DIMENSIONAL METROLOGY
LABORATORY MANUAL
Industrial and Manufacturing Engineering
University of Rhode Island
2nd edition - October 1988
Revised - March 1996
CONTENTS
Module 1Introduction to Dimensional Metrology
Module 2Instruments and Support Equipment
Module 3Preparing to Take Dimensional Measurements
Module 4Analyzing Dimensional Measurements
REFERENCES
APPENDIX
BIBLIOGRAPHY
MODULE 1: INTRODUCTION TO DIMENSIONAL METROLOGY
1.1 Dimensional Metrology Defined
Metrology is the science of measurement. Dimensional metrology is that branch of metrology which deals solely with the measurement of lengths and angles- "dimensions" of a workpiece or part being investigated.
1.2 Dimensional Metrology's Role in Meeting the Needs of Manufacturing
Dimensional measurement is a crucial task for the manufacturing engineer. The "dimensional measurement process" is as basic as any other manufacturing process: without dimensional measurement, nothing can be manufactured to specification. The manufacturing engineer should be well-educated in the basic concepts of dimensional metrology.
The manufacturing engineer using dimensional metrology, (engineering metrologist) is concerned with different aspects of dimensional metrology than the "reference standard metrologist." The engineering metrologist is concerned with constrained or qualified optimization, where the goal is to economically make reliable measurements at the required level of accuracy (based on part tolerances). The reference standard metrologist, on the other hand, is concerned with unconstrained optimization, where the goal is to make measurements to the maximum level of accuracy attainable with available instruments or with those which can be developed. The title of reference standard metrologist is derived from the fact that the measurements made usually become "reference standards" for others to use. For example, the reference standard metrologist will calibrate gage blocks for an engineering metrologist who will use them to calibrate other instruments.
As a general rule, the accuracy requirements of the engineering metrologist are not as stringent as those of the reference standard metrologist; but with the current trends towards tighter and tighter tolerances and reduced costs, the accuracy requirements of the engineering metrologist are approaching those of the reference standard metrologist.
The trend toward tighter tolerances is due to several manufacturing needs. These needs include: high-speed automatic assembly which demands components with high accuracy; compacting and miniaturization of products to satisfy customer demands especially in the areas of computers and electronics; material and weight savings in parts for operational and economic reasons; interchangeability of parts and components; reliable operation of the finished products.
Dimensional measurements at the required level of accuracy, are an essential link between the designers' intent and the delivered product. Designers communicate their intent for a product through dimensioned and toleranced drawings. Manufacturers use these drawings and dimensional measurements during the manufacturing process to create a finished product. Quality Engineers or inspectors continuously check the finished product and process by comparing the designers' drawings with dimensional measurements of the product usually made under stricter conditions than in the manufacturing environment. Thus, the quality of the product which is finally delivered to the customer is controlled and the capability of the process is monitored.
1.3 The Dimensional Metrology Needs of Industry
Before the basic concepts of dimensional metrology are discussed, the dimensional measurement needs of industry must be identified. There are five basic categories of industrial dimensional measurement needs as follows:
(a)Linear measurements are required for almost every manufactured part. External and internal linear dimensions are particularly important on part features that must fit into or around other parts in some way. The aluminum workpiece used in some of the experiments and exercises is an example of a part which requires several linear measurements.
(b)Angular measurements are required for several manufactured parts. Turning tools, screw threads, cams and drill jigs or fixtures are some examples (Fig. 1).
(c)Geometric form measurements are required for several parts. A specific geometric form control refers to one feature on a part. That feature must hold that geometric form within the tolerance stated independent of the other features on the part. That feature must hold that geometric form within the tolerance stated independent of the other features on the part. Geometric forms are round, flat, straight, cylindrical, or some other specialized profile. Roundness is required primarily on spherical and cylindrical surfaces like ball bearings, shafts, and holes. Roundness is required on parts for several reasons, including the following: to ensure running fits where a minimum clearance is required between mating parts; to ensure press fits where a minimum interference is required between mating parts; for maintaining uniform distances between the outer and inner races of a bearing; and for maintaining the smooth operation of a ball bearing (Fig. 2). Cylindricity is actually a combination of roundness and straightness controls. A cylinder may be round at any particular cross-section; but over its entire length, its axis may not be straight. This would yield a bowed form with poor cylindricity, which depending on the severity of the bow, would probably lead to assembly and operational difficulties. Straightness is most commonly applied to the axis of cylinders and other forms with a center line or axis, and its importance in ensuring smooth assembly and operation has been well-established. Flatness measurements are required for surfaces of parts that must come into full contact with mating parts such as in a seal; that must be parallel to other surfaces and parts; that must be used as reliable datum features of contact gage datums for linear dimensions; that must be used as locating planes to mount or assemble the part. Specialized profiles that may be required on parts may be regular or irregular, curved and/or straight surfaces on parts and are measured by a variety of methods (Fig. 3).
(d)Geometric interrelationships exist on parts in many instances. Features on a part can be perpendicular to one another, parallel to one another, concentric to one another, or "runout" from one another. The surface of a cylinder, cone, or other surface of revolution which is circular in cross-section or has surfaces constructed at right angles to a datum axis is said to "runout" when any of its elements lie outside of a tolerance zone (usually two concentric circles) indicated by the runout tolerance specified.
(e)Controlled surface texture, which is actually a special case of geometric form, is required in several instances - some are shown in Fig. 4. As with geometric form controls, controls on surface texture are independent of other part features. Excessive surface roughness on machine guideways for example, will limit the area available to carry the load between mating parts and may cause an increased wear-rate. Excessive surface roughness on the plunger of a hydraulic cylinder may cause it to corrode more easily, because the pits of a rough surface serve as nucleating points of corrosion. The surfaces of fixed gages, like cylindrical plug gages for example, must be smooth to maintain their reliability. Controlled surface texture is also necessary for aesthetic and safety considerations. Sheet metal for automobile bodies for example must be smooth to be of a pleasing appearance when painted. Other metal parts that are to be frequently handled by humans must be smooth in order to prevent injury.
1.4Communicating Dimensional Metrology Needs
The communication of these dimensional measurement needs throughout the design, manufacture and inspection of parts is accomplished through the use of Geometric Dimensioning and tolerancing (GDT), the "language of dimensional metrology" and standard surface texture specifications. "Geometric Dimensioning and Tolerancing is a means of specifying engineering design and drawing requirements [dimensions and tolerances] with respect to actual 'function' and 'relationship' of part features. Furthermore, it is a technique which, properly applied, ensures the most economical and effective production of these features. Thus, GDT can be considered both an engineering design drawing language and a functional production and inspection technique." Standard surface texture specifications ensure the proper communication of surface texture requirements.
In this curriculum, emphasis is placed on the functional inspection technique of GDT. A functional inspection technique is a natural outgrowth of proper GDT specifications, since inspectors must be guided by the drawing of a part when measuring it. The GDT specifications for industry's most common dimensional measurement needs will now be discussed and the functional inspection techniques of GDT will be covered in the instrument descriptions of Module 3.
First, linear dimensional measurement needs are expressed in GDT specifications as shown in Fig. 5. The newest standard for linear dimensional measurement needs is shown at the top of Fig. 5 and the old (but still commonly used standard) is shown at the bottom. In the old standard, a basic size and a tolerance is specified. A basic size on a part is usually the target size on a part is usually the target size for that particular feature, and then the tolerance or range of acceptable feature sizes is applied to it. So, at the bottom of Fig. 5, the basic size or distance from the top surface to the bottom surface is 0.49 and the tolerance applied to it is 0.01. It is more appropriate, however, to simply express a dimension on a drawing as a range- ie., the maximum and minimum limits of size. Then, manufacturers and inspectors know that any size of the feature within that range is acceptable. In this way, if one has a large production volume, it is economical to measure the parts using a fixed gage like a plug, ring, or snap gage. At the top of Fig. 5, the minimum distance between the top and bottom surfaces of the part is 0.48 and the maximum distance between the surfaces is 0.50.
Angular dimensional measurement needs are expressed in GDT specifications as shown in the top of Fig. 6. The basic angle between the top surface and sloped surface of the workpiece is 30. The tolerance on this basic angle is given in the "feature control frame" (see Fig.) which contains the geometric form or interrelationship being controlled (angularity in this case), the tolerance (0.1 in this case), and the datum surface(s) (in this case, A). The meaning of the tolerance as indicated by a "tolerance zone" is shown at the bottom of the Fig. The tolerance zone is bounded by two parallel planes 0.1 apart and 30 from the top surface.
The linear and angular size tolerances described above control the form of an individual feature on a part to a certain extent, but if further refinement of the form is necessary (like flatness, straightness, roundness, profile, cylindricity, or surface texture), a separate geometric form control must be applied (Figures 7 - 12) Also, if interrelationships among features exist (like perpendicularity, parallelism, concentricity, runout, or position), they must be stated in the form of separate geometric interrelationship controls (Figures 13 - 17). These three types of geometric tolerance controls (linear or angular size, geometric form, and geometric interrelationship), then, will fully describe the "function" and "relationships" of the part and its features and it is important to note that a feature on a part should have one or more of these geometric tolerance controls applied to it only once on a drawing- ie., in only one view.
Note that each geometric interrelationship control has a datum reference (e.g., Datum A in Fig. 13). A datum reference is the point, line, plane, or cylinder which is the reference or origin for dimensional measurements involving the feature with the control specification. A distinction must be made at this point between a "datum feature" and a "datum." A datum feature is the actual, often inexact part feature which has been designated as a reference or origin for dimensional measurements. All features on a part are potential datum features. A datum is the theoretical, exact geometric form (point, line, plane or cylinder) of the datum feature. The datum is simulated by the very accurate point, line, plane or cylinder on a gage, instrument, or piece of support equipment. For the purpose of taking dimensional measurements, the datum feature will be brought into contact with the simulated datum (Fig. 18). This contact establishes a nearly exact datum from which to take dimensional measurements and it simulates the functional contact of the part feature with a mating part feature.
Also note that geometric form controls do not require a datum reference, since "form" is an "individual feature" control. The form of a feature is not related to another feature on the part as the "interrelationship" of a feature is. Profile control (a geometric form control) is one exception to the rule, because it may have a datum reference if the profile is defined in terms of the datum feature- ie., the profile could be defined with basic dimensions from the datum feature (Fig. 10b).
Datum features are selected on the basis of the functional design requirements of the part. In other words, those features which are functionally the most important on the part are designated datum features. Functionally important features on a part are those which locate and mount the part in its assembly or in its use. The part shown in Fig. 19 for example, mounts on surface or plane A and is located by surface or cylinder B. Consequently, surface A is selected as the primary datum feature, because it will establish the end-to-end orientation of the part. Surface B is chosen as the secondary datum feature, because it locates the part in the assembly once the part's end-to-end orientation is established.
Fig. 1Examples of Angular Measurement Needs [Fa68]
Fig. 2Examples of Roundness Measurement Needs [Fa68]
Fig. 3Methods of Profile Measurement [Fa68]
Fig. 4Parts Needing Controlled Surface Texture [Fa68]
Fig. 5GDT Specifications for Linear Tolerances
Fig. 6GDT Specifications for Angular Tolerances
Fig. 7GDT Specification for Flatness
Fig. 8GDT Specification for Straightness
(Note: Straightness only applies in the view shown)
Fig. 9GDT Specification for Roundness
(Note: Roundness applies at every cross-section of the workpiece)
Fig. 10aGDT Specifications for Profile Without Datums
Fig. 10bGDT Specification for Profile of a Surface with Datums (Note: One may proceed in a similar manner for Profile of a Line with Datums)
Fig. 11GDT Specification for Cylindricity
Fig. 12Standard Specifications for Surface Texture [Ass 78]
Fig. 13GDT Specification for Perpendicularity [Fo82]
Fig. 14GDT Specification for Parallelism [Fo82]
Fig. 15GDT Specification for Concentricity [Fo82]
Fig. 16aGDT Specification for Circular Runout & Method for
Checking It [Fo82]
Fig. 16bGDT Specification for Total Runout [Fo82]
Fig. 17GDT Specification for Position [Ho87]
Fig. 18GDT Specification of a Datum and a Datum Feature in Contact with a Simulated Datum [Fo82]
Fig. 19Datum Selection Example [Kr86]
MODULE 2: INSTRUMENTS AND SUPPORT EQUIPMENT
Industry needs to make linear, angular, geometric form, geometric interrelationship, and surface texture measurements as was discussed in Module 1. Support equipment and measuring instruments that can meet each of these needs will be introduced in this module and experiments in Module 5 will cover their usage. The factors to be considered when choosing an appropriate instrument for a particular need will be outline. All the instruments and support equipment mentioned are either on display in the laboratory or their photographs are available.
2.1Support Equipment
Support equipment is equipment which aids the measurement process. This equipment usually establishes datums- lines, planes, points, and axes. The most common piece of support equipment is the surface plate. A surface plate is a block of granite, cast iron, or glass. The top surface of the plate serves as a flat reference plane to establish planes and horizontal and vertical axes when measuring. The top surface is lapped flat with diamond dust to within a few ten-thousandths of an inch or less. Surface plates are generally manufactured to three "grades" or
flatness tolerances: these grades are Laboratory, Inspection, and Toolroom with tolerances of 25 microinches, 50 microinches, and 100 microinches, respectively. Variations on these grades and their tolerances do exist among manufacturers; therefore, the specific flatness of any surface plate being used should be determined. The URI laboratory is equipped with two granite surface plates which have been calibrated and re-lapped in the last year. One plate (24" x 36") is flat to within 0.000075 inches ( 75 microinches). The other plate (30" x 48") is flat to within 0.000225 inches ( 225 microinches).
Besides serving as a reference plane, a surface plate can serve as a foundation on which to support precision instruments; as a heat sink - ie., all instruments and workpieces on the surface plate will stabilize in temperature within about 20 minutes if a cast iron plate is being used or within about 60 minutes if a granite or glass plate is being used provided they were at similar temperatures and provided the surrounding atmosphere is being controlled (for either type of plate, several hours may be required for temperature stabilization if the original temperatures of the plate, instruments, and workpieces were not similar); as a stabilizer reducing and nullifying vibrations; and as an aid in work organization and professionalism - the inspector is more organized and tends to be more concerned about cleanliness and accuracy when using a surface plate.
Granite and glass plates have several advantages over the cast iron plate, one being that a dig or scratch on the granite or glass causes only a below-surface depression: the fine particles of granite or glass are removed by the scratching and can be brushed away. However, a dig or scratch in cast iron, as with other metals, causes deformation in the form of metal ridges at the surface level as well as a below-the-surface depression. This is because scratching metal only causes the metal to be displaced, not removed as with granite or glass. The result looks much like a ploughed furrow, with the ridges being a few ten-thousandths or even thousandths of an inch high. Consequently, these ridges can cause serious inaccuracies. Another advantage of granite and glass is that they do not rust. Now, comparing granite and glass, granite is a better choice since granite is not as fragile as glass when subjected to heavy instruments and workpieces. Granite, therefore, is usually the preferred material for surface plates.