Gage Yourself

Author James Mullineaux
Published
December 01,1999 - 11:00am

Find out how much you know about the proper use of measuring tools.

Is a micrometer more accurate than a set of calipers? See the answer to question 1.

People who manufacture parts should know how to properly use and care for the most common types of gages: steel rules, calipers, indicators and micrometers. If they don’t, consistent part quality is not possible.

To help reduce the production of unsatisfactory parts, companies should regularly assess their operators’ knowledge of gages. Below are 10 true-or-false questions designed to help them do that. The correct answers follow the quiz.

True or False?

 

1.When measuring tight-tolerance parts, an operator should use a micrometer instead of calipers, because a micrometer is more accurate.2.An operator learning how to use a gage on a practice part should be given the correct dimensions in advance.3.The amount of pressure a micrometer exerts on the part that it’s measuring should be sufficient to allow the operator to pick up the micrometer without the part falling out.4.High-quality gages are designed to withstand harsh manufacturing environments.5.Moisture can adversely affect a gage’s performance.6.Gages that are made from hardened steel will not wear when used under normal conditions.7.Operators should hold onto their gages between readings in order to minimize non-machining time.8.Today’s gages are manufactured so that they don’t need regular calibration.9.An experienced operator doesn’t need gage-usage training.10.An operator who uses proper measuring techniques can get incorrect readings.

 

Answers to the Gage Quiz

1. When measuring tight-tolerance parts, an operator should use a micrometer instead of calipers, because a micrometer is more accurate.

FALSE. Calipers can be as accurate as, or more accurate than, a micrometer. Some operators think that a micrometer is more accurate because it has higher resolution. Resolution is defined as the smallest unit of measurement on a gage. Accuracy refers to how the reading on a gage compares to the value of a known standard, such as a gage block. (If a gage does not conform to a known standard, it has no accuracy.) When operators say that a micrometer is accurate to 0.0001", what they really mean is that the micrometer can measure a unit that small. Remember this rule of thumb: The resolution of a gage should be at least one decimal place greater than the smallest dimension being measured. So if the part tolerance is ±0.01", for example, the gage resolution should be at least 0.001".

2. An operator learning how to use a micrometer on a practice part should be given the correct dimensions in advance.

TRUE. Some people believe that if the operator has this information in advance, he or she will tend to "force" a correct reading. This is a valid concern when measurements are being taken during a production run. But knowing the correct dimensions during a practice run provides immediate feedback, which shortens the learning curve. It is natural for operators to be unsure of themselves when using a micrometer for the first time. They must learn how to simultaneously hold the measuring device and the part—not set the part on a table or other piece of equipment. Left-hand operators face additional challenges, because most gages are designed for right-hand users. Learning to use the non-dominant hand usually requires extra practice.

Foreign material that enters a gage’s working mechanism can cause it to jam. A frequent target for contaminants is the backside of a set of calipers.

3. The amount of pressure a micrometer exerts on the part that it’s measuring should be sufficient to allow the operator to pick up the micrometer without the part falling out.

FALSE. Too much pressure can damage the gage or give artificially low readings. Conversely, lack of sufficient pressure can produce artificially high readings. The somewhat subjective "touch" needed to properly measure a part takes some practice to acquire. Also, touch can vary according to a specific part’s features or the material from which it’s made. For example, plastic parts require a different touch than hardened steel parts.

4. High-quality gages are designed to withstand harsh manufacturing environments.

FALSE. All gages must be handled carefully. Improper handling or use can significantly shorten a gage’s life. Steel rules should not be used to stir liquids. Micrometers are not C-clamps and gage blocks are not shims. A measuring tool should never be left on a table. It should always be stored in its case, and it should never be dropped or tossed into the drawer of a toolbox. Even a lightweight object that drops onto a gage can destroy it.

5. Moisture can adversely affect a gage’s performance.

TRUE. Gages are not sealed, so they must be kept dry and dirt-free. Most gages are made from steel. The use of cutting lubricants or water will cause a gage to rust and will shorten its useful life. Gages should be wiped clean after each use. Foreign material that enters a gage’s working mechanism can cause it to jam. (A frequent target for contaminants is the backside of a set of calipers.) A gage should move freely through its entire measurement range. If it stops and must be forced to continue, remove it from service. It is also a good practice to wipe the contact surfaces of the gage and the part before taking a measurement. This will increase the validity of the measurement.

6. Gages that are made from hardened steel will not wear when used under normal conditions.

FALSE. If certain areas of a gage’s contact surfaces repeatedly touch a steel part, they will eventually wear. It is impractical to take measurements by trying to avoid worn contact areas. Eventually, someone will produce a false reading. Check contact surfaces regularly for wear. When the contact surfaces of a closing-style gage are together, the operator shouldn’t be able to see light between them. Operators should also regularly check surfaces for burrs.

7. Operators should hold onto their gages between readings in order to minimize non-machining time.

FALSE. Operators should only hold gages as long as it takes to measure a part. When finished with a gage, the operator should wipe it clean and return it to its case. Humans transfer sweat and dirt to gages, as well as body heat. The latter can have an especially detrimental effect on the measurement of a part with very tight tolerances. Body heat can cause sensitive gages to misread a dimension by 0.0001".

8. Today’s gages are manufactured so that they don’t need regular calibration.

FALSE. To ensure that accurate and repeatable measurements are made consistently, gages should be calibrated regularly. The frequency depends on the type of gage, the required precision, the operating environment and the skill level of the operator. As a rule of thumb, calibrate gages at least once a year. A trained technician or a certified in-house employee should perform the task. Many inexperienced operators believe that "checking zero" is an acceptable calibration method. It is not. The gage must be checked throughout its entire measurement range, not just at zero. If a gage is dropped or damaged in any way, it should be recalibrated before being used again.

 

9. An experienced operator doesn’t need gage-usage training.

FALSE. Some employers believe that operators who have several years of experience always use gages correctly. Although experienced operators may know the quickest way to get readings, that may not be the correct way. All operators should be taught proper techniques and should be tested periodically. Some companies conduct repeatability and reproducibility studies to detect improper gage usage. An operator bias study can determine how much operator error affects measurement results. In this type of study, the operator is asked to measure the same part 10 times and record the readings. The average is then calculated and compared to the target value. The difference between the average and the target value is known as "operator bias." Someone with a "negative operator bias" has what is commonly called a "heavy touch." He or she tends to get readings that fall below the target.

Electronic direct-read measuring devices help reduce operator error, because they eliminate the calculations required when using certain types of indirect gages.

10. An operator who uses proper measuring techniques can get incorrect readings.

TRUE. Incorrect readings usually result when operators use poor techniques. But that’s not always the case. Some indirect gages, such as vernier micrometers, require the operator to make one or more calculations to find a measurement. While good results are possible, mistakes occur more frequently with indirect gages. Therefore, it may be less expensive in the long run to invest in electronic direct-read gages. Whether such an investment in equipment is made or not, all companies should spend time training their operators to properly use and care for gages. The payoff will be fewer scrapped parts, increased productivity and greater profits.

 

How Did You Do?

If you took the preceding quiz, add up the number of questions you answered correctly. Then find the corresponding number below to assess your knowledge about the use and care of gages.

9-10You have a good grasp of gaging fundamentals.7-8If you are an experienced operator, it’s likely that you’ve developed some bad habits. Carefully review your incorrect answers and make a conscious effort to apply what you have learned here. If you are a new operator, you’re doing well. Keep learning and practicing.5-6If you’re an experienced operator, you need to take a refresher course or study a text on gaging. Newcomers should keep at it. Time and persistence will pay off.<5You should probably not be conducting measurements in production without some direct supervision.

Related Glossary Terms

  • calibration

    calibration

    Checking measuring instruments and devices against a master set to ensure that, over time, they have remained dimensionally stable and nominally accurate.

  • micrometer

    micrometer

    A precision instrument with a spindle moved by a finely threaded screw that is used for measuring thickness and short lengths.

  • tolerance

    tolerance

    Minimum and maximum amount a workpiece dimension is allowed to vary from a set standard and still be acceptable.

Author

President

James Mullineaux, CQE, is president of JTS Associates, and training and consulting firm based in St. Louis. He also teaches gage classes for the Tooling & Manufacturing Association in Park Ridge, Illinois.