Get With the Program: Right from wrong

The world of a CAM programmer may seem straightforward and orderly. Programmers build toolpath strategies in logical sequences, select cutters from a library and calculate and simulate results.

But there are complex challenges when moving from the sterile programming office to the shop floor. The theoretical process becomes real.

About 10 years ago, the goals of CAM technology were quite basic. Everything fell into two important categories: Improve the experience of either the programmer or those on the shop floor. Software that provided faster calculation times, an easier and more-organized user interface and new strategies to simplify programming aided the programmer. Shop floor personnel, however, sought software that generated toolpaths able to eliminate or minimize collisions and that enhanced toolpath efficiency to extend tool life.

A compressor impeller shows deviation after first-article inspection. Image courtesy of Open Mind Technologies USA.

Reducing engineering costs or making parts faster and with higher quality is certainly a positive development. But a shop’s overarching goal is to make a part that is accepted by the customer. That requires controlling the many variables that enter the scene when the programming world meets the shop floor.

Machine tools may be aging, for instance, or may have experienced a spindle crash that left them out of factory alignment. The shop environment, such as room temperature, might not be controlled. Also, practical tolerance issues exist. Tools may have been ground or re-ground incorrectly. Some toolholders have runout. Machine spindles might grow when running under load. Tool lengths can vary.

Then consider the realities of machining: A thin-walled surface or a long cutter may deflect, or a worn cutter may not behave predictably.

Modeling these variables can be useful to producing an accurate part. In a sense, it is akin to making a part wrong to ultimately make it right. People attempt to model these situations, whether through examining spindle warm-up each morning, probing to check tool length or updating the controller offset tables. But the reality is that making a part to meet all tolerances and objectives is not easy.

To achieve total part quality, additional modeling is needed. This modeling is available with advanced CAM software. Although specific software compensations are not provided for each of the previously mentioned process issues, the best solution is to target the totality of process issues so the workpiece meets the required quality standards.

The following examples begin to create a paradigm for how to make a better part. In one case, a deformed model was programmed and machined. In the other, the toolpath was programmed intentionally with sophisticated offsets and blending algorithms to improve part quality.

Production example: Turbomachinery compressor impellers are complex parts that must be machined to a tight positional tolerance. These components operate as high-speed rotational devices. In addition, blade failure can cause instant damage and even loss of life. Therefore, the manufacturing process must be consistent for all blades.

Typically, a centrifugal compressor has a long leading-edge surface and, compared with the hub, thinner sections near the shroud (outer contour), which leads to machining challenges.