work out what motors you need, then decide on the drivers... then go back and refine your thinking to what you can afford... doing it any other way will end in disappointment...
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