Machining Centers

Most machining centers on the market feature numerical control (CNC) and serve more than one purpose. Many can perform combinations of operations such as milling, drilling, boring, tapping and reaming in a single setup. Machining centers come in three general types: horizontal three-axis, vertical three-axis and five-axis machines (four- and six-axis machines exist, but are less common).

For a vertical machining center, the X-axis controls left-and-right movement, parallel to the workholding surface; the Y-axis controls front-and-back movement, perpendicular to the X- and Z-axes; and the Z-axis controls up-and-down movement. Most machines use a fixed spindle and a moving table, or a fixed table and a moving spindle. Spindle rotation is never considered an axis.

Five-axis (and four-axis and six-axis) machines introduce additional axes that enable the table or spindle head to rotate and pivot. The A-axis involves X-axis rotation, while the B-axis is paired to the Y-axis and the C-axis is paired to the Z-axis.

Source: Machining 101: What are Machining Centers?

Machining centers come in three general types: horizontal three-axis, vertical three-axis and five-axis machines (four- and six-axis machines exist, but are less common).

Horizontal and vertical three-axis machines differ primarily in the inclination of the spindle, with the spindles of horizontal machines parallel to the surface of the machine table and the spindles of vertical machines perpendicular to the surface, although individual constructions vary widely to support different applications.

Source: Machining 101: What are Machining Centers?

Accuracy and repeatability are both vital, but these specifications can be especially difficult to determine because different manufacturers use different definitions. In general, there are three standards for accuracy: unidirectional forward, unidirectional reverse and bidirectional (which is the average of the two). Repeatability — which is the distance between accuracy samples, tested over the full range of data points — generally has four standards: forward repeatability, reverse repeatability, bidirectional repeatability and scatter.

“Lost motion,” also called “mean reversal error,” is the difference from center found when comparing marks made with forward and backward repeatability. Data collection typically repeats processes seven times, then creates a bell curve of results, calculating both the standard deviations and the mean. Different measurement standards use the standard deviations in different ways.

Source: Machining 101: What are Machining Centers?

Standard adapters and tooling are normally satisfactory at spindle speeds up to 8,000 rpm. At faster speeds, specially balanced tooling can be critical for high tolerances and surface finishes.

Unbalance is a tool’s mass times its eccentricity (which is the distance from the tool’s center of rotation to its true center of mass). Eccentricity is measured in microns and tool mass in kilograms, so unbalance is measured in gram-millimeters. ISO 16084 is the standard for setting targets for tool and toolholder balance.

To evaluate unbalance in processes, users can perform trial runs one at a time with tools balanced to a variety of different values. Such an evaluation might start at an unbalance of 10 g-mm, then progress through a series of increasingly balanced tools until it achieves proper tolerances or accuracy and surface finish fail to improve any further.

Source: Machining 101: What are Machining Centers?

Eccentricity is the distance from the tool’s center of rotation to its true center of mass. Eccentricity is measured in microns and tool mass in kilograms, so unbalance is measured in gram-millimeters.

Source: Machining 101: What are Machining Centers?

Machine tools use linear and rotary encoders to measure their own movements and stay on target. There are three types of encoder contacts — photoelectric (also called optical), magnetic and mechanical — but photoelectric encoder contacts are the most common.

Rotary encoders measure rotational movement drives, but spindles and recirculating ballscrews can also enable them to measure linear movements. Rotary encoders can be incremental or absolute.

Incremental rotary encoders have output signals that are evaluated by electronic counters that measure “increments.” For general length measurement applications — particularly the measure of slide movements using a recirculating ballscrew as the scale — shaft encoders that incorporate digitizing electronics are standard.

Absolute rotary encoders derive angular positional value from the pattern of a coded disc that provides values immediately after power switches on. The Gray coder and coders which use natural binary are most common, with many modern computer programs using the binary system to support high speeds.

Source: Machining 101: What are Machining Centers?

1. Know The Spindle

2. Measure The Process Instead Of The Part

3. Raise The Bar On Drawbar Attention

4. Control Chatter

5. Inspect With A Reference

Source: How to Improve Machining Center Accuracy