Time To Re-Evaluate Your Gaging?

Let’s say you’ve been making the same part for decades, running hundreds or thousands of parts every day, from the same machines, using the same process and the same gaging purchased when the process was started. If this is the case, then you’re not alone.

Columns From: 6/1/2006 Modern Machine Shop,

Let’s say you’ve been making the same part for decades, running hundreds or thousands of parts every day, from the same machines, using the same process and the same gaging purchased when the process was started. If this is the case, then you’re not alone. It’s also not uncommon to find that manufacturing tolerances have been tightened as the demand to run parts more efficiently increased. Perhaps when the parts were first made, the bearing diameter was ±0.002", then it was changed to ±0.001", and now you are expected to hold ±0.0005"—all with the same machines and the same gages that were bought years ago.

Again, this is not an infrequent situation. Although it may not always be a problem, it is a clear signal that you should take another look at your gaging. Gages designed to measure to thousandths are substantially better than those built for lesser tolerances. With a little tender care, these gages are often able to measure a few more ten-thousandths than if they aren't properly cared for. Here are some pointers for ensuring that the performance of your gaging can keep up with the demands of changing part tolerances.

First, check the readout device. Maybe the gages were originally supplied with dial indicators having grads of 0.0002" or 0.0001". For a ±0.0005" tolerance, this will not be a good fit. The operator will have difficulty discriminating the tolerance band and the indicator will not use enough of the dial span. Change to a 0.0001" grad indicator or, better yet, put in a digital indicator having 50 microinches or 20 microinches digital resolution. Now the operator can resolve the measurement sufficiently to make better decisions.

This method is not the catch-all for tighter tolerances, however. The gage needs to be capable of doing the task, or else there will be far too many repeat errors for the gage to be useful.

Checking with the gage manufacturer will help you understand what the real limits of your gages are before you go too far down this path. Maybe the gages were not designed to meet the new tolerance requirements. This could mean that an investment in new gaging is required. However, for some gaging, such as fixed body gages (air plugs or mechanical plug gages), body clearance will be the deciding factor on whether the gaging is capable. The gage may need to be reworked to reduce the body clearance and thus reduce the amount of centralization error in the plug. This reduction will greatly improve the repeatability and performance of the tooling.

Often with this type of gaging, a simple added piece of fixturing can remove most of the operator influence on the measurement. For example, with the same fixed plug gage, the addition of a stop collar can help square up the part and bring the measuring contacts to the same spot, thus greatly reducing the influence of part irregularities on the measurement.

At the end of the day, however, you are most likely to improve your gaging performance by applying a bit more tender loving care. Gages that have been serving you well for the past 5 or 10 years are probably a little frayed around the edges. The only time they were ever looked at were those once-a-year calibrations. While they still pass good parts, they may not be so good with tighter tolerances. It may be time to get the gages on a more regular and rigid inspection schedule.

It’s also a good time to start looking for those enemies of precision gaging that can steal away a large percentage of your tolerance. On fixed plug gaging, for example, worn bodies increase clearance and reduce performance. After measuring thousands of parts and exploring their diameters, contact points can also become worn. Large worn spots on a pair of contacts can cause two errors—they can report different sizes than a similar gage with fresh new contacts, and the flat area can span gaps or imperfections in the part. These are often the types of errors you would see when two lines are using the same gaging, but one line is using a newer gage than the other. The gages “see” the parts differently, and the error is not discovered until the parts are being assembled, when it’s too late.

Finally, the real enemy in the shop is the environment. Treat the gages better by wiping them off and blowing them clean more often. Clean the parts as well. You may want to make wiping the part clean before measurement a required part of the operator’s measurement process.

With a little more care and tolerance consideration, those old gages may still be able to meet increased performance requirements based on new part tolerances.

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