Quality in Manufacturing

On-machine measurement

Software makes on-machine probing a viable option

By Stephen Logee Director, Business Development  Wilcox Associates Inc.

Manufacturers are reluctant to use measurement probes even though they have been available for CNC machines for years. The biggest impediment to using them is the sometimes difficult and always time-consuming task of writing probing macros. In addition, the mistaken ideas that "doing anything but cutting on a machine tool wastes precious time" and "you should never measure a part using the same machine that made it," also pose obstacles.

With new software introduced by at least two manufacturers in the United States this year, the primary barriers to on-machine measurement are crumbling. For example, one of these programs, our PC-DMIS NC, provides interactive graphic tools for automating the task of creating probe macros on a workstation away from the machine tool. By pointing and clicking on a CAD model, programmers create probing routines in a fraction of the time. Then operators can launch these routines as they are needed or cutting programs can call them automatically. Now that it is easy to generate on-machine measurement routines, consider what they are good for.

By measuring a few key features at setup, machine operators can detect a range of errors before they become costly mistakes. Probing makes it easy to diagnose problems related to improper tool and work offsets, erroneous feeds and speeds, buggy NC programs, and the like. Unlike conventional inspection, it is not necessary or even desirable to measure all features of the part. Most critical errors can be found by checking a select group of features. The trick is to identify the measurements most likely to highlight problems.

Tool wear is another major contributor to scrap and rework. By periodically measuring select features while machining, users can monitor the degree of tool wear and make informed decisions as to whether they should replace a tool, adjust their cutting program, or leave things alone. As a result, manufacturers can now use actual process data, rather than some arbitrary schedule to decide when to replace their tools.

Today, operators spend an inordinate amount of time aligning fixtures and parts during setup. For short runs, for example, more time can be spent on setup than on actually removing metal. By using automated, on-machine measurement, the operator significantly reduces the time required to set up fixtures and parts. In addition, he or she can quickly calculate and enter tool and work offsets (also a big time saver).

Manual, on-machine first piece inspection is another major setup bottleneck. First the machine is idle waiting for an inspector to arrive. Then additional time is lost during the inspection process and while results are being calculated and entered. With automated on-machine inspection the machinist only needs to load and launch previously developed probing routines. If adjustments are needed, some in-process metrology software, like PC-DMIS NC, can, in many cases, make them automatically. If not, the operator can easily interpret the results and make appropriate corrections quickly. Once production is under way, automated inspection, integrated with part cutting, alerts operators of actual and potential problems before they result in scrap or rework.

When parts, particularly large ones, have to be removed from the CNC machine for inspection, productivity can really suffer. The machine may stand idle for hours (sometimes days) while the part is removed, transported to the inspection machine, set up, measured, evaluated, removed, transported back, set up again, and adjusted. Using in-process metrology software to automatically measure the part on the machine puts an end to this convoluted process and dramatically reduces the potential for errors.

In addition to giving immediate feedback, the in-process metrology package can direct its output to external databases and software systems to perform such functions as: SPC, machine and process capabilities studies, generation of graphic reports, etc.

Solving difficult problems
Given a powerful tool for automating CNC probing, users are coming up with creative solutions to complex and otherwise insolvable problems. For example, one early adopter is investigating the use of on-machine measurement to ensure that parts actually exist within near net shape castings. Probing data from PC-DMIS NC goes to a third-party best-fit analysis package that does the calculations. This makes it possible to scrap inexpensive, off-spec castings before they undergo an expensive machining process that has no hope of succeeding.

Another user is developing a unique iterative process for manufacturing high-quality lens molds. The machine tool alternately cuts and automatically measures the part until the desired curve is achieved. This integrated operation replaces a manual, off-machine measurement process that had a scrap rate higher than 50 percent.

Users of this new technology can quickly take advantage of these cost-saving functions. Furthermore, by integrating on-machine measurement with powerful database and analytical tools the system becomes even more capable. In the near future, second generation systems will not only provide users with a real time window into manufacturing processes, but will also make automatic process corrections on the fly. As users develop a better understanding of these capabilities, probes that are now gathering dust will deliver higher metal cutting yields and, more important, increase productivity.

Companies in this article

Wilcox Associates Inc.