Five-Axis CNC Machining Centers

Five-axis machining adds rotational movement to the traditional three axes of linear motion (X, Y and Z). These machines can incorporate an A-axis that handles rotation of the X-axis, a B-axis that does the same for the Y-axis, or a C-axis that rotates about the Z-axis. Some employ rotary, tilting tables, which provide mechanical and thermal stability alongside improved access for swarf removal. Others employ a pivoting spindle head to accommodate larger, pallet-loaded parts.

Source: Machining 101: What is Five-Axis Machining?

Besides the linear axes X, Y and Z, 5-Axis Machining Centers also move in two rotary axes, often identified as A and B axis. The rotary axes tilt the tool with respect to the part. Physically, it can be either the tool that tilts or the part that tilts.

Granting users access to five sides of a part in a single setup enables tighter tolerances. Parts with 3D contoured surfaces (such as mold cavities and electrodes) tend to be great candidates for five-axis machining. For example, flexibility in accessing deep cavities enables shops to use tool assemblies with reduced lengths and aspect ratios for these operations.

Improved tool orientation control also helps with tool breakage and chatter when working with difficult-to-machine materials. For example, the surface speed of a ball mill depends on the contact angle between tool and surface. On three-axis equipment, this angle changes as machining proceeds. When the bottom of the ball mill contacts the part, surface speed becomes an unproductive zero. Surface speed then reaches its peak when the tool’s equator contacts the part. These variations in surface speed cause variations in surface finish, create premature and unpredictable tool wear, and reduce productivity.

Active orientation enables five-axis contouring: the angle of the tool changes relative to the machine axes in real time. This enables a constant angle and surface speed, leading to higher programmed feed rates and better surface finishes.

Orientation control also boosts performance of flat-bottom and bull-nose end mills that need a constant 5- or 10-degree angle between the cutter axis and the plane normal to both the surface tangent plane and the direction-of-motion plane. The angle increases the effective radius, reducing the scallop height, improving surface finish and reducing the number of tool passes required to complete the operation. Controlling tool orientation also enables a constant depth-of-cut and accounts for variations in the stock-on condition of parts.

Source: Machining 101: What is Five-Axis Machining?

Five-axis functionality comes with a learning curve — shops adopting five-axis for the first time cannot expect plug-and-play success. Machining strategies will require reconfiguring tailored to the specific part family under production. The machine, the CNC and the CAM software are likely to be more expensive than comparable three-axis versions. Additional axes of motion introduce additional sources of error and potentially compromise rigidity. In short, five-axis is a high-yield process, but it is also higher-risk.

Initially operating a five-axis machine like a three-axis machine may seem like a waste of capability, but it can help users become more familiar with the machine while they learn.

Compound rotary tables are the most straightforward and economic way of getting five-axis performance, but they come with several significant drawbacks. These tables can be added to three-axis machine tools, but are less stiff and possess higher angular errors than more integrated approaches. Stacking the rotary axes on top of each other further magnifies angular errors, and the large stack-up between workpiece and rotary axes with a double-tilt rotary means high-torque loads affect the gear sets, reducing life and increasing deflection.

Two-axis spindle heads, also called spindle heads, typically see use on gantry-type machining centers. They suffer the same accuracy and stiffness weaknesses as double-tilt rotary tables, and can have limited rotational range (for vertical machining, the C-axis will typically have an unlimited range of motion, while the A-axis might be limited to ±90 degrees of motion from the vertical position).

Single-axis spindle heads, often called “pendulum-type” heads, generally provide better accuracy than double-tilt tables or multi-axis spindle heads because the rotary axes are not stacked.

The tilting spindle, when combined with a single-axis rotary table, is generally the best choice for five-axis machining of small- to mid-size concave and convex components. Prismatic and long, slender parts are particularly well-suited for this configuration. The B-axis enables the spindle to rotate around the tool tip, minimizing angular errors and keeping torsional loads on the B-axis to a minimum.

Further reading: Machining 101: What is Five-Axis Machining?