Manufacturing Processes- Week 1 ch. 5

Unilateral Tolerance

Variation from the specified dimension is permitted in only one direction Either positive or negative, but not both

What is a dimension?

"a numerical value expressed in appropriate units of measure and indicated on a drawing and in other documents along with lines, symbols, and notes to define the size or geometric characteristic, or both, of a part or part feature"

Dimensions and Tolerances Factors that determine the performance of a manufactured product, other than mechanical and physical properties, include:

-Dimensions - linear or angular sizes of a component specified on the part drawing -Tolerances - allowable variations from the specified part dimensions that are permitted in manufacturing

DIMENSIONS, TOLERANCES, AND SURFACES

-Dimensions, Tolerances, and Related Attributes -Conventional Measuring Instruments and Gages -Surfaces -Measurement of Surfaces -Effect of Manufacturing Processes

Four Elements of Surface Texture

1. Roughness - small, finely‑spaced deviations from nominal surface -Determined by material characteristics and processes that formed the surface 2. Waviness - deviations of much larger spacing -Waviness deviations occur due to work deflection, vibration, tooling, and similar factors -Roughness is superimposed on waviness 3. Lay - predominant direction or pattern of the surface texture (picture below) 4.Flaws - irregularities that occur occasionally on the surface -Includes cracks, scratches, inclusions, and similar defects in the surface -Although some flaws relate to surface texture, they also affect surface integrity

Dimensions (ANSI Y14.5M‑1982)

A dimension is "a numerical value expressed in appropriate units of measure and indicated on a drawing and in other documents along with lines, symbols, and notes to define the size or geometric characteristic, or both, of a part or part feature" The dimension indicates the part size desired by the designer, if the part could be made with no errors or variations in the fabrication process

Cutoff Length

A problem with the Ra computation is that waviness may get included To deal with this problem, a parameter called the cutoff length is used as a filter to separate waviness from roughness deviations Cutoff length is a sampling distance along the surface -A sampling distance shorter than the waviness eliminates waviness deviations and only includes roughness deviations

Tolerances (ANSI Y14.5M‑1982):

A tolerance is "the total amount by which a specific dimension is permitted to vary The tolerance is the difference between the maximum and minimum limits" Variations occur in any manufacturing process, which are manifested as variations in part size Tolerances are used to define the limits of the allowed variation

Accuracy

Accuracy - the degree to which a measured value agrees with the true value of the quantity of interest -A measurement procedure is accurate when it avoids systematic errors (positive or negative deviations that are consistent from one measurement to the next)

Why Surfaces are Important

Aesthetic reasons Surfaces affect safety Friction and wear depend on surface characteristics Surfaces affect mechanical and physical properties Assembly of parts is affected by their surfaces Smooth surfaces make better electrical contacts

Alternative Surface Roughness Equation

Approximation of previous equation is perhaps easier to comprehend (see pic) where Ra has same meaning as above; yi = vertical deviations (absolute value) identified by subscript i; and n = number of deviations included in Lm

Surface Roughness Equation

Arithmetic average (AA) based on absolute values of deviations, and is referred to as average roughness (see picture) where Ra = average roughness; y = vertical deviation from nominal surface (absolute value); and Lm = specified distance over which the surface deviations are measured

Dial Indicator Setup to Measure Runout

As part is rotated about its center, variations in outside surface relative to center are indicated on the dial

Surface Roughness

Average of vertical deviations from nominal surface over a specified surface length

Measurement of Angles

Bevel protractor with Vernier scale (courtesy L. S. Starrett Co.)

Surface Technology

Concerned with: Defining the characteristics of a surface Surface texture Surface integrity Relationship between manufacturing processes and characteristics of resulting surface

Dial Indicator

Dial Indicator showing dial and graduated face (photo courtesy of L. S. Starrett Co.)

Micrometer

External micrometer, standard one‑inch size with digital readout (photo courtesy of L. S. Starret Co.)

Electronic Gages

Family of measuring and gaging instruments based on transducers capable of converting a linear displacement into an electrical signal -Electrical signal is amplified and transformed into suitable data format such as a digital readout -Applications of electronic gages have grown rapidly in recent years, driven by advances in microprocessor technology, and are gradually replacing many of the conventional devices

Plug Gage

Gaging of a hole diameter (difference in diameters of GO and NO-GO plugs is exaggerated)

Snap Gage

Gaging the diameter of a part (difference in height of GO and NO‑GO gage buttons is exaggerated)

Metallic Part Surface

Magnified cross section of a typical metallic part surface

Measurement of Linear Dimensions

Measuring instruments are divided into two types: -Graduated measuring devices include a set of markings on a linear or angular scale to which the object's feature of interest can be compared for measurement -Nongraduated measuring devices have no scale and are used to compare dimensions or to transfer a dimension for measurement by a graduated device

Mechanical Gages: Dial Indicators

Mechanical gages are designed to mechanically magnify the deviation to permit observation Most common instrument in this category is the dial indicator, which converts and amplifies the linear movement of a contact pointer into rotation of a dial -The dial is graduated in small units such as 0.01 mm or 0.001 inch -Applications: measuring straightness, flatness, parallelism, squareness, roundness, and runout

Surfaces

Nominal surface - designer's intended surface contour of part, defined by lines in the engineering drawing -Nominal surfaces appear as absolutely straight lines, ideal circles, round holes, and other edges and surfaces that are geometrically perfect -Actual surfaces of a part are determined by the manufacturing processes used to make them -Variety of processes result in wide variations in surface characteristics

Limit Dimensions

Permissible variation in a part feature size consists of the maximum and minimum dimensions allowed

Precision

Precision - the degree of repeatability in the measurement process -Good precision means that random errors in the measurement procedure are minimized

Conventional Measuring Instruments and Gages

Precision gage blocks Measuring instruments for linear dimensions Comparative instruments Fixed gages Angular measurements

What is a Measurement

Procedure in which an unknown quantity is compared to a known standard, using an accepted and consistent system of units Measurement provides a numerical value of the quantity of interest, within certain limits of accuracy and precision

Stylus Instruments

Similar to the fingernail test, but more scientific In these electronic devices, a cone‑shaped diamond stylus is traversed across test surface at slow speed As the stylus head is traversed horizontally, it also moves vertically to follow the surface deviations The vertical movement is converted into an electronic signal that can be displayed as -Profile of the actual surface -Average roughness value

GO/NO‑GO gages

So-named because one gage limit allows the part to be inserted while the other limit does not -GO limit - used to check the dimension at its maximum material condition -Minimum size for internal feature such as a hole -Maximum size for external feature such as an outside diameter NO‑GO limit - used to inspect the minimum material condition of the dimension in question

Tolerances and Manufacturing Processes

Some manufacturing processes are inherently more accurate than others -Most machining processes are quite accurate, capable of tolerances = 0.05 mm ( 0.002 in.) or better -Sand castings are generally inaccurate, and tolerances of 10 to 20 times those used for machined parts must be specified

Surfaces and Manufacturing Processes

Some processes are inherently capable of producing better surfaces than others -In general, processing cost increases with improvement in surface finish because additional operations and more time are usually required to obtain increasingly better surfaces -Processes noted for providing superior finishes include honing, lapping, polishing, and superfinishing

Precision Gage Blocks

Standards against which other dimensional measuring instruments and gages are compared -Usually square or rectangular blocks -Surfaces are finished to be dimensionally accurate and parallel to very close tolerances and are polished to a mirror finish -Precision gage blocks are available in certain standard sizes or in sets, the latter containing a variety of different sized blocks

Stylus Traversing Surface

Stylus head traverses horizontally across surface, while stylus moves vertically to follow surface profile

Surface Changes Caused by Processing

Surface changes are caused by the application of various forms of energy during processing -Example: Mechanical energy is the most common form in manufacturing -Processes include forging, extrusion, and machining Although its primary function is to change geometry of work part, mechanical energy can also cause residual stresses, work hardening, and cracks in the surface layers

Surface Roughness and Surface Finish

Surface roughness - a measurable characteristic based on roughness deviations Surface finish - a more subjective term denoting smoothness and general quality of a surface -In popular usage, surface finish is often used as a synonym for surface roughness -Both terms are within the scope of surface texture

Surface Integrity

Surface texture alone does not completely describe a surface There may be metallurgical changes in the altered layer beneath the surface that can have a significant effect on a material's mechanical properties Surface integrity is the study and control of this subsurface layer and the changes in it that occur during processing which may influence the performance of the finished part or product

Surface Roughness Specification

Surface texture symbols in engineering drawings: (a) the symbol, and (b) symbol with identification labels

Surface Texture

The topography and geometric features of the surface -When highly magnified, the surface is anything but straight and smooth -It has roughness, waviness, and flaws -It also possesses a pattern and/or direction resulting from the mechanical process that produced it

Measurement of Surface Roughness

Three methods to measure surface roughness: 1. Subjective comparison with standard test surfaces -Fingernail test 2. Stylus electronic instruments 3. Optical techniques

Measurement of Surfaces

Two parameters of interest: Surface texture - geometry of the surface, commonly measured as surface roughness -Surface roughness Surface integrity - deals with the material characteristics immediately beneath the surface and the changes to this subsurface that resulted from the processes that created it

Calipers

Two sizes of outside calipers (photo courtesy of L. S. Starret Co.)

Bilateral Tolerance

Variation is permitted in both positive and negative directions from the nominal dimension Possible for a bilateral tolerance to be unbalanced Ex: 2.500 +0.010, -0.005

What does the dimension indicate?

the part size desired by the designer, if the part could be made with no errors or variations in the fabrication process