Losing Accuracy One Micron at a Time

Most gages on the shop floor provide a specified level of accuracy in conditions for which they were designed. However, it’s critical to think about gaging requirements before putting instruments in tight-tolerance production environments, and possibly having them not meet expectations.

Columns From: 5/14/2012 Modern Machine Shop,

Editor's Commentary

From the monthly column: Quality Gaging Tips
In short, if you’re trying to measure to microns, every micron counts. Thus, it is very important to ensure that proper thought is given to the gaging process. Ask yourself, “How do I account for losing a micron here or there, and how do I prevent that from happening?”
 
When working to improve gaging performance, you’ll see how quickly you can pick up some microns. However, you will also find that as you increase performance, the microns get harder to find.
 
The following is an example of a manufacturer that recently came to us with a requirement to inspect a wide variety of hole sizes on a line of valve bodies.
 
Some relevant parameters for this gaging situation included:
• Throughput. With hundreds of thousands of parts to measure, inspection had to be fast and foolproof.
 
• Output. The manufacturer required automatic collection of data for SPC.
 
• Ease of Use. The gage had to be moved to the large parts, not vice versa.
 
• Accuracy. Most hole tolerances were ±25 µm, but some were as tight as ±12 µm.
 
Hand instruments, such as calipers and micrometers, are important tools in the shop. Even though they are versatile and quick, they didn’t have the performance required for this particular application. They used too many microns of error.
 
Adjustable bore gaging wouldn’t do the job either. While the adjustable bore gage had the flexibility to cover a large range of hole sizes, it was rejected because sweeping through the part was time consuming, and the operator had to have a high skill set.
 
The manufacturer specified a gage repeatability and reproducibility (GR&R) requirement of 20 percent or better on holes with tolerances of ±25 µm. This meant that the gaging system had to perform to 4 µm or better. This requirement was met using standard gage plugs and standard digital indicators with 1-µm resolution. The GR&R achieved with this setup was less than 16 percent.
 
However, on holes with tolerances measuring ±15 µm and ±10 µm, the manufacturer required a GR&R of 10 percent, which translated to gaging system performance of 1 µm. Given the other parameters of the application, mechanical plug gages remained the only practical approach, so we had to find a way to stop the flow of lost microns and “squeeze” more accuracy out of the situation.
 
Plug gages are typically engineered for 50 µm of material clearance in the holes they are designed to measure. This accommodates undersized holes and eases insertion. However, the greater the clearance, the greater the amount of centralizing error (when the gage measures a chord of the circle instead of its true diameter). Reducing the designed clearance minimizes centralizing error and saves a few parts of a micron, albeit with some trade-off against ease of insertion.
 
We engineered a special set of plug gages with minimum material clearance of 15 µm. The standard digital indicators were also replaced with high-resolution units, capable of 0.5-µm resolution. This combination satisfied the
requirements, generating a GR&R of less than 8.5 percent.
 
Remember SWIPE? This acronym stands for the five categories of gaging variables:
1. Standard (the master)
2. Workpiece
3. Instrument (the gage)
4. Personnel
5. Environment
 
In the case of the valve-body manufacturer, we tweaked the instrument, thus reducing one source of gaging variability. We reduced a second source by providing higher-quality masters for these gages. If throughput had not been such a high priority, we also might have considered altering the inspection environment or providing more personnel training. If portability hadn’t been an issue, then the solution might have been a different instrument altogether.
 

The five categories of gaging variables encompass dozens of specific factors. (For example, within the workpiece category, there are variables of surface finish and part geometry that might influence dimensional readings.) To squeeze more accuracy out of a gaging situation, look for opportunities to reduce or eliminate one of more of these factors. In the end, those lost microns will be used to improve gaging accuracy. 

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