Gaging for SPC: Now Even Simpler
Data collection for statistical process control can be automated and integrated into your existing inspection process.
Author’s note: This is an updated version of a column that originally ran in December 1992. I was surprised 1) at how little updating was needed and 2) that readers who keep asking me about this apparently didn’t pay attention the first time!
After all that has been written about statistical process control (SPC) over the years, I am continually surprised at how often shop owners tell me they would like to do data collection for SPC, but they are “just too small” a shop or they “can’t afford all that fancy equipment.” There seems to be a mistaken perception out there that 1) SPC is a lot more complicated than it is and 2) you need expensive hardware and a sophisticated computerized system just to get into it.
This misperception is unfortunate, because small shops can often benefit the most from SPC, and there really is no reason they should not. The simple fact is that you don’t need a big investment in hardware, and you don’t need to be a statistical genius to do data collection for SPC. Really, all you need to start is an indicating gage, a pad and a pencil. And today, even the pad and pencil can be eliminated, and data collection can be automated and easily integrated into your existing inspection process. So if you’re one who is still hesitant about collecting data, try this easy-to-follow recipe for small-shop SPC.
Before you start, you do have to understand a little bit about the process itself, but this is not difficult. The basic principles of SPC were developed back in the 1930s—long before computers were invented—and have not really changed since. An X/R chart then was the same as an X/R chart now. You do not need to understand all the theory behind the process—just the basics of frequency distribution and charting will do to start—and there are many guides to help do this.
Next, get the process in place. This involves three steps. First, look at the part you’re going to measure. What are its important dimensions, and what part of the process controls them? This is what you want to measure and where you want to start the process. But keep it simple; start with a single measurement until you get the feel for it.
Second, look at the gaging equipment you are going to use. Make sure you follow all the gaging basics we’ve talked about in this column and you have the right gage for that type of measurement. Do you need the flexibility of a digital caliper, or do you have tighter tolerances that require variable gaging and digital indicators? You should also run a series of gage repeat and reliability (GR&R) studies to make sure your measurements are as accurate as possible. Remember, whatever “analysis” you do can only be as accurate as the measurements you start with.
Now you are ready to think about the data collection part. What you are basically looking to do is take a few simple measurements, average them and then record the results. Although this seems simple, it is often the most daunting part of the process for the operator. Manual recording takes much effort on the part of the machinist or inspector and is fraught with the potential for errors such as writing down the wrong number. A simpler, faster and more accurate approach is to transmit measurement data electronically. Today’s digital calipers and indicators can transfer their measurement results wirelessly to PCs already located at the workstation. Thus, there is no high cost barrier to begin the process of collecting data. With today’s wireless gaging, you can get going for the price of the gage.
The final step in setting up the data collection is to benchmark your machining process. You simply need to find out if your machine is capable of holding the tolerances or control limits you require. Your SPC guide can help you do this.
Now you are ready to actually gather data. Simply start feeding it into the existing computer that the operator is already using. Most everyone today is familiar with Excel worksheets, and, using wireless gaging, it’s easy to drop measurement data right into the spreadsheet. With this simple automation, operators are not asked to do any more work. They still measure the parts—only now they can watch their work being collected in the spreadsheet.
With a little creativity and the power of Excel, much can be done with the collected data. This is often an eye-opener to the machinist. He sees that there is a relationship between what he is doing and what the spreadsheets show. He also sees that his process does have variation, something that is not apparent to the naked eye. And with a few simple charts, he can learn to predict how the process changes and then be able to control it. He sees his wheel is wearing, for example, and knows when and when not to compensate. Charting empowers the operator, and that, ultimately, is what SPC is all about.
That’s it—simplified SPC for the small shop. Start small, take it one step at a time, and keep your operators involved. Remember, SPC is not a thing you buy, it’s a thing you do.
A gage that meets performance specs will ensure better measurement results.
A centers-based method can be useful for checking for out-of-roundness without the need for a dedicated roundness machine.
Long-wavelength waviness and short-wavelength roughness require a wide variety of instruments and parameters for proper measurement — and lower numbers aren’t always better.