Owing to the variations that occur in every process, the goal of Statistical Process Control (SPC) is to make rational adjustments to correct for shifts and changes in the process. SPC is useful in determining when and what adjustment should be made—but it’s not the best solution as we will show. Consider a milling operation involving a tool holder, and tool tip, as we cut a groove into a part. Look at the BEFORE in the milling example below: As the tool is spinning, a groove is cut in the part. Tool wear (exaggerated in image on left) occurs with each cut. Before the tool is exchanged because it is completely worn out, the operator will have to make a series of downward adjustments to the tool holder and the tool. To do this, SPC uses X-bar and R charts (among other SPC tools) to predict when such adjustment will be needed—and how large each adjustment should be.
A far better approach is to eliminate adjustments entirely! That’s what we do in the third segment of our SMS Method: We turn adjustments into settings so we never have to figure how much to adjust again—or when. This is exactly what happened when the operator decided to insert a spring between the tool and the tool holder in our milling example (see AFTER below).
- After a predetermined number of cuts and long before tool wear results in an out-of-tolerance part, a counter alerts the operator to remove the tool and tool holder and place them in a fixture that holds them exactly as they are held on the machine.
- The tool grip is then released, and the spring pushes the tool down to the pre-determined fixed stop—and restores the tool holder and tool to their original positional relationship.
- When the tool holder and tool are placed back on the milling machine, the tool tip is in the same precise position as before the wear occurred. This is done without making a adjustment. We’re ready to cut the groove in the next part at precisely the same depth as the first part. In SMS, we turn adjustments into settings so we never have to adjust again. You can do this across every production process
Here’s another example of how you can convert adjustments into setting—this time in machine change over. Traditionally, at every setup change the replacement fixture is located in a different position. Skilled machinists then make adjustments to assure the fixture is oriented correctly with the motions of the bed, and centered with proper alignment to the tool head. But the need for such adjustments can be entirely eliminated through SMS.
Look at the example on the right. Three holes are machined into the back of the milling machine bed, and stout precision pins are inserted into these holes. When the setup is changed, the fixture on the milling machine is removed, and the new fixture installed has a V notch that centers on the center pin. When the installed fixture touches the back two pins, it is located within 0.0001” of the position when it was previously used to make the same part. It is already squared up with the motion of the milling machine, and no vertical adjustments are required because the fixture is in the exact same position each time it is placed on the machine.
By converting adjustments to settings, variation is controlled at stream—and the need for any data collection or downstream analysis required by Six Sigma and process control is entirely eliminated. But more importantly, all adjustment errors are eliminated, and setup times and errors can be reduced from 97 to 99 percent. This results in improved quality and huge savings.