Sometimes You Just Can’t Get There From Here

We recently took a look at how digital indicators have greatly increased their capabilities while maintaining relatively low cost. The upgrades included better and larger displays, dynamic features, calculation capabilities, and longer battery life, along with higher resolution and accuracies. They have advanced to the point where they can almost be characterized as small portable bench-gaging amplifiers, but at a fraction of the cost. But these advances can only get you so far.

One of the biggest issues these days is customers trying to squeeze more performance out of their existing gaging. What were once ±0.002 in. tolerances have gone to ±0.0002 in. or even ±0.0001 in. Many users think all they need to do to improve performance is replace their old dial or digital indicator—which has a 0.0001 in. resolution—with a new high performance digital indicator that might read to 50 µin or even 10 µin. Unfortunately, all this replacement is likely to do is cause gage operators to lose faith in the gaging they have used for the past 20 years or so.

It’s a whole other world when it comes to making millionth measurements. There are many hidden factors that must be considered when going from 100 µin resolution to 50 µin, 20 µin or even 10 µin. Let’s look at a fairly common test case to see what pitfalls we might find.

One of the most common gages sold is the comparative snap gage. The comparative snap gage is the second step up on the ladder of dimensional OD measurement. It is an improvement over the simple micrometer because it is faster to use and eliminates all the operator influences one would see with a handheld micrometer. The snap gage is virtually hands-off when making its comparative measurement. It has a very repeatable gaging force, and with its backstop, positions the part at the same location.

When snap gages are sold today, they typically come with a dial indicator that has a 0.001 in. or 0.0001 in. graduation. For tolerances such as ±0.002 in., this snap gage is a good solution. But on the shop floor, the ±0.002 in. tolerance has now become ±0.001 in. or ±0.0005 in., and with a 0.0001 in. grad on the dial indicator, there is not a lot of range on the dial indicator being used. Therefore it becomes difficult for the operator to make a judgment if the part is good or bad. Add to that some of the errors in the gage itself, and you start to test the operator’s ability to make good measurements.

Also there is the requirement for collecting data for pre-process or statistical process control. Now the dial indicators are definitely out as a means of better resolution, and digital indicators are the choice for improving the gage performance. With the capabilities and value of the digital indicator, it becomes simple to upgrade to the latest indicator with a much higher resolution.

So the purchase is made, the new indicator goes on the 10-year old gage, and all of a sudden the operators are bringing the gages into the gage repair department because they don’t repeat like they used to. The operators may even question the performance of the indicator on the snap gage. But what’s happening is that the digital indicator simply has the ability to magnify and show the errors of a gage that was not built to perform such high resolution measurements.

On a snap gage, the original anvil parallelism—that is perfectly acceptable for 0.0001 in. grad indicator—will now begin showing as repeat errors. Most snap gages with dial indicators have a parallel tolerance of 0.0001 in. on the whole surface of the anvil. On a dial indicator, you would be hard pressed to see the movement within one graduation. However, when a digital indicator having 20 µin of resolution is used, that 0.0001 in. parallelism could be 5 flips of the digit (or more) simply by placing the part on different locations on the anvils. Show that to a discriminating operator and it would make him quickly lose confidence in his gage.

There are many more errors that might also get magnified with the higher resolution, such as part geometry, dirt, temperature, deflection due to gaging force, and tooling marks.

There is only one way to ensure that this step up will work and that is to do a thorough gage study first. Put your best readout on the gage. Do repeat tests, do GR&Rs and analyze the results. If the gage cannot make the measurement, the analysis will be clear. And it will prevent bad gages from being put on to the shop floor only to come back later on.

George J. Schuetz, Director Precision Gaging, has been employed with Mahr Federal Inc. for 40 years in the metrology business. During that time, he has worked in the application’s areas for mechanical and digital indicators, mechanical gages, air tooling, electronic products, special gaging designs, surface finishing, and geometry gaging, and has worked with many companies to solve specific gaging problems. Presently, George is responsible for Precision Gage Product Management at Mahr Federal. Sign up to receive George’s Gaging Tips eNewsletters at www.mahr.com/gagingtips.

This blog originally appeared on www.mahrfederal.com. It has been republished on Design Engineering’s website with permission from Mahr Federal.