If you had to pick, NEMA17 or NEMA23 and why? I messed around with NEMA17 on 24V and they seem powerful enough for small table top router. What do you think?
That is totally depends on the project, but if I have to choose I would prefer a more powerful version (NEMA23).
so NEMA23 is not too big for tabletop CNC?
These have different length too. For example
http://www.linengineering.com/LinE/contents/stepmotors/5709.aspx
Click on Dimensions tab.
My tabletop printrbot 3D printer has both NMEA 23 and 17 motors for it. The 17s are used bonded (2 tied together) for the Z axis, while the X, Y and E (Extruder) are all 23s.
NEMA17 is probably enough, but more powerful motors mean faster travel is possible without risking missed steps. I see no reason not to use NEMA23, unless a few dollars saved is really that important.
It also depends on if you use belt drive or leadscrew…
Belt drive is fast, but needs a bigger motor. Leadscrew is SLOW, but the force you can get out of even a small motor is phenomenal.
A motor is sized by the type of load it is driving and the motion profile the device needs. What type of mechanism is being moved? What is its mass? How fast do you need to move it and how fast does it need to accelerate. It is impossible to say that a NEMA 17 or 23 frame is better, by default, it all depends on what you are trying to do.
Using too large or too small a motor for an application will cause problems. I ‘upgraded’ a customers Taig mill a few months ago with smaller stepper motors. He had some HUGE 380oz-in NEMA23 motors that he got with his original control package and they were limiting his performance. I replaced them with my ‘tiny’ 166 oz-in steppers (in X and Y) and a 276oz-in for the Z axis (his machine was also fitted with an obnoxiously large/heavy spindle motor.) These smaller motors allowed for increasing both the acceleration and velocity resulting in about a 25% reduction in machine time. He made lithopanes on this machine, which have a machining time measured in hours so a 25% reduction is a big benefit. A lithopane is a 3D relief carving of a photograph on a translucent material, when help up to light it looks like a black and white photo. With this type of work acceleration is king as a 3D relief is done with hundreds of thousands of small line segments requiring the machine to accelerate/decelerate each axis rapidly.
Not really…since a stepper motor is driven with ‘pulses’ the inductance of the windings play a major role in how fast it can accelerate and its maximum velocity. The inertia (rotating mass) of the motor is also a factor. Bigger motors, i.e. higher torque rating, will either have higher inductance or require a lot more current. Given the same machine, stepper driver and power supply a bigger motor will quite often be slower and not accelerate as quickly.
The inductance problem can be partially overcome by using a higher voltage power supply and current limiting to the coils. The higher voltage allows the magnetic field to build up more quickly. This is how most modern stepper drivers work. There is a practical limit on voltage though, at some point the increased voltage just creates more heat in the motor without improving performance.
A stepper motor will NEVER miss steps unless you ask too much of it, any motor will stall out when you ask too much from it so it is not something that is unique to stepper motors. Choosing the right motor, power supply and driver for the application is the key.
In a nutshell bigger motors are not better…the right motor for the job is the best.
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Let me rephrase the question then. What is most commonly used motor size for tabletop DIY CNC machines?
For 3d printers, the most common by far is Nema 17. Some machines are moving to Nema 14 where possible. Anywhere you are actually moving the motor mass smaller is better.
The problem Gus is that it all depends on what you are doing. The requirements for a 3D printer are different than for a milling machine. I carry two sizes of NEMA 23 stepper motors in stock (166 oz-in and 276 oz-in) that cover most of the table stop sized machines and small routers. For folks with different requirements I have to special order the correct size. I’m evaluating some larger motors now, around 360 oz-in which is about all that is practical for a NEMA 23 frame.
Think of it like sizing a power supply transformer, there is no one size that works for everything.
I just meant that more powerful (bigger) motors can drive faster mechanisms, such as belts, or steeper lead screws, because they have more torque, not that they could run at a higher RPM.
It sounds to me that GUS is preparing to offer a KIT of some sort…CNC Kit perhaps? 
What give you that idea?
My Zenbot has NEMA 23s.
The Arduino compatible CNC controller I got on Kickstarter came with 17s.
It really depending on how big/heavy of a spindle, whether it’s belt or screw driven (as mentioned before) and really how fast or cheap (they are sometimes mutually exclusive!) you want it to be.