I have been seeing questions asking why the CNC spindle is stalling during cuts. They claim the machines have plenty of horsepower [HP]. People have a tendency to simplify things, and HP is not immune.
With the famous bell curve we all learnt in school, the HP rating of a CNC is bound to that same shape of rising and falling power and torque. Newer CNCs all use a “variable frequency drive” as a way to obtain high motor control while reducing “mechanical” costs. Let us take a moment to understand what this variable frequency drive is for laymen. My best analogy is a merry-go-round. Let us call it MGR for the sake of simplicity. The MGR represents the spindle motor, and the people pushing will represent the electricity. One person can hold a rail and run in circles indefinitely. While the person does this, another person tries to grab and slow the MGR down. You can imagine how the MGR may slow down if the second person is stronger than the first person. To alleviate this, larger people were asked to push. Eventually, more people were asked to push also, and here is where the fun of physics and electricity meet. Instead of people pushing forever, what if they stood stationary and pushed at a specific moment? That moment is separate from any other moment another pushes. The bell curve would look like little humps when a person pushes and then the MGR starts to slow and then another person pushes and power is returned again. This rising and falling bell curve becomes just the tip of that curve. This is a simplistic understanding of a variable frequency drive. It becomes the stationary people and pushes the motor thousands of times a minute with small pulses of energy. The flaw with this particular technique is advertising.
Since this motor type can easy achieve higher rpms without any transmission gears to help, and the formula for HP = rpm*torque, advertising sells the best HP moment available with complete disregard to the real-world; the increased rpm is masking the lower torque ability.
Backgears… What is it? The name implies it is a gear of sorts. I like to imagine an old manual transmission in a car. Maybe a simpler design or more complex, it doesn’t really matter because any machine with a back gear has more torque that a machine with a variable frequency drive at low rpms. The back gear uses mechanical leverage to add torque to a spindle drive. These machines with transmissions historically run lower spindle speeds and excel in harsh machining environments. The CNC may not achieve 10k RPM but when cutting an alloy with an optimal RPM range around 100, it doesn’t matter.
So which is better? I prefer to have both available for any situation which may arise. I often run an old mill with a back gear over a newer one when I need large amounts of torque at low rpms to cut through materials. Leave the highspeed work to the newer generation of machines.