Metrology
Metrology
New Parts, Tighter Tolerances
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New Parts, Tighter Tolerances:  You Can Gage Them, But…
George Schuetz, Mahr Federal Inc.


If you are planning on manufacturing to tolerances of .0001" or less, and have not done so before, your introduction to high order gaging may be an eye opener. Gaging at this level is a very different animal, especially when considering how to measure the new parts you will be producing.

When gaging parts with the usual tolerances (0.0005" or broader), the quality of the gage is usually the main consideration. However, when you become interested in tolerances of say 30 µinches, there is much more than the gage to consider. You must place just as much importance on creating a special environment in which it will operate.

But even before thinking about the gage or the environment, a decision must be made about what should be measured: the size of the part, or the relationship of the part to another part. The answer to this can make a big difference in the cost and difficulty of getting the desired results. At these tighter tolerances, it is much less difficult and often more effective to check part relationships—clearance for example—rather than part dimensions.

If the decision is to check size, new methods of production measurement will be needed. Using the old 10X rule of thumb, for a 30 µinch tolerance, the repeatability of the gage needs to be in the 3 µinch range; and to be sure both the gage and the master have such accuracy we would need a master that is accurate to 0.3 µinches. Since no one would guarantee a master or even a gage block to have this accuracy, the best you can do is use the best standard available, which is apt to be a set of carefully cleaned and properly wrung Lab Master or Grade 0.5 (AAA) gage blocks with a ±1 µinch tolerance.

Now you need to focus on the environment. We all know the influences, but unfortunately, they are too often underrated. Particular attention must be paid to temperature and cleanliness. Within limits, temperature itself is not as important as its consistency and the degree to which it is shared by the gage, the master, the part, and the surrounding atmosphere. For example, if you are checking the diameter of a 0.5 inch bore, a temperature difference of only one degree between the part and the master is likely to introduce an error of about 3 µinches. In addition, if the room temperature shifts a few degrees, or the operator's body heat is allowed to reach the gage, it is easy to accumulate a few more millionths of error.

Here are a few tips that can help control the needed environment.

Parts, master and even the gage should be part of one large heat sink, if possible, so they can all be brought to the same temperature and held there. The larger the mass of the gage and heat sink, the less likely it will change during sudden air temperature changes. Even with the heat sink in place, it may take hours for the parts to normalize. Gaging must take place in a constant temperature room and the parts must be measured without touching them: the use of insulated gloves or tweezers is a must. 

Assuming these recommendations are followed, it will still be necessary to check the gage frequently and compensate accordingly. No one can predict how the opening of a door and the resulting draft that occurs will affect the measurement. It may even be necessary to try to isolate the operator from the gaging process by placing a thermal shield between the operator and the gage—all in an effort to save microinch errors.

The problem of dirt can be just as troublesome. Dirt particles or film, which is virtually invisible, can easily cause serious repetition errors. It will likely be necessary to chemically clean the entire gaging area periodically (there is no way to predict how often—only experience will tell). Part cleaning will vary somewhat depending on shape, material and size, but methods and cleaning fluids will have to be studied since residue can't be ignored at these tolerances. And don't forget that residue may also build up on gage contacts after repeated measurements.

Taking these steps into account—with thorough planning, careful personnel selection and training, along with significant investment in proper equipment—will help achieve measuring to these tight tolerances. Anything less, i.e., cutting corners, can ruin the economics of the job and cost more in the long run.

This ID/OD gage has all the elements necessary for millionth measurements—when used in a temperature controlled room—including a large granite base, set up with gage blocks, and millionth readout.