Most of us in the United States are VERY satisfied to work in the inch system for everything we do. If we happen across a part dimensioned in metric, we simply convert all the dimensions to inch and run the part in inch. While we have all heard that the metric system is slowly replacing the inch system, we have been very slow to change. Very few companies in the U.S. work exclusively with the metric system.
The most basic reason we have stayed away from the metric mode has to do with motivation. We have seen no real benefit in changing to metric. Also, our company’s measuring devices are made to measure in the inch system. This means a company would incur an expense when switching. This short discussion will show you one benefit that may convince you to use the metric mode of your CNC machine for your next close tolerance job.
Almost all current CNC machines allow you to input coordinates in either inch or metric mode, depending on which mode you elect to use. Usually, a G code allows you to set the desired mode by programmed command. But even on older CNC machines, the operator usually has a way of activating inch or metric by some form of toggle switch.
The benefit of working in the metric mode has to do with the least input increment allowed in the metric mode of your CNC control. In the inch mode, the least input increment allowed by almost all controls is 0.0001 inch (one ten-thousandth of an inch). This means you can input program coordinates and tool offsets down to 0.0001 inch.
But in the metric mode, the least input increment is 0.001 millimeter (one thousandth of a millimeter). When converted to the inch mode, 0.001 mm is equal to 0.00003937 inches, which is less than half the least input increment when using the inch mode.
For programming, this means that you can target the coordinates going into the program to be much closer to what is desired for accurate parts. You can come up with die “mean dimension” of the tolerance more precisely.
For operation, this means you can adjust offsets more precisely to hold the desired size of the part. In the inch mode, many times the operator must have the offset adjusted perfectly in order for the part to come out “on size”. But in the metric mode, the operator can “split the tolerance” in a way that allows the offset to be specified more accurately. Over two increments of the offset in the metric mode are needed to equal one increment in the inch mode.
As an example, say you have an overall tolerance on a turned part diameter of 0.0001 inch. In the inch mode, the offset controlling this diameter would have to be set perfectly, since there would be no “margin for error” if using the inch mode. But if you converted the program to metric, you would be able to “split the tolerance” into two parts.
Note that we are NOT saying that the machine is more accurate in the metric mode. The machine will make its positioning movements within the specifications of the machine tool builder’s accuracy quotation regardless of which mode is used. We ARE saying that you can TARGET the end point of each movement to a finer increment” in the metric mode. We liken this to the resolution of a computer’s display screen. If the resolution is poor, objects on the screen may appear “fuzzy” or “jagged”. If the resolution is fine, objects can be described more accurately. Using the inch mode is like using a computer display with poor resolution. Your desired tolerances cannot be accurately described. The metric mode allows much finer “resolution”.
When working in the metric mode, remember that the whole programming environment must switch. That is, coordinates going into the program, the program zero measurement, tool offsets, and even feeds and speeds must be in metric. If you are used to working in the inch system, it may be somewhat difficult for you to adapt to the metric system. But if you have extremely close tolerance work, it would be well worth the time it takes you to learn the metric system thoroughly.
http://findarticles.com/p/articles/mi_m3101/is_n8_v63/ai_9339190
