Losing accuracy, one micron at a time
LOSING ACCURACY, ONE MICRON AT A TIME
Losing accuracy – Most gauges on the shop floor designed to provide a specified level of accuracy. However, in the production environment, where tight tolerances are a way of life. It is critical to think about gauging requirements before putting instruments out there and possibly having them not meet expectations.
In short, if you are trying to measure to microns, every micron counts. Thus, it is very important to ensure that proper thought given to the gauging process. Ask yourself, “Where may I lose a micron here or there that I should be accounting for, and how do I prevent that from happening?”
When starting on the path of improved gaging performance, you will see how quickly you can pick up some microns, but also find that as you increase performance, the microns get harder to find. Case in point, a manufacturer recently came to us with a requirement to inspect a wide variety of hole sizes on a line of valve bodies.
Some of the relevant parameters for this Gauging situation included:
- With literally hundreds of thousands of parts to measure, inspection had to be fast and fool proof.
- The manufacturer required the capability of automatically collecting data for SPC.
- Ease of Use. The parts gauged were large, so the Gauge had to come to the parts, not vice versa.
- Most hole tolerances were ±25 µm, but some were as tight as ±12 µm.
Hand instruments, such as calipers and micrometers, are important tools used in many places in the shop. However, while versatile and quick, they just did not have the performance required for this particular application: they simply used too many microns of error.
The next step up in performance, adjustable bore Gauging, would not do the job either. While the adjustable bore Gauge had the flexibility to cover a large range of hole sizes, it was rejected because of the time consuming operation of sweeping through the part, and a high skill requirement for the operator.
The manufacturer specified a GR&R (Gauge repeatability and reproducibility) requirement of 20% or better on holes with tolerances of ±25 µm. This meant that the Gauging system had to perform to four µm or better. This requirement met using standard Gauge plugs, and standard digital indicators with resolution of one µm: the GR&R achieved with this setup was less than 16%.
On holes with tolerances of ±15 µm and ±10 µm, however, the manufacturer required a GR&R of 10%, which translated to Gauging system performance of one µm. Given the other parameters of the application, mechanical plug Gauges remained the only practical approach, so we had to find a way to stop the flow of lost microns to “squeeze” more accuracy out of the situation.
Plug gauges typically engineered for 50 µm of material clearance in the holes they designed to measure, to accommodate undersize holes and to ease insertion. Nevertheless, the greater the clearance, the greater the amount of centralizing error. This is when the Gauge measures a chord of the circle, and not its true diameter. By reducing the designed clearance, centralizing error minimized, thus saving a few parts of a micron, albeit with some trade-off against ease of insertion.
We engineered a special set of plug Gauges, with minimum material clearance of 15 µm. The standard digital indicators 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%.
SWIPE? This acronym stands for the five categories of Gauging variables: Standard (i.e., the master); Workpiece; Instrument (i.e., the Gauge); Personnel; and Environment. In the case of the valve body manufacturer, we tweaked the instrument, thus reducing one source of Gauging variability. We reduced a second source by providing higher-quality masters for these Gauges. If throughput had not been such a high priority, we might have also considered altering the environment where inspection performed, or providing more training to personnel. If portability had not been an issue, then the solution might have been a different instrument altogether.
The five categories of Gauging variables encompass dozens of specific factors. (For example, within the category of Workpiece, there are variables of surface finish and part geometry that may influence dimensional readings.) To squeeze more accuracy out of a Gauging situation, look for opportunities to reduce or eliminate one or more of these factors, and in the end those lost microns used in improved gauging accuracy, not dropped on the floor somewhere.
The type of gauging selected for the application determines how much of the tolerance zone is used for production as opposed to being lost because of poor Gauge performance.
George Schuetz, Mahr