Imperial vs. Metric: Navigating Measurement Systems in CNC Machining

Most CNC machinists have already chosen a measurement system for their CNC machine. When they find a work piece dimension with a different system, they simply convert the print to their system of choice and run the job in that measurement system. However, there are some critical considerations when switching measurement systems.

Imperial vs. Metric Basics

In the Imperial system, the most basic unit is the inch. In the Metric system, it is the millimeter. These units apply to axis and related specifications such as X, Y, Z, R, I, J, and K.

Feed rate specification in the Imperial system is measured in inches per minute/revolution, while in the Metric system, it is specified in millimeters per minute/revolution. Spindle speed in constant surface speed mode (for turning centers) is measured in surface feet per minute in the Imperial system and in meters per minute in the Metric system. Likewise, thread pitch is specified in threads per inch in the Imperial system and in millimeters in the Metric system.

Resolution and Precision

The least input increment (smallest programmable value) is usually 0.0001 inch in the Imperial system and 0.001 mm in the Metric system, though high-precision machines can measure to 0.00001 inch and 0.0001 mm.

Consider the dramatic difference in resolution: 0.001 mm is less than half of 0.0001 inch. Specifically, 0.001 mm is 0.000039 inch, or 39 millionths of an inch. This gives the Metric measurement system a much better resolution than the Imperial system. It often allows the mean value of tolerance bands to be more precisely specified in the program and provides more offset settings within a given tolerance band.

Offset Adjustment Example

For example, a 0.001-inch overall tolerance has about ten offset positions in the Imperial mode that will place the machined attribute within its tolerance band. The same tolerance in the Metric mode will have about 25 offset positions. This finer resolution for offset adjustments can be crucial when holding very small tolerances. Some companies perform close-tolerance work on workp ieces dimension in the Imperial system but convert the dimensions and tolerances to Metric to gain this offset setting resolution advantage.

Multi Axis Motions

The resolution advantage of the Metric measuring system also extends to multi-axis motions. Within any interpolated motion, the machine divides the motion into a series of single-axis steps. The step size for many CNC machines is related to the current measurement system. Machines using the Metric system follow a more precise tool path.

Practical Testing

Here’s a quick test to see how your machines behave. On a vertical machining center, give this command in the Imperial mode:

G91 G20 G01 X1.0 Y0.001 F5.0

Monitor the position display when this command is run. The machine will likely move in X (only) for about 0.1 inch—then the Y axis will step by 0.0001 inch. This will be repeated nine times. Now give the equivalent command in the Metric mode:

G91 G21 G01 X25.4 Y0.025 F125.0

The machine’s steps will be much closer together—(0.1 mm, or about 0.004 inch apart). The tool path remains closer to its programmed path.

Switching Measurement Systems

When switching measurement systems, CNC machines are initialized to start up in the user’s chosen measurement system mode, controlled by a parameter. Two G codes commonly specify the measurement system mode (G20 for the Imperial system and G21 for the Metric system).

Most CNC machinists agree that incorporating both measurement systems modes within the same program isn’t feasible, although it is done by some. Many companies switch measurement system modes regularly based on how a given workpiece drawing is dimensioned.

When changing measurement systems, it’s essential to understand what happens to all values within the CNC control, such as axis displays, offsets, and parameters. With older machines, the decimal point may move one place to the right or left, making the switch complex as every offset value must be changed. Newer machines ensure true conversions, showing 10.0000 inches as 254.00 mm when switching from Imperial to Metric, eliminating the need to change offsets or other values after the measurement system mode is changed.

Understanding the differences and implications of the Imperial and Metric measurement systems is crucial for CNC machinists. Each system has its advantages, and choosing the right one can enhance precision, efficiency, and overall success in machining operations. Whether you stick to one system or switch between both, being informed will help you make the best decision for your specific needs.