Over the years there has been a substantial debate going on over which stepper motor (and stepper motor driver) should be used to drive a CNC router. Much has been written. Most of it true, but some information is still being bandied about as fact when the data shows that it is not quite reliable.
First, let's remember that a stepper motor works because of the principle of magnetism. We all remember, back in grade school, how we played with magnets and how difficult it was to separate two properly aligned magnets. Stepper motors work in basically the same way. When properly sized and properly implemented, a stepper motor can be counted on to move a load a finite distance each time a pulse is applied to the stepper driver. The key words are "properly sized" and "properly implemented".
Just like we learned, as grade-schoolers, we could pull apart two magnets if we applied enough force. We can also cause a stepper motor to "stall" if we expect it to do more than it was designed to do.
Some methods have been developed to compensate for those times when we expect a stepper motor to do more than it was designed to do. One of those methods is the feed-back loop built into the Oriental Motor "Alpha" series of stepper motors. The feed-back loop incorporates an encoder (to measure movement) and some kind of timer (to verify that movement took place within a certain time limit). When properly implemented, the "Alpha" system can guarantee that a command to move an axis a certain distance will actually move that distance, or else the controller will enter a fault condition and the operator will be notified that something went wrong.
On the surface, that concept is a good concept. In practice, it leaves much to be desired. Remember that a CNC router is a multi-axis machine. Typically a CNC router uses four motors, 2 X-axis motors, 1 Y-axis motor and 1 Z-axis motor. Many "moves" require that all four motors work together, at different speeds, to perform a required task. It is not hard to imagine what happens when one of the four motors starts to lag behind the other three motors. The commanded move deviates from the desired path and a "glitch" appears in the work. Depending on many factors, that "glitch" might be inconsequential or it may be large enough to ruin the part.
The traditional way of controlling stepper motors relied on the fact that the programmer knew how hard and how fast he could move an axis. If he commanded a motor to move too fast or required that the motor move a load too large for the motor's design, the motor stalled and the part was ruined.
All of this brings us to Shopbot and the design philosophy behind the two controllers they sell.
Shopbot sells one mechanical model with the option of two different controllers. One controller uses stepper motors without feed-back and the other controller uses stepper motors with feed-back. Of course, the model with feed-back costs substantially more than the basic model. It should cost more because it used technology that costs much more to purchase.
Finally, with that back-ground, we can look at some of the differences between the two controllers. Both controllers use Oriental Motor stepper motors, which is a good thing. Oriental Motor makes excellent motors.
The original standard model used a motor equivalent to the PK296-01. That motor is an excellent motor, but it was designed to be used with a stepper driver that could operate with a voltage of about 175VDC. That's a lot of voltage for any stepper driver and more than 2X the voltage that the highly acclaimed Gecko G20x driver can handle. Basically, that meant that the standard motor was not very well matched to the newer stepper driver and that both speed and torque produced by that motor and the stepper driver would be lower than expected. That was not necessarily a design flaw caused by Shopbot, but simply the result of using two components that were not perfectly matched. When Shopbot introduced the 4g upgrade, they allowed users who already had the original Oriental Motor steppers to upgrade to a much nicer controller for a moderate price. Feed speeds were about doubled for a very reasonable cost.
Those users that needed a higher production machine were steered towards the "Alpha" model which used the much more expensive Oriental Motor "Alpha" stepper motors and drivers. Those motors were tuned to special drivers that caused the motors to deliver both high speed and high torque - at a substantially higher price.
I bought the PRT-Alpha model and it does run fast and hard.
Since buying that PRT-Alpha, I've spent more than three years testing various stepper motors and stepper motor drivers to see what could be used to get good performance at a reasonable cost. So, for the "developers" who don't mind building their own electronics, here is what I've found.
The AS98 motor used as the basis for the 7.2:1 geared motor on the Alpha models produces about 300 oz*in of torque at 250 RPM. That torque drops to about 250 oz*in at 1,000 RPM. The PK296-03 motor, wired half-coil, produces about 250 oz*in at 200 RPM. That torque drops to about 100 oz*in at 1,000 RPM.
What is not shown in the charts is the fact that the AS98 motor, when used with the 7.2:1 gearbox, is limited to 80 lb*in of torque and the PK296A2A-SG7.2 motor, with a 7.2:1 gearbox is limited to about 40 lb*in of torque. In other words, looking at the chart, the PK model gives substantially flat-line torque up to the feed speed of about 8-ips when driving a 1.5" pitch diameter spur gear (30-tooth). The AS98 motor gives essentially 80 lb*in of torque up to a speed of about 11 ips, using that same 1.5" pitch diameter spur gear.
On the surface, it looks like the AS98 motor and driver is the winner for any high-production user.
Well, if you only had those two choices, then I would agree that the AS98 motor/driver is the optimum motor/driver for a CNC router used with a stepper motor; however, there is a third choice.
Oriental Motor also makes the PK299-F4.5 motor, which can be wired serial, parallel or half-coil. It is an excellent motor, but it is not offered with a gearbox. My own experience with my PRT-Alpha showed that adding a gearbox was necessary to get acceptable edge quality. I built several models of belt-driven gearboxes and then purchased the 7.2:1 upgrade offered by Shopbot.
The PK299-F4.5 motor, when coupled with a 3.6:1 belt-drive gearbox develops more than 100 lb*in of torque at more than 20-ips. Those specs are substantially better than either the PK296A2A-SGxx motor or the AS98 7.2:1 motor.
The PK299-F4.5 motor costs $207 each. The Gecko G203v stepper driver costs $147 each. A homebuilt power supply (45VDC @ 20A) will cost about $200. Belt-drive transmissions for the PK299-F4.5 motors will cost about $200 each, depending on what you make and what you buy. In other words, all four PK299-F4.5 motors with stepper drivers, a power supply and belt-drive transmission will cost about the same as one AS98 geared motor purchased directly from Oriental Motor.
Resolution between a 3.6:1 belt-drive and a 7.2:1 gear drive will be exactly the same, given the fact that the AS98 uses 1,000 steps per revolution and the PK299-F4.5 uses 2,000 steps per revolution.
The PK299-F4.5 does NOT have encoder feed-back, but it does have at least 20% more torque than the AS98 motor/driver, so if properly programmed, it can do at least as much as the AS98 motor/driver before it stalls.
Repair prices for the PK299-F4.5/driver/transmission would be about $200 for any one component. The AS98 is purchased as a complete unit at a cost of well over $1,000 per unit.
So, with that much conjecture, what is the purpose of this post? It's simply to say that a do-it-yourselfer, who is willing to do a lot of building, can build a very robust electronics package at a substantial savings.
Do I recommend building your own electronics package? Well, I'm perfectly happy using the Alpha controller that came with my PRT-Alpha. It has worked flawlessly since July, 2004. I don't plan on replacing it until it breaks - then, depending on how much time I have on my hands, I might build my own controller.