How much power do I have or need?
This is a tough question that can only be answered by the user. That said the vast majority of OEM models use a NEMA34 stepper with a holding torque between 500 and 650 ozin holding torque. Since this is most likely the most popular used NEMA34 motor, lets get it into understandable numbers. I will use a 600 ozin typical motor as a generic reference.
The 600 ozin number is holding torque. That means when stopped. What is not often mentioned is that the number is measured at full steps. If, and we all do, we choose to use microstepping that holding torque is reduced. More levels of microstepping, more torque reduction. Here are the numbers:
One other scenario: stepper drives that use 1/10th microstepping are very popular. I recommend against them for router use. Reason is that 1/10th microstepping reduces available torque to 20% vs 70% while in motion for most of the other settings. Tenth does a great job to provide very quiet, very smooth motion, especially with a consistent load. Medical applications come to mind here. Smooth quiet motion, vs the growly motion we may accept in a stepper driven small router table. Our usage has ever changing, sometime unpredictable loads, I prefer to have as much power available as possible to keep my from losing steps.
So, we have 84 ozin of torque, what does this mean in "real world" numbers? Lets convert to "push/pull" at the bit. Using 5:1 gear or belt reduction is a middle of the road solution. A rack and pinion setup with a 24T (1.2" Pdia) pinion is common also. Our cals would yield: 84 x 5 or 420ozin at the reduction shaft. dividing by the radius of the pinion 420 / .6 yields 700 oz of linear torque (less friction and other resistances) or 43.75 pounds of linear force at the bit. or ~12.5 # if using 1/10. Some of my previous testing (with UPS and fish scales) shows that a 1/4" bit, 1/4" deep, at 250ipm & 15K rpm (.008" chipload) places around 50 pounds of lateral force on the blank. As you can see, with the above mentioned combo, chipload must be reduced to ensure no steps are lost.
Or, higher torque motors could be used. Upgrading to 900 or 1200 ozin NEMA34 motors will allow cutting in the higher chipload ranges. FYI... adding higher torque motors to the same gear ratio will REDUCE not INCREASE top speed. You will be able to cut at a higher chipload maybe even faster than before without loss of steps, but overall max speed will be reduced a bit.
What about higher torque with closed loop motors and drives?
600 ozin is 600 ozin. For the most part other than the fact that a closed loop stepper system will alarm and stop the machine if a preset threshold has been exceeded, there is little or no torque advantage over an open loop motor with the same torque rating. Some drives claim that once a "full step" is lost they will exert 100% torque to regain position. True, but in most cases where steps are lost, 100% of torque was being used and there is no "extra" to exert.
That said, stopping the machine and allowing the operator to adjust cutting parameters to stay within available torque limits and thereby NOT losing steps and destroying an expensive material blank can be everything. Even tho I build economical systems for users that have open loop steppers, I only use closed loop steppers or servos in my complete systems.
What about servos?
First off there are 2 types. Hybrid and Full closed loop. The hybrid versions have an alarm that performs similar to those on closed loop steppers. This is the type we can use with Acorn. Full closed loop servos use encoder feedback right to the controller allowing the controller to use additional torque to regain position. This usually happens so fast that it cannot be noticed.
Teknic ClearPath servos are common here. How do they compare to a stepper?
Many users will assume they have 600 ozin steppers so they want to buy much more powerful servos. Who wouldnt? The problem is they are cutting with 84 ozin, not 600. Lets take the smallest, single stack NEMA34 ClearPath, the 3411 frame. Continuous torque between 57 and 150ozin, peak torque between 192 and 638ozin. I will use the "S" configuration as it most closely resembles the rpm range of a stepper, so gear reduction would not have to be changed. 150ozin of continuous torque up to 2000 rpm. This will virtually double the "normal circumstances" torque (78 pounds linear) up to 1100 in/min AND provide up to ~250 pounds for brief time periods, if needed. This is close to 6 time what the stepper had up to 375 in/min, even more if under 100 ipm.
This torque stays consistent no matter how the resolution (steps per rev) is set. Imagine that, 6 times the torque at 4 times the resolution (1600 vs. 6400)
Other ClearPath configurations (P & D) will perform similarly if proper gear reduction is used.