Messtechnik
Messtechnik
Degrees of Performance
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Degrees of Performance
George Schuetz, Mahr Inc.

 

Whether I am cutting my grass in the sweltering heat of summer or hunkering down getting ready for freezing temperatures with snow up to my waist, I'm constantly reminded of the effects of temperature…not only on us but on precision measurement.

During the winter I frequently get early morning calls from my sales partners who are anxious to demo a bench or height gage. How long do they have to wait, they want to know, before they can start making accurate measurements with their demo equipment? Why wait I ask? Just plug it in and start measuring? Oh, well there's just one detail they didn't mention: the gage was in the back of their car the night before. While they were warm and toasty, the car was sitting outside where it was a brisk 5˚F below zero. In this case, the gage will be barely capable of movement—being literally frozen in place—let alone able to give accurate readings. Give it a day, I say, and I think you will be good to go.

Dimensional control is an important part of manufacturing, yet those trying to achieve precision measurements often forget the single largest source of measurement error: temperature.

In most shops, temperatures change over time and space. As temperatures change, so will the measured values of critical dimensions. As tolerances get tighter, along with the ability of gages to resolve to smaller values, it's important to consider how temperature may affect your results.

Here are two common examples of how temperature affected gage performance.

A customer was experiencing a problem with his height gage: measurements appeared neither accurate nor repeatable. A factory representative noted the problem and arranged to send the gage back for warranty repair. This was not a normal event as these gages are very reliable, and the repair team could not find any problem. Since it met all performance specifications and tested out thoroughly, it was returned to the customer.

But the customer again complained that the gage was not accurate. Again the height gage was returned and inspected. Again no issues were found and the gage was returned, this time with a new certification of performance.

For the third time the gage was set up only to have the same situation observed. This time though, with the help of a new set of eyes that accompanied the gage's return, observations were made about the location and environment the gage was used in. While the gage was being used in a typical shop, it was located in the receiving department, right next to a shipping bay door.

When the gage was moved to a different department, farther from the fast temperature changes caused by the opening and closing doors, the problem was solved. The gage met all performance criteria.

It may be pretty clear that the larger a part or gage gets, the more it is apt to change with temperature swings. However, it is also true that if a gage can provide very high resolution it can pick up the smallest temperature variation.

This was seen in another case where, during a morning demo of a small and very precise length measuring machine, all was going well. While not a true "millionth" measuring system, the gage was accurate when the customer and salesman tested a few of the customer's gage blocks.

Later in the afternoon, however, they noted that the gage started drifting high when they measured the same gage blocks. After a frantic call to the factory and routine questioning from the technical guys, they were asked if insulating gloves were being used while handling the blocks. Of course the answer was "no." What had happened was that between handling the blocks with their hands and with two users standing in front of the gage for six hours, their combined body temperature was having a significant effect on the results the precise length gage was reporting.

After a short break to give the gage and the blocks a chance to stabilize, and with a change in the measuring and handling process, the gage once more performed as expected. And the salesman scored another successful demo!

Gloves, tweezers and a breath shield are three high tech weapons against temperature.