Case Studies

SPC & CNC Sinker EDM hit a homerun!


By employing CNC Sinker EDM together with SPC in the production of a complex, very tight-tolerance Missile component, dramatic process improvements were achieved resulting in dramatically superior parts and greatly reduced cost.

The Challenge:

RS Precision received an order to manufacture a small, round, aluminum waveguide component for a guided missile. The key to the part was four rectangular pockets with three concentric steps located at 90-degree intervals that had +/-.0005” tolerances on 48 surfaces simultaneously from a small datum hole and slot.

Our initial approach was to produce the three steps one at a time on two in-line pockets, then rotate the part 90 degrees and repeat the process to complete the part. This resulted in parts of acceptable quality, but the cycle time was double what we had originally estimated, resulting in a cost overrun. However, we ran off the first 20% of the parts in this manner to satisfy the customer’s production requirements.

RS Precision retooled the job to make it more productive. However, the customer related that the quality of these parts was only marginally acceptable. Since we were not checking the parts in the same manner as our customer, we went to their facility to learn how they were inspecting them. It turned out that our customer’s method was superior to ours, although much more time consuming and cumbersome. However, there was no arguing with their data, and we decided to start over again.

The Solution:

By adopting their method of inspection, which consisted of using gage blocks and indicators, and entering the data into a spreadsheet that compensated for minute errors in the inspection fixturing, RS Precision was able to replicate their findings. We then enhanced the spreadsheet to provide the operator with direct feedback on setup and process errors in skew, centering and internal variances between the 48 surfaces within each part.

Using this SPC data, we identified and eliminated several major sources of error. After we optimized the setup, we identified that the new electrode holder was not holding the electrodes in the proper relationship to each other. We redesigned it, and made a new one. We also made electrodes with all three steps on them so we could produce all four pockets with all three steps at the same time. The real breakthrough, however, came when we started using the CNC Sinker EDM’s orbiting capability to dramatically improve the geometric accuracy of every surface of every step of every pocket. All of these factors combined to enable RS Precision to produce parts that were dramatically superior in quality to the first 20%, while simultaneously reducing the cycle time by 75%, resulting in superior quality parts for our customer.