ME C40-3
Computer Integrated Manufacturing
- Manufacturing Automation -
LABORATORY EXERCISE #1
DIMENSIONAL, FORM, AND SURFACE METROLOGY
"When you can measure what you are speaking about and express it in numbers, you know something about it; and when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory thing. It may be the beginning of knowledge but you have scarcely in your thought advanced to the stage of a science.
Lord Kelvin, 1848
The quotation of Lord Kelvin above suggests why we measure. The science of measurement is called METROLOGY; a formal definition of which is the science of measuring time, length, and mass.
I. OBJECTIVES:
1. To get acquainted with the principles of Geometric Dimensioning and Tolerancing (GDT), in accordance with ANSI standard Y14.5-1995, and its metrological implications.
2. To become familiar with elementary and advanced principles of dimensional, form, and surface metrology to verify the design intent achieved on manufactured parts.
3. To gain further expertise in the principles of dimensional metrology and measurement methods using the coordinate measuring machine (CMM).
II. BACKGROUND:
Geometric dimensioning and tolerancing can be described in its simplest terms as a means of specifying the "geometry" or "shape" of a part on an engineering drawing. In more broad terms, it can be described as a modern technical drawing language which has uniform meaning to all; it can vastly improve communication in the cycle from design to manufacture. It has become a universal engineering drawing language and technique which companies, industries, and government are finding essential to their operational well-being. Over the past 30 years, this subject has matured to become an indispensable management tool; it assists productivity, quality, and economics in building and marketing products around the world. Appendix 1 offers a brief introduction to GDT.
Our emphasis in this exercise will be on the implementation aspects of this technology as it pertains to the metrological verification of a part's compliance to the stated requirements.
III. EQUIPMENT:
- Brown and Sharpe Coordinate Measuring Machine with Micro-Measure III software
- Rank, Taylor and Hobson Talyrond 150 Roundness Tester
- Tokyo-Seimitsu Surface Analyzer with Surfcom
- SPC Micrometer, Calipers, Protractor
Miscellaneous instruments (partial listing)
Thread Measurements: thread micrometer, optical comparator, toolmaker’s microscope.
Outside Diameter Measurements: vernier micrometer, vernier caliper, toolmaker’s microscope.
Angular Measurements: bevel protractor, sine bar, optical comparator.
Height Measurements: vernier height gage, height bars.
Surface Flatness Measurements: optical flats, digital level.
Gaging: inspection gages, gage block set(s).
NOTE:
Information on the use of the Micro Validator, Surface Analyzer, and Roundness Tester will be available in the laboratory.
IV. ASSIGNMENT AND REPORT FORMAT:
Familiarize yourself with the use of the Coordinate Measuring Machine (CMM) in the Metrology Lab and become familiar with the operation of the Surface Analyzer and Surface Tester. As a group, formulate a strategy for verification of the various features of the test pieces provided. Verify at least one of each of the distinct characteristics of the parts (dimensional tolerance, form, orientation, location, surface characteristics, etc.). Also, if feasible, use several alternative methods to verify a particular feature.
For this laboratory exercise the requirements for credit are the following:
1. Record how well the dimensional characteristics, taken by the CMM and/or by other means, of the test pieces actually reflect the design intent.
2. Verify how well the design intents of the test pieces actually met with respect to roundness and surface characteristics measured with the Roundness Tester and Profilometer.
3. On a copy of the engineering drawing of the test pieces, record the measurements taken on the features of the test pieces. Compare them to the indicated dimensions.
4. Discuss whether or not the test pieces meet specifications.
Copies of the engineering drawings of the test pieces will be provided in the laboratory.
V. PROCEDURE
V.I.G.s {Very Important Guidelines}:
Please return all test pieces back to the box when finished. DO NOT LOSE ANY!
Please don’t mark, tear or wrinkle the engineering drawings.
Please follow the TAs instructions when using metrology equipment (the unit prices for the surface analyzer, roundness tester and CMM are >$30,000!)
To VERIFY a dimension or geometric specification, make the appropriate measurements (if possible by several alternative methods) to ensure that the part’s features comply with the drawing’s tolerances. Use the available measuring devices to verify at least one of each distinct dimensional and geometric specification. Report the results of your measurements in graphic format, that is, on the provided copies of the drawings of the test part. Be sure to clearly indicate your measurements and on a separate piece of paper report on deviations and compliance to specifications.
Measurements needed for lab credit:
Test piece Measurements
Cam Follower A few dimensions to specified tolerances
PLUS Flatness
Cylinder A few dimensions to specified tolerances
PLUS Flatness, Parallelism, and Ra
Machined Cylinder Head A few dimensions to specified tolerances
PLUS Concentricity, Flatness, and Parallelism
Push Rod A few dimensions to specified tolerances
PLUS Flatness and Ra
Rocker arm Pivot A few dimensions to specified tolerances
SUGGESTION: Do tests on one piece exhaustively, then move to the next piece. Don’t do all CMM measurements, then all profilometer measurements, etc.
APPENDIX 1.
A BRIEF INTRODUCTION TO GEOMETRIC DIMENSIONING AND TOLERANCING
Geometric dimensioning and tolerancing can be described in its simplest terms as a means of specifying the "geometry" or "shape" of a piece of hardware on an engineering drawing. In more broad terms, it can be described as a modern technical drawing language which has uniform meaning to all; it can vastly improve communication in the cycle from design to manufacture.
Geometric dimensioning and tolerancing, also often referred to in colloquial terms as "geometrics," is based upon sound engineering and manufacturing principles. It more readily captures the design intent by providing the designer or drafter better tools with which to "say what they mean." Hence, manufacturing or production can more clearly understand the design requirements. In practice, it becomes quite evident that the basic "engineering," in terms of machining, fixturing, inspecting, etc. is more logically consistent with the design intent when geometric dimensioning and tolerancing is used. As one example, functional gaging can be used to facilitate the verification process and, at the same time, protect design intent.
Geometric dimensioning and tolerancing has become a universal engineering drawing language and technique which companies, industries, and government are finding essential to their operational well-being. Over the past 30 years, this subject has matured to become an indispensable management tool; it assists productivity, quality, and economics in building and marketing products around the world.
The many further technical reasons for, and advantages of geometric dimensioning and tolerancing will be discussed in detail in the classroom lectures. Each phase and every aspect discussed will add further logic and practicality to the subject. The reader and user will, thus, be guided both technically and psychologically to become more competent and confident in application.
RATIONALE OF GEOMETRIC DIMENSIONING AND TOLERANCING
Geometric dimensioning and tolerancing builds upon previously established "drawing practices." It adds, however, a new "dimension" or capability to drawing skills in defining the part and its features which the older methods cannot provide.
An effective way to introduce the technical aspects of geometric dimensioning and tolerancing is to first examine and analyze a drawing without such techniques used. In this way, the meaning, or interpretation, of such a drawing is put to the test of clarity. Whether or not the part requirements on such a part have been adequately stated and can, thus, be produced with clearest understanding is the question. In other words, is geometric dimensioning and tolerancing really necessary?
The Flange Mount drawing in Figure 1 illustrates such a part with all the major features dimensioned using the methodology of the common "coordinate system." The designer, or drafter, it is assumed, has stated the requirements on the part adequately. The manufacturing personnel will be expected to understand these requirements also, and thus, produce the part satisfactorily.
The first step in the analysis of this part notes that most dimensions and tolerances are to control "size" of the features. The 0.210 holes are also given some "location" control.
Figure 1. Part Drawing Using Only Coordinate Dimensioning and Tolerancing
SIZE CONTROLS AND THEIR LIMITATION
The first question which can be raised relative to the size controls is, precisely what do these "size" tolerances actually mean?
It is reasonably certain that the designer intended each size and its tolerance to establish an absolute limit, a sort of boundary, beyond which each feature could not transgress. Otherwise, interference or distortion would occur.
Production would, no doubt, produce the "size" features within tolerance to the best of its ability. However, would the inspection operations use caliper or micrometer methods, or would full form (ring or plug) gages be used to verify the sizes? Different results could occur, of course, dependent upon which basic method of verification is used. For example, the 0.820 diameter could be produced with a "bow" actually violating an outer cylindrical boundary of 0.820. If a micrometer type of measurement were used, the part could be accepted as good; i.e., the cross sectional measurement is within the size tolerance. Yet, if a full form, or ring, gage were used, the "bow" would be detected and the part rejected.
A search of the drawing reveals no specific instructions or answers to this question other than an "assumption" that an outer boundary of size is "probably" intended. If, for example, a quantity production run on this part had been completed and conflicting interpretations arose between producer and receiver (e.g., vendor and company), who is right? A serious question of interpretation of size control is inherent on the "old fashioned" coordinate drawing. Further, all too often no standard or basis of authority is referenced on the drawing which could be of some help in resolving these kinds of problems.
Another series of questions which arise with regard to the 0.980, 0.315, 0.820, 1.375, 1.125-12 UNF, and 2.75 diameters concerns any required relationship of these feature to one another. These features are depicted on the drawing as on the same drawn axis. Are they expected to be "perfect" in their relationship to one another; or, should there be some control or "tolerance" in their axis or surface relationship to one another? If so, which feature is related to which other feature? Size tolerances control only size; nothing else. Therefore, there is no control on this drawing to indicate any such requirements. In a produced part situation, are these features to then ignore any concern for relationships?
If the thread diameter dies have some relationship, is the criterion for verification the O.D. or the pitch diameter? Again, no answer can be derived from the drawing.
Still further questions regarding "size" controls on this drawing could ask; do the length (size) dimensions (i.e., 1.750, 0.500, 1.120, etc.) adequately depict the design requirement? That is, does the ±0.005 tolerance of the overall 1.750 length dimension, for example, ensure needed precision on the end surfaces of this length?
LIMITATIONS OF THE COORDINATE SYSTEM
All of the foregoing questions, and more, relative to the example "coordinate" drawing, are left unanswered.
At this point it must be assumed that a heavy dependence is apparently placed upon "somehow" building this part to the drawing as indicated. The major factors involved in such a situation where parts are "somehow" produced without definitive specification are:
1. Workmanship -- normal workmanship is assumed.
2. Common sense
3. Probabilities -- the part "probably" will not vary to an extreme.
4. Title block, workmanship, or contractual standard invoked -- if these are invoked formally, some clarification occurs. The example does not include such options.
The first three of the preceding factors have always been assumed in such a situation and no doubt will continue into the future. Yet, can today’s more sophisticated products and advancing technology depend upon such questionable communication? There appears to be far too much left to chance.
Further exploration of the "coordinate" drawing in the top (right) view presents additional questions. The eight 0.210 holes are located by toleranced angular (45°) and the 2.125 bolt circle requirements. Some questions which must be asked, are:
1. What is the basis for establishment of the ± 0°-30’ angular tolerance and the ±0.005 bolt circle tolerance? (Answer: it probably was a conversion, or a foreshortening, of a calculated diametrical relationship between the hole and mating feature (pin, screw?) as determined by the designer; the method shown is in keeping with the "coordinate" system methodology.)
2. Where is the functional center, axis, or vertex of the 2.125 diameter and the 45° angle? (If the answer is the centerline or axis of the part, is the part anticipated to be "perfect"?; are the axes of all features shown on the drawing axis expected to be exact in their coaxial relationship?; if there is actually some functional relationship of the hole pattern to any specific reference feature, it is not indicated.)
3. Is there any consideration of the effect of hole size variation on the requirement? (Answer: if there has been, it is not indicated; nor is it possible to do so without such specification on the drawing.)
4. How would the hole pattern be produced or inspected? Where would the part be picked-up for fixturing or inspection? (Answer: the drawing give no assistance.)
These questions are all left without clear answers or even minimal clarification.
It is very evident that the "coordinate" dimensioned and toleranced drawing is inadequate, ambiguous, and leaves the design to manufacturing procedures heavily dependent upon chance. Such drawings may serve a purpose where conceptual, experimental, or prototype hardware is being fabricated on a very limited and controlled basis; i.e., as "Level 1" documentation under a military contractual requirement. However, where repeatability, quantity production, or a complete and self-sustaining drawing is to be produced, the coordinate system method falls far short of providing complete and clear communication.
A drawing may be required to go almost anywhere and still be able to "speak for itself." In other words, "it must say what it means and mean what it says." Each drawing must avoid ambiguity and be as complete as necessary to ensure fulfillment of the requirements.
The odds favoring successful completion of the design to production cycle are vastly improved with the use of geometric dimensioning and tolerance and established standards.
GEOMETRIC DIMENSIONING AND TOLERANCING INTRODUCES "FUNCTION" -- AND ADVANTAGES
From the foregoing discussion it is readily seen that the many problems of misunderstanding, or lack of communication, are brought about by out-dated drawing methods.
The FUNCTION of the part and its feature RELATIONSHIPS are simply not captured in the purely coordinate system. Whereas, the geometric dimensioning and tolerancing system is actually based upon FUNCTION and RELATIONSHIP as the fundamental principles.
FUNCTION and RELATIONSHIP are the key words--and the key principles.
Therefore, geometric dimensioning and tolerancing, being based upon the principle of "function" and "relationship," prevents many of the before referenced problems; it further can claim many advantages instead.
First and foremost its use saves money via:
1. Maximum producibility ensured
2. Ensures integrity of the design requirements
3. Ensures interchangeability
4. Provides uniformity of drawing delineation and interpretation.
Also:
5. Accommodates today’s more sophisticated design; brings drawing capability abreast of the product state-of-the-art.
6. It is increasingly becoming the universal "spoken word" in worldwide engineering documentation.
Function and Relationship
Referring to the subassembly drawing in Figure 2, some insight is gathered regarding the functional requirements of the Flange Mount part shown earlier. Relationships of key features between the assembled parts and also on the individual parts are clearly suggested. However, if this is the nature of the design requirement, there is no indication of such detail on the coordinate toleranced drawing in Figure 1. The function and relationships of the key feature, clear in the designer’s mind and represented on the subassembly drawing, have been lost in the "translation" to the coordinate drawing; lost because geometric dimensioning and tolerance was not used.
Geometric Dimensioning and Tolerancing Application
In Figure 3, the FLANGE MOUNT part discussed earlier using the coordinate system, is now shown using the geometric dimensioning and tolerancing system (geometrics).
Note first that all the "size" controls are retained and stated as before. Then evident, of course, are the geometric tolerancing controls added which do capture "function" and "relationship" of the part features. Further, and of equal importance, is the uniformity of meaning, no ambiguity exist on this drawing.
The details of application can be studied in greater detail within supplementary texts and handouts. For the purposes of this introductory example, however, only the major improvements and clarification brought about by the use of geometric tolerancing are emphasized. Thus, the reader can grasp the reasoning and principle of the system in comparison to the "coordinate" example.
Clarity and Uniformity of Meaning of "Size"
With reference to the "size" dimensions and tolerances, and as previously noted, a boundary and limitation of the size of each feature has been determined by the designer. By using the geometric dimensioning and tolerancing system and the basic United States standard, the designer can now be assured of better uniformity in meaning and understanding of design requirements.
The NOTE on the drawing, or otherwise clearly invoking the authority and basis of the United States standard ANSI Y14.5M-1982, ensures the "perfect form at MMC boundary" of each individual feature; the very same principle the designer originally assumed but was not assured under past methods. Although not always policed as based upon Quality’s or Inspection’s discretion, this principle (actually known as RULE #1 in the system) will protect the design intent and will provide more uniform
Figure 2. Flange Mount, Shaft, and Housing Subassembly
Figure 3. Part Drawing Using Geometric Dimensioning and Tolerancing
guidelines in production and inspection.
The first magnitude of control, "size," has now been given standard meaning; yet production methods retain all the necessary freedom of choice as before. By using the geometric tolerancing system and the authority of ANSI Y14.5, a disciplined and uniform meaning of "size" is established.
According to RULE #1, "size" controls "form" of individual features (i.e., the 0.315, 0.820 etc. diameters) as well as size. Therefore, as the size features are produced, "form" control is automatically established to the extent of that size tolerance.
Stating Geometric Tolerances
Since a "size" tolerance invokes control of the "form" of the individual feature only, any interrelationships of features must be stated. The geometric tolerancing system provides the tools to so indicate such requirements. The "coordinate" system leaves such considerations to chance or are simply ignored.
The specified form and location controls are supported by other RULES (i.e., #2, 3, etc.) which also helps establish uniform meaning. The drawing, of course, used symbolic methods (the pictorial language of the system) to indicate these various geometrical controls.
Note that the "function" and "relationship" of the part and its component features are captured by the specific geometric tolerance controls indicated. This includes the Flatness of the end face of the flange, Parallelism of the related opposite surface from the Flange, Runout of the counterbore, Perpendicularity of the through hole, and Position of the other indicated features. Required datums (where the relationship is taken from), and appropriate conditions (RFS or MMC) are included as per the design requirements.
With the use of geometrics, the design intent has been captured and stated more clearly and production can better understand an implement manufacturing and inspection. The communication loop is more effectively completed with the use of this system.
Understanding the System
Correct and effective use of geometric dimensioning and tolerancing, it hardly need be said, requires an "understanding" of the system. This point is emphasized because misuse and problems arise when the user does not clearly understand the fundamentals of the system. Experience and use will be the "best teachers."
The geometric characteristics, symbols, and terms are given in Figure 4.
NOTE: As an emphasis to the reader and user of this text, it is recommended that if geometric dimensioning and tolerancing be used, the exact principles of the established standards (principally ANSI Y14.5-1992) be carefully followed. To deviate, or customize, the principles and symbols defeats the purpose of the long-sought standardization now established. Any necessary deviation for unique purposes should be carefully explained and documented
Figure 4. Geometric Characteristics, Symbols, and Terms