Power, Speed, Accuracy
The objective of every metalworking operation is to remove metal within tolerances as quickly as possible. The issue for every shop is to define how much metal removal, how quickly, and what tolerances are required of a VMC. There are a lot of interrelated factors that affect a VMC's power, speed and accuracy, but the three basics include the spindle drive system, machine operating system (computer numerical control), and axis drive system. The spindle drive system provides power to the cutting tool to remove metal. The control or "machine operating system" is the brain of the VMC and coordinates machine motion. The axis drive system is "the ride." How smooth is the motion of the VMC and how does that translate into parts that are consistently accurate with acceptable surface finish quality? The quality of "the ride" or axis drive system is a function of the construction of the frame and the X-Y-Z way system. This is the hardware of the machine and it determines rigidity, vibration damping capacity, and resistance to side thrust. It's the balance between these three critical areas (power, speed, accuracy) that you must evaluate against your shop's needs to get the best buy for your money.
Basic requirements for your VMC, such as spindle rpm, low speed torque, and high speed horsepower are established by the materials that you machine. For example, soft materials require higher speeds for finishing, while hard materials require low-speed torque, as well as rigidity to reduce the effects of side thrust.
We can all agree that throughput is important. But throughput of prototypes and short runs requires different features than long production runs. If you're machining prototypes, then anything that makes setups faster and easier is going to be important: program editing, access to the control from the work envelope, table height, and a cooling system for thermal stability. If the VMC is for long production runs or dedicated production runs, then automatic loading and chip removal are going to be high on your list.
Quality is a function of the control, encoder, ways system, construction, and rigidity. The control has to be accurate and should be calibrated periodically. There are several different types of encoders available, including rotary encoders, glass scales and laser scales. They provide progressively higher accuracy at higher speeds.
Another issue is the ways system, which affects rigidity, vibration damping, and the ability to withstand side thrust during heavy machining operations.
The VMC features that are needed to machine an aluminum mold with 3D contours, such as high program execution speed, spindle concentricity and ramp-up/ramp-down are not necessarily the same features needed to drill holes in brass. If you're doing 2D parts then high feed rates and tool change speeds will be important. You have to match your needs with the VMC.