@chipinator it can be a bit overwhelming trying to spec and design a CNC system. There are really very few guidelines out there to help with the decision making. I hope that this will be of some help.
I have pretty much stayed out of this discussion on purpose, because I have no experience with benchtop machines, and haven't researched the parts. I normally work on commercial equipment, but the same general principals apply. I had a bit of time and took a look at what others have done on similar machines. I also actually have a benchtop CNC machine but it just sits on the shelf because I have no need or room on the floor for it right now. It uses NEMA 23 and 34 stepper motors. I'll do a controls retrofit on it someday.... Maybe.
Ignoring budget constraints, the three most important factors to consider when purchasing parts is quality, documentation, and support. In most cases if the documentation and support is there, quality will be there also. In other words, purchase from known trusted vendors that have a North American telephone number and email address. You pay a bit more for hardware purchased with good documentation and support, and it's worth every extra penny. This also applies to the software and controller.
Accuracy. Repeatability. Resolution.
Accuracy: If you tell the table to move 6.00001'' then if the machine is accurate it will move 6.00001'', no more, no less. In a perfect world it would hit that target, but the world is not perfect so there is always going to be some error in the move. To get the error smaller the cost of the hardware goes up exponentially. A common accuracy for hobby class equipment is +/- 0.002'' over 12''. Or maybe a bit better.
Repeatability: More important than accuracy is repeatability. Will the machine move to the same place every time? You can compensate for accuracy errors if the machine will repeat. I expect that repeatability should at least be in the +/- 0.0002'' range if you have enough resolution.
Resolution: The smallest unit that your system is capable of measuring/moving. The more steps or encoder pulses per revolution of the ball screw the smaller division the system will measure. Normally you want the resolution to be at least 10X the target repeatability. So let's take a 5mm pitch ball screw and a 200 step/revolution stepper. I'm going to use metric measurements here. 1000 microns = 1mm or 0.001mm = 1 micron =0.000039'' . So 5000 microns/200 steps = 25 microns or about 0.001'' per step. So reasonable resolution will be in the range of 2.5 microns or about 0.0001'' or better. I use 1 micron resolution scales on my machines, and expect repeatability of +/- 0.0001'' or better.
Having said all of that, there are many other factors that enter into the overall tolerance and what you can expect out of the machine.
Mechanical: There is no substitute for a tight machine with no backlash. Just like perfect accuracy, 0 backlash is not going to happen, but you can most likely get it close to not measurable with tools normally found in most shops. The backlash can come from not only the ball screw but also the thrust bearings as well as flex in the overall system. The ultra precision ball screw kit from Precision Mathews would probably be my first choice. It comes equipped with double nut ball screws and looks to be just a simple bolt in installation, as well as being designed to exactly fit your machine. It doesn't get any simpler than that. Looks like it could be installed in a day. $1200 for the complete kit seems reasonable.
Axis Motors: There are two types of motors that are in common use today; steppers and BLDC (Brushless DC). The BLDC motors are commonly called ''servo motors'', but that is not technically correct. The marketing department of the various vendors have muddied the waters with their own naming conventions. A servo system is any closed loop system, so any motor that operates with a closed loop is a servo motor. Your home heating system is a closed loop servo system, as is your car a multi axis system where you are the controller.
Stepper Motors: This is the low end of the axis motors. Cheap and easy to set up. Normally limited to Step & Direction open loop control. Can easily lose position if over torqued by missing steps, or just completely magnetically decouple and quit turning while growling at you. Open loop only. Base resolution of 200 steps per revolution, but normally operated in half or quarter step mode for 400 or 800 steps/rev. Steps/rev can be set higher at some cost to performance, normally up to about 56,000 steps/rev. Rated torque is holding torque at 0 RPM. Torque drops off rapidly as RPM increases.
Closed Loop Steppers: A small step up from a standard stepper, the difference being that they hung an encoder on the back of the motor then feed that encoder signal to the drive to compare the actual position to the commanded position, and make small adjustment if needed. These are still stepper motors and subject to the same performance limitations as normal stepper motors. These are a closed loop servo system with the loop closed at the drive, but with no possibility of feedback to the controller.
BLDC Servo Systems, or more commonly called servo motors: More expensive than steppers or closed loop steppers. Generally much higher performance. The torque curve is normally flat from 0 up to near the motor rated speed, with 2x to 3x short duration overload capacity in the low to mid speed ranges. They will not ''lose steps'' like a stepper. Most modern drives can be controlled by your choice of Step & Direction, Analog torque or speed signal, or direct communication using some derivation of a ModBus protocol. Depending on your choice of control, the loop can be closed at the drive or the controller via the drive encoder output or an external encoder. All commercial machines use these systems today. There are probably 25 or 30 servo system manufactures. Prices range from affordable for the hobbiest to crazy prices.
Controllers
Motion Controllers: The motion controller handles the trajectory planning for the desired move. You tell the motion controller where to move to in any number of axes, give it the speed and acceleration and the Go command. It figures out how to make the move so the tool follows the desired path to get to the target point. The moves are only straight lines, arcs are generated by very short straight line moves, so small that you can't see them. This target position could be only 0.001'' or less from the current location or it could be several inches to the target position. In the case of a closed loop system, the drive or controller is tightly monitoring the tool path and is making adjustments as needed, normally with a few microsecond update rate. In addition the controller needs to perform PLC type functions like being able to turn on coolant, maybe controlling a VFD for the spindle and other peripheral functions.
Gone are the days of the parallel port machine control used the old Mach3 systems. It's pretty hard to even find a modern PC with a parallel port. Most systems today use USB, Ethernet, or PCI card to interface with the PC. Ethernet and PCI cards are the most robust systems. USB systems can be sensitive to electrical noise. Many of the cheap ''motion controllers'' sold on eBay and Amazon are not actually motion controllers, but rather just breakout interface between the PC and the world. A hardware motion controller is a computer dedicated to the machine control duties and uses real time control. They communicate with the PC, but they do the heavy duty math thus relieving the PC from those duties.
I am only going to address the systems most commonly found in the home shop here, there are many others available. Mach3/4 use the PC as the motion controller, Centroid Acorn?, Centroid Oak, Dynomotion, CamSoft, and others use hardware motion controllers.
Open Loop Systems: The cost for these systems is generally less than a full closed loop system. Mach3, Mach4, Centroid Acorn, and Dynomotion Kflop are all open loop step & direction systems, there is no provision for position feedback to the computer/controller. The computer/software just assumes that the motors are doing what they are being told to do, and for the most part this works just fine. If the loop is closed at the drive level then these CNC systems work good, but keep in mind that the DRO display shows where the controller thinks the position is, not necessarily where it actually is, but for the most part the displayed and actual position agree. CamSoft can be used as open or closed loop because they use a Galil Motion Control controller that has the capability of both step & direction and full closed loop analog control.
Closed Loop Systems: Centroid Oak, Dynomotion Kflop/Kanalog, CamSoft, and others close the loop at the controller. All of these use a dedicated hardware motion controller. The DRO display is the actual position based on the encoder feedback from the motors or external encoders. It is my preference to close the loop at the controller level, and use external encoders for position feedback, although I have used the drive encoder output as the feedback encoder with good success. My prefered encoders are 1 micron resolution linear magnetic scales. With linear encoders you pretty much take mechanical ball screw/backlash errors out of the system and directly read the table position. But there is no substitute for a tight mechanical system.
CNC software: Mach3/4 is CNC operating software of course. Dynomotion, Centroid, CamSoft and others all come with their own CNC software. I have not used any of them, but they all work and have various levels of user friendliness. You mentioned wanting conversational programming. I have used that before and found it to be more trouble than it's worth. CAM software is so much easier to use, I even use it for quick jobs like facing off a part. No way I'm going to stand there and punch commands and numbers into the computer when I can generate the same thing with a few mouse clicks.
CAD/CAM software: There are so many options available that I'll only touch on a couple that I use. Prices range from free to only SpaceX can afford it. Today Fusion 360 is my go to CAD/CAM software. Free to hobbyists, although the free version is somewhat broken, but what do you want for free. It still works and has mostly full functionality but a few limitations. I used to use AutoCAD and let that license expire when I bought a Fusion 360 license, but for a substitute I use NanoCAD V5.0 (free) which is virtually an AutoCAD clone. For CAM software CamBam is a good value with a short learning curve. It has some rudimentary CAD functionality. At $150 for a lifetime licence it is a good value and I sometimes still use it. They have a very generous trial period program.
Electrical: Once you decide what hardware you want, you are going to need to stuff it in a box and wire everything together. Buy all of the components that you need, lay it all out on the table, then buy the enclosure 20% larger than you think you need. Nothing worse than running out of panel real-estate. You want to generally follow NEC, UL, and OSHA guidelines, however for home shop use, strict adherence to any of those is not required. But you want to keep things safe and not burn down your shop. This means using proper components, fusing, and wire sizing. In addition proper safety shutdown circuits. Automation Direct is my go to vendor for all things electrical panel related, however I am not opposed to buying enclosures and other non-critical parts from eBay or Amazon. Do not buy circuit breakers from anyone but known vendors, there are a lot of fakes out there that look like breakers but are in fact just cheap switches.
As far as electrical diagrams, there is no generic diagram for a CNC machine, they are all different. There are however some generally accepted circuits for power handling, control power, and E-stop circuits. The hardware will come with wiring diagrams.
I am tired of typing now so I think I'll go make some chips. I hope this helps. I'll be happy to try to answer any questions.
Tuning is just a matter of tightening up the action of the system with the PID values until the machine goes unstable, then back off a bit.