Accelerate into HSM: General Industry Coverage

The motion of a machine tool’s axes is often characterized by the range of motion and maximum feed. That’s because that information tells the user the part size the machine can accommodate and how long it will take to make the required axis motions.

However, as spindle speeds and feed rates have increased, axis acceleration is becoming a limiting factor, and axis acceleration largely determines machining time. If the acceleration is not high enough, then the machine does not reach the maximum feed during some—or most—machining operations.

To see how this works, let’s consider a permanent-magnet DC servomotor, a common type of machine tool axis drive. For DC servomotors, the achievable acceleration is generally limited by the maximum current the motor can support, and the motor controller prevents the current from exceeding that limit. The current produces torque, and torque produces acceleration.

There are two algebraic equations that relate the acceleration, the velocity and the position of an axis:

v = at + v0

x = 12 at2 + v0t + x0

where a is axis acceleration, t is time, v is axis velocity (feed), and x is axis position. The subscript “0” refers to the starting velocity or position.

Let’s assume the machine tool in question will be used for high-speed machining of aluminum. As a general principle, we desire the highest spindle speed possible, but the spindle speed is limited by the bearings, and commercially available spindles with CAT 40 or HSK 63 tapers are typically available on machines in the 20,000- to 25,000-rpm range.

The milling feed for such a machine is calculated by the feed per tooth (chip load) times the number of teeth per revolution times the spindle speed. Using a tool with four teeth and a chip load per tooth of 0.2mm, the required feed is:

Only 20 m/min. is required for the working feed, yet many high-speed machines advertise substantially higher maximum feeds. Why? One reason is to speed up noncutting motions, such as rapid traverse and tool changes. Another reason is to account for chip thinning in small radial immersion cuts with ballnose tools.