Which Offset Should You Adjust?

Companies vary when it comes to how they use CNC people. In some companies, such as contract shops, one person is responsible for the entire CNC job, including programming the job, setting it up and running production. This person is familiar with the job and knows which cutting tools machine each workpiece surface.

In other companies, such as many product producing companies, one person programs the job, another sets it up and yet another runs production. The person running production is called the CNC operator. For large lots, several CNC operators may be involved—first shift, second shift and third shift, for example.

A CNC operator will not be nearly as familiar with a job as the programmer, and yet many programmers provide minimal (if any) production-run documentation to help CNC operators figure out what they’re supposed to do during a production run. They depend upon word-of-mouth to answer any questions that CNC operators may have. One especially troublesome area has to do with sizing adjustments that are required during long productions runs.

When a workpiece surface is growing or shrinking because of tool wear, and as it nears a tolerance limit, the CNC operator must make a sizing adjustment. To do so, he or she must know which offset controls the workpiece surface in question. Because an offset is specified by a number, and the offset number is usually made to correspond in some way to the tool station number, the CNC operator must know which cutting tool machined the surface (and its tool station number) to determine which offset must be adjusted.

While this may sound like a simple task to an experienced programmer or a setup person who is intimately familiar with the job, it can be quite challenging for CNC operators. The more tools used in the job, the more challenging it will be. Consider a 20-tool machining center program. If the depth of a pocket must be adjusted, the CNC operator may take several minutes to determine which milling cutter in the automatic tool changer magazine machines the pocket.

Most setup documentation will not help. While the setup sheet will provide a list of cutting tools and station numbers, nothing ties cutting tools to the workpiece surfaces they machine. If the operator makes a mistake and changes the wrong offset, the results can be disastrous.

You can dramatically simplify the task of determining which offset must be used to make a sizing adjustment in the production run documentation. Consider this drawing shown at the left:

Although we are showing a machining center application, the same techniques can also be used for turning center applications. Notice that every machined surface is documented. This documentation names the gaging tool that must be used to measure each workpiece surface, as well as the offset number that must be adjusted when sizing is required.