Section 6 The Science and Skill Of Measuring

Overview  In this and the next section, you’ll be learning one of the most central issues facing quality manufacturing the unending quest to achieve better accuracy, more reliably, without increasing manufacturing cost.

Section 6 Goals Unit 6-1, Dimensions and Tolerances >List the six categories of Measuring >Use Dimensions & Tolerances On Prints > Apply Tolerances to Measuring > Recognize Protracted and Geometric Angles on Drawings Unit 6-2, Managing Accuracy > Define resolution and repeatability > Recognize and Control in-accuracy factors found in each tool.

Goals Continued Unit 6-3 Use Five Measuring Instruments > Micrometers to a.0001 inch accuracy > Rules to.015 inch > Calipers to.002 inch > Height Gages various accuracies (dial, vernier or electronic) > Dial Test Indicators to or inch.

The Personal Accuracy Habit  Developing the ability to measure precisely requires more than learning the parts of the instruments, and how to read them, although that’s a good place to start.  To be the best you can be, there are six elements required:

Elements of Accuracy  Uncompromising Attitude More than anything else, the decision to be as accurate as possible, must be first.  A Scientific Approach Staging and participating in experiments to zero in on accuracy.  Unit 6-2 Learning Inaccuracy Factors We’ll look at the potential spoilers in all measuring processes. Each tool or process has it's list of things that can degrade results.

More Elements  Unit 6-1 Knowing the Accuracy Requirement After reading the drawing, choose the right tool for the tolerance. In other words, know how demanding the task is then initiate the right actions to meet the challenge.  Identify the Inaccuracy Factors in the chosen tool and process. Then take the right actions to control them.  Unit 6-3 Practice, Refine, Practice, it's a life- long kind of thing! Adapting to new technologies comes into play here.

Unit 6-1 Dimensions and Tolerances  “To be regarded a marksman, you have two good options;  Either do what it takes to hit the target every time,  or never tell anyone where you were aiming.” Will Rogers

 There are 6 measuring targets that machinists must hit every time! Size Position Relative to a feature or datum Form Orientation Relative to a feature or datum Surface Roughness  The measuring instruments and processes studied here will achieve many but not all of them.  Section 7 will round out the ability. Shop Talk Then in Section 16, we’ll look at the 6 th measuring skill, hardness testing.

The Measuring Process  Often, there are two or more ways to use a given measuring tool, the process. Some are easy and others take more time but deliver more accurate results.  For example, using a height gage to measure part height, there are 4 different methods. One requires only a scribe attachment, while others require setting up more tools and accessories – more time required, but increasing accuracy as well.

 The chosen measuring tool and process, should deliver at least ten times finer results than the task tolerance.  We call that the Rule of Ten: 10X  For example the tolerance is in. so the micrometer chosen should deliver in. results.  Truthfully, 10X isn’t always possible when tolerances become very tight, still it's a good guideline! 10X (The Rule of Ten )

Five Basic Measuring Tools  In Unit 3, you’ll learn the five most universal measuring tools.  Several of which you will buy for your toolbox.  We’ll discuss each ones features and its inaccuracy list.  Then how to control or eliminate each factor toward best accuracy.

Height Gages With Indicators or Scribes Precision Rulers Yes rules can be fairly precise in skilled hands. Question? How precise would you think? Digital Calipers Dial Calipers Outside Micrometers Dial Indicators

Calipers The Universal Instrument  Calipers will probably be your first measuring tool purchase.  There are a lot of choices as to the kind to buy.  Discuss the options with your instructor or a journeyman where you’ll be working.  They can be used in a wide variety of situations.

Outside Measurement

Inside Measurement

Depth Measurement

Step Measurement Step measurement isn’t strongly recommended other than as a double check of some other process. It requires an estimate of the upper jaw’s match to the surface, and a math calculation to add to the reading, the offset size between the actual jaw reading and the head of the tool. Offset distance

Digital or Mechanical?  Most measuring instruments are found in both electronic and mechanical versions.  Which is best?  They both have advantages.

Comparing  Mechanical tools are generally less costly and more suited to harsh environments, but that’s changing. Newer digital tools are getting tougher and more affordable.  Mechanical tools don’t become obsolete  Digital tools can do data management A) Metric/Inch conversion easily B) Nominal zeroing – the useful ability to set zero at the part’s target size. C) Data offload to PC – even wireless  There is a no best choice, they both are good instruments. Perhaps one of each is the ultimate answer?

Shop Talk Employers mention that new machinists often over-use calipers when another instrument would deliver better results. Calipers are easily used, affordable, universal measuring tools, but they have limits. Unless there’s no other tool to do the job, don’t trust calipers much finer than inch, even though most can be read to (mechanical) or inch. (electronic) Why? The inaccuracy stems from closing and aligning them to the workpiece, the same way every time, a real challenge using either kind. Caution - Caliper Over-Use

Resolution and Repeatability  You’ll hear these two terms many times more in this text and in the CNC manufacturing world in general.

Resolution  Is a simple issue in most cases  It is the finest graduation or reading on the tool, without estimating  For machine tools, it is the finest movement the machine can make.

For example This micrometer’s resolution is inch. This one has an added scale bringing the resolution to in Graduations Grads. Question The second micrometer’s resolution is how much finer than the first? Answer Ten times finer Answer Ten times finer

Repeatability  Repeatability is a more complex mix of factors for measuring and for machinery.  It is the ability to deliver the same result over and again. The ultimate goal.  Repeatability is the sum of many factors including resolution.  For a machine it is the ability to move the axes away from and return to any given location, at varying speeds, within a given tolerance. Shop Talk For measuring, there is a tool repeatability but the only one that truly counts is with the measuring tool in your hands. It’s your Personal Repeatability which introduces human factors as well as mechanical!

Beyond the Text Book  Textbook, “paper” micrometers and calipers can only take your skills to the first level – understanding.  Beyond that, here’s a few ideas on how to continue improving your personal repeatability.

Challenge, Collaborate & Enjoy  Make it a personal challenge to improve continuously  Improve the measuring environment always excellent lighting, comfortable positions to take the measurement, clean and organized with good instruments.  Challenge fellow students to accuracy contests. Have fun while zeroing in on best repeatability.  Compare results to more a skilled person. Watch their measuring technique.

For Example In holding this micrometer correctly, I can reach both the friction driver and the lock ring, while joggling (nulling) the mic in two directions to get the final reading.

More Improvement Ideas  Check your results with a tool or process following the 10X Rule for comparison – a finer resolution tool.  Buy the best measuring tools you can afford, as soon as possible.  It's a fact, the best repeatability comes when you have a feel for the tool – but owning your own has another advantage,  Control over the condition and pride in ownership. Shop Talk One quick, but surprisingly revealing test employers sometimes give to job candidates, is to ask to see their toolbox and especially their measuring tools. With a glance, attitude, pride and organization are revealed (or the lack of them)!

Conclusion  Learn the tools and process of Section 6 well.  They along with the inaccuracy factors are the foundation of all that you will do in measuring.  We’ll build upon these skills in Section 7

Good Measuring! Personal Repeatability