In many companies, CNC machinists do not work directly from workpiece drawings because those types of drawings show only the finished workpiece. Instead, a manufacturing engineer will create other drawings from which the shop personnel can work. One engineering drawing is made for each step in the manufacturing process. This is commonly called a process drawing. It shows only those operations that must be accomplished in a given step of the process.
Process drawings are often used by companies that manufacture workpieces with complicated (or sometimes confusing) processes. For example, a CNC turning center’s operation is machining a surface that must eventually be ground. The CNC turning center operation that precedes the grinding operation must leave grind stock on the workpiece. How much grind stock must be left? By creating a process drawing for the CNC turning center operation, a manufacturing engineer (or a process engineer) can specify the exact size (including the tolerance) for each surface machined by the turning center.
Again, the primary reason for process drawings is to allow the manufacturing engineer to be explicit about each step of the process used to produce the workpiece. They can treat each step independently, making it clear to the machinist responsible for actually producing the workpiece. Process drawings show how/what the workpiece should look when the current step is completed.
Because your engineering team is recreating the drawing for each step in the process, they have complete control of tolerancing methods. However, I’ve seen many process drawings that use the same complicated and confusing tolerancing methods that are used on the workpiece drawing. This causes everyone in manufacturing to perform calculations to determine the needed tolerance values. In addition to taking time and causing mistakes, this results in scrap. Those in manufacturing need three values for each tolerance: the high limit, the target value and the low limit. The high and low limits are self-explanatory. The target value is, of course, what you want the CNC machinist to aim for when he/she makes sizing adjustments. The target value is commonly, but not always, the mean value of the tolerance.
To make things as simple as possible for those in manufacturing, specify these three values for every dimension. On top, write the high limit. In the middle, write the target value. On the bottom, write the low limit.
By definition, the process engineer who develops process drawings must be familiar with the machine on which a workpiece is produced. Yet most don’t specify something that is of critical importance to the person actually setting up and running the machine.
Let’s say a given workpiece attribute, maybe a turned diameter being machined on a turning center, requires an adjustment. An adjustment must be made to the machine so that the turning tool machines this diameter correctly. While all CNC machinists know that offsets are used to make such adjustments, setup personnel and operators may have difficulty determining which offset must be adjusted.
Admittedly, offsets are commonly specified with the same number as the tool station number, meaning if the CNC person knows which tool is machining the surface, then he or she also knows which offset to adjust. However, there are times when two or more offsets are required for a given tool. Also, while it is relatively easy for the setup person (who loaded all the cutting tools) to know which tool machines a given surface, it is not nearly as easy for a CNC operator. Time is often wasted while the operator scans through the program or studies the documentation so he or she can make a needed adjustment.
You can easily tie each surface being machined to the specific offset used to adjust it right on the process drawing. One method is to color code each surface. Each colored surface is related to a different offset—brown for offset one, red for offset two, blue for offset three, and so on.
Some process engineers may feel that this kind of documentation is more the responsibility of the CNC programmers. The CNC setup staff and operators will be working closely with the process drawing. Because the process drawing is done by people in your company, why not include the documentation right on the process drawing?