Gaging For SPC: Keeping It Simple

After all that has been written about statistical process control (SPC) in the past few years, I am continually surprised at how often shop owners tell me they would like to conduct this process, but they’re “just too small” 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 a complex computerized system and expensive software just to get into it.

This is unfortunate because small shops can often benefit the most from SPC, and there really is no reason why they should not. The simple fact is, you don’t need a complex computer system and you don’t need to be a statistical genius to do SPC. I know this latter point for a fact because I can do it. All you need, really, is a gage with data output (wired or wireless) and a computer running Excel. So if you’re one of those who is still hesitant about collecting data and charting, 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. Bear in mind that the basic principles of SPC were developed back in the ‘30s—long before computers were invented—and have not really changed since. Don’t be intimidated by all the bells and whistles, however. Collecting data and creating a chart now is the same as creating a chart was then. In fact, it’s a bit easier today because you can let Excel do it for you. You don’t need to understand all the theory behind the process, just start with the basics of collecting data and charting. There are many guides to help you do this. What you’re basically looking at is collecting a few simple measurements, averaging them and then charting the results. 

Next, get the actual SPC 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? 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, and follow all the gaging basics we’ve talked about previously in this column. Make sure you’re using the right gage for that type of measurement, and that you follow the standard “10 to 1” rule for resolution (to measure 0.001" tolerance, you need a gage with at least 0.0001" resolution) or other guides that recommend the proper indicating device. 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.

What’s even easier for the operator is that most of the gages today have the ability to send data electronically, either through a cable or wirelessly to a computer. This is not an expensive feature  you have to add but is standard on most calipers, micrometers and digital calipers.

The final step in setting up the SPC process is to benchmark your machining process. You need to find out, simply, if your machine is capable of holding the tolerances or control limits you require. Your machine capability studies should help you do this.

Now you’re ready to gather data. There is nothing sophisticated here and often nothing to buy. For example, some suppliers of gaging equipment provide a free basic data collection software tool when a customer buys a data collection cable (or the same software tool is free if a wireless system is chosen). This software tool acts as the interface between the gage and Excel, so, with every Send Data command used on the measuring tool, the results pop up in the Excel spreadsheet. The great thing about this is that most operators are becoming comfortable using Excel and are familiar with most of its basic features. Collecting data in the spreadsheet and even being able to present a simple chart of the data is becoming no more difficult than sending an email.

Charting electronically like this puts the operator immediately in contact with the process. He is able to see that there is a relationship between what he’s doing and what the charts show. He sees when his process changes, and the charts provide a prediction of how it changes. He is then able to control it. He sees that his wheel is wearing, for example, and knows when and when not to compensate. Charting easily while the process is going on empowers the operator, and that ultimately is what SPC is all about.

The other advantage of collecting data and using the results is that you can use just about any type of indicating gage, so long as it is accurate. Since you probably already have many gages with output capability in your shop, your SPC investment cost is reduced virtually to the price of the cable and the use of the computer, which is probably already on the bench by the machine.

That’s it: simplified SPC for the small shop. Start small, take it one step at a time, and keep your operators involved. And remember, SPC is not a thing you buy, it’s a thing you do.