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66 MMS April 2017 mmsonline.com CNC TECH TALK Columnist Companies that run the same jobs over and over often go to great lengths to engineer exactly what must happen on the manufacturing floor. CNC setup people and operators in this kind of company may not work from design engineering drawings. Instead, a manufacturing engineer provides a process drawing and related documen- tation that limits content to a given operation, machine or manufacturing cell. The engineer will do this for each operation/cell involved in produc- ing a workpiece to completion. If an operation involves lathe machining on a turned workpiece, for example, only turned work- piece attributes will be dimensioned and docu- mented. Workpiece attributes machined by other machines/cells will not be detailed. A huge amount of clarifying information can be included on a process drawing. For example, a process engineer can precisely identify machined surfaces and the cutting tools that machine them. He can provide quality control information used for gaging and can relate specific (sizing) adjust- ments to the CNC methods used to adjust them. This kind of information is never included on a design engineering drawing. In my experience, process drawings tend to have a biased emphasis. If a quality control person develops them, the emphasis may be on gaging. If a CNC programmer develops them, the empha- sis may be on making sizing adjustments. If a tooling engineer develops them, the emphasis may be on cutting tools and workholding devices. Shopfloor people who actually use the process drawing rarely have a say. When developing the template for your process documentation, be sure to involve everyone who plays a part in your man- ufacturing environment. Only then can you ensure that you don't forget something important. Here are my suggestions: 1. Identify machined surfaces. This informa- tion is especially helpful for entry-level workers when machining operations have been previously performed on the workpiece. One may not be able to identify what the machine has done by simply looking at a completed workpiece. And, as I'll show, the method for identifying machined surfaces often leads to further clarifying documentation. One effective method is to use a color-coding system. Mark surfaces that have been machined by different cutting tools using different colors. In a color-code legend table, correlate each surface with information about the cutting tool used to machine it, like station number and offset number, as well as each of the details suggested below. 2. Provide target values for adjustments. Be consistent with dimension/tolerance specifica- tions on the process drawings/documentation, and make them as simple to interpret as possible. I recommend providing three values for each: the low limit, the target value needed when a sizing adjustment is required (often the mean value of the tolerance band) and the high limit. This will keep workers from having to calculate tolerance values, which in turn will save time and minimize the poten- tial for mistakes when measuring workpieces. For a dimension and tolerance of 4.255 inches ±0.002 inch, for example, and assuming you want your people to target the mean value, specify the dimen- sion for the workpiece attribute as 4.253/4.255/4.257. 3. Specif y the workpiece at tributes to measure. Often, multiple workpiece attributes are machined by a single cutting tool, yet only one Process Documentation Drawings that include these seven details can help clarify specific operations. MIKE LYNCH FOUNDER AND PRESIDENT CNC CONCEPTS INC.