SPC
Statistical Process Control, or SPC, is a valuable tool when used correctly. SPC in some companies means Sort Parts Continually, until you make the data read what you what. It has had its abuses but when used properly it can be a great asset. In this article we will look at a process and show the basics of SPC and CMM data. This is only a glimpse and will be very basic in its scope but you will get the idea.
Process
Whether you realize it or not each part coming off a machine center has a process. The things that go into making that part are the building blocks of that process. If one of these things breaks down or needs attention then the whole process is affected and it will directly affect the quality of that part or a series of parts coming off that same machine.
Chiron America uses a Zeiss CMM with Calypso software. Each inspection result is written directly to the company’s internal server both as a PDF, for permanent record storage but also a .chr and .tab file which can be pulled directly into Excel. This allows Chiron to be totally paperless. A SPC program has been developed in Excel to give each machinist the ability to analyze the data for their machine based on the CMM results.
One aspect of the SPC program is the ability to graph run charts which give a telling picture of the actual machine process. This is what we will examine in this article.
Terms To Be Familiar With
CP = How much of the specification limits are being used.
CPK = How close your process is to the target value.
You might a good CP but have a low CPK meaning the process meets the specified limits but is far from the targeted value.
True Position Charts
Let’s look at some charts and see what data is revealed in them. In figure 1 we see a true position run chart. At first glance we see the data is at the top of the chart above the UTOL, or upper tolerance limit. Look at the bottom left corner of the chart and you will see the CPK is a -3.26. These parts are out of tolerance but this chart reveals something else. Look at the range between the parts, it is 0.045 microns.
This is acceptable and within the tolerance of this process so while these individual parts will be scrapped the process can be targeted. However this one chart can’t reveal what exactly needs to be done other than the obvious that one of the axis that make up the true position needs to be better targeted.
This is the Z axis and it shows the axis is out of tolerance but capable. Look at the CP this shows we have a good process but we need to target Z better toward the mean and it will bring in the true position. These charts were created with a machinist in mind so the KOR, or correction value, is shown on the far lower right of the graph. If the machinist moves Z in the negative direction 0.311 he will be at the mean value for this feature. This will correct the CPK value also.
Out of Process
Figure 4 shows a process that is out of control. Obviously we would want to re-clean and recheck parts 21, 22 and 24 to make sure we have the proper data.
If we get the same results then this process needs serious attention. With a CP of 0.87 we can not simply make a targeting move but look at the last four parts. They seem to have an acceptable range. This creates some questions. Did the process settle down? Was the tooling inserts replaced prior to machine and now have settled in? Is the part moving in the fixture? Is there sufficient clamping pressure in the fixture? Do we need to run a larger sample to get the proper overall view of this process?
With so many variables involved in the machining process, such as fixtures, tooling, coolant consistency, hydraulic pressures, etc.. it will take time to get the answers to these questions. As a Quality Inspector you may not be involved in the individual tasks of finding the solution but you need to be equipped with the tools to give a reasonable answer as to what data is coming off your CMM.
Conclusion
Starting out with the right SPC program is key in giving you an overall view of your process. There are plenty of programs out there and you need to look at reporting functions of all of them to rightly discern which one will work for your situation. This is just one tool you can use to improve the quality of the parts coming across your CMM.
Figure 2 shows the X axis that make up the above true position. The data is distributed along the median of the tolerance. In this chart that is the MW line. Both the CP and CPK look very good for this individual axis. This feature is fine and we can move to the next axis.
