Imperial Versus Metric

Most CNC machinists have already chosen a measurement system for their machine tools. When they find a workpiece dimensioned 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 things to watch out for when switching measurement systems.

In the imperial system, the most basic unit is the inch. In the metric system, it is the millimeter. This unit will apply to axis and related specifications (such as X, Y, Z, R, I, J and K).

In the imperial system, feed rate specification is measured in inches per minute/revolution, and 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.

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 there are high-precision machines that measure to 0.00001 inch and 0.0001 mm.

Think about the dramatic difference in resolution here: 0.001 mm is less than half of 0.0001 inch. Actually, 0.001 mm is 0.000039 inch, or 39 millionths of an inch. This gives the metric measurement system as it is applied to CNC machine usage a much better resolution than the imperial measurement system. It often allows the mean value of tolerance bands to be more precisely specified in the program, and it provides more offset settings within a given tolerance band.

For example, a 0.001-inch overall tolerance has about ten offset positions in the imperial mode that will put the machined attribute within its tolerance band. This same tolerance will have about 25 offset positions in the metric mode. This provides finer resolution for offset adjustments, which can be especially important when holding very small tolerances. When performing close-tolerance work on workpieces dimensioned in the imperial system, some companies will actually convert the dimensions and tolerances and run the part in metric just to gain this offset setting resolution advantage.

With many CNC machines, the resolution advantage of the metric measuring system carries over to multi-axis motions. Within any interpolated motion, the machine will internally divide the motion into a series of single-axis steps. The step size for many CNC machines is related to the currently instated measurement system. Those machines using the metric system will actually follow a more precise tool path.

Here’s a quick test to find out 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. This will be repeated nine times. Now give the equivalent command in the metric mode:

G91 G21 G01 X25.4.0 Y0.025 F125.0

The machine’s steps will be much closer together—(0.1 mm, or about 0.04 inch apart). The tool path is kept closer to its programmed path.

So, what happens when measurement systems are switched? CNC machines are initialized to start up in the user’s measurement system mode of choice, which is controlled by a parameter. Also, two G codes can be used to specify measurement system mode (commonly G20 for the imperial system and G21 for the metric system).

Most CNC machinists agree that it isn’t really feasible to incorporate both measurement systems modes within the same program (though people are probably doing it). However, there are many companies that will switch measurement system modes regularly based on how a given workpiece drawing is dimensioned.

When the measurement system is changed, it is important to know what happens to all values within the CNC control, such as axis displays, offsets, parameters and others. With some machines, especially the older ones, the decimal point will simply move one place to the right or left. When switching from imperial to metric on these machines, a value of 10.0000 inches will be changed to 100.000 mm. However, 100.0 mm is not equivalent to 10.0 inches. This makes switching measurement systems quite difficult because every offset value must be changed.

Also, remember that certain program-related parameters are affected by the current measurement system mode. For example, using the G83 peck drilling cycle, the clearance distance during each peck is controlled by a parameter. If the machine doesn’t truly convert values during a measurement system change, these parameters must also be changed. This makes switching measurement systems all the more difficult.

Newer machines ensure true conversions. A value of 10.0000 inches will be shown as 254.00 mm when switching from imperial mode to metric mode. This eliminates the need to change offsets or other values after the measurement system mode is changed.

Note that a parameter setting might control what happens in this regard. So if the conversion does not take place when switching modes—especially with newer machines—be sure to search out the parameter that controls this
function.