Chapter 5

Surface Technology

(1) defining the characteristics of a surface, (2) surface texture, (3) surface integrity, and (4) the relationship between manufacturing processes and the characteristics of the resulting surface.

Area Surface Measurement Techniques

A limitation of stylus instruments is that the measurement is taken along the line that is traversed by the stylus, which means it is a two dimensional measurement and a very small sample of the surface. One possible way of doing this is by taking a series of closely spaced profile measurements with a stylus-type instrument and then combining these into a topographical map of the surface.

Surface Finish

A more subjective term denoting smoothness and general quality of a surface.

Fixed Gages

A physical replica of the part dimension to be assessed. 1) Master Gage 2) Limit Gage

Measurement

A procedure in which an unknown quantity is compared with a known standard, using an accepted and consistent system of units. 1. The International System of Units (SI or metric system) (2) the U.S. customary system

Vernier Caliper

A refinement of the slide caliper (movable jaws)

Micrometer

A widely used and very accurate measuring device, the most common form of which consists of a spindle and a C-shaped anvil. 1) Internal or Inside Micrometer 2) External or Outside Micrometer 3) Depth Micrometer (Holes)

Average Roughness

An arithmetic average (AA) is generally used, based on the absolute values of the deviation

Divider

An instrument similar to the caliper is a divider, except that both legs are straight and terminate in hard, sharply pointed contacts; they are used for scaling distances between two points or lines on a surface and for scribing circles or arcs onto a surface.

Flaws

Are irregularities that occur occasionally on the surface; these include cracks, scratches, inclusions, and similar defects in the surface.

Dimensions

Are the linear or angular sizes of a component specified on the part drawing

Surface Integrity

Concerned with the definition, specification, and control of the surface layers of a material (most commonly metals) in manufacturing and subsequent performance in service.

Snap Gage

Consists of a C-shaped frame with gaging surfaces located in the jaws of the frame

Simple Protractor

Consists of a blade that pivots relative to a semicircular head that is graduated in angular units

Slide Caliper

Consists of a steel rule to which two jaws are added, one fixed at the end of the rule and the other movable

Nongraduated Caliper

Consists of two legs joined by a hinge mechanism.

Bevel Protractor

Consists of two straight blades that pivot relative to each other; the pivot assembly has a protractor scale that permits the angle formed by the blades to be read.

Surface Roughness

Defined as the average of the vertical deviations from the nominal surface over a specified surface length.

Waviness

Defined as the deviations of much larger spacing; they occur because of work deflection, vibration, heat treatment, and similar factors.

Nominal Surfaces

Defined by lines in the engineering drawing. The nominal surfaces appear as absolutely straight lines, ideal circles, round holes, and other edges and surfaces that are geometrically perfect.

Mechanical Gages

Designed to mechanically magnify the deviation to permit observation (most common is the dial indicator).

Gaging

Determines simply whether the part characteristic meets or does not meet the design specification.

Electronic Gages

Electronic gages are measuring and gaging instruments based on transducers capable of converting a linear displacement into an electrical signal. (1) good sensitivity, accuracy, repeatability, and speed of response; (2) ability to sense very small dimensions—down to 0.025 μm (1 μ-in); (3) ease of operation; (4) reduced human error; (5) electrical signal that can be displayed in various formats; and (6) capability to be interfaced with computer systems for data processing.

Master Gage

Fabricated to be a direct replica of the nominal size of the part dimension.

Limit Gage

Fabricated to be a reverse replica of the part dimension and is designed to check the dimension at one or more of its tolerance limits.

Surface Plate

For best results, gage blocks must be used on a flat reference surface, such as a surface plate. A surface plate is a large solid block whose top surface is finished to a flat plane

Microhardness Profile

Hardness differences near the surface can be detected using microhardness measurement techniques such as Knoop and Vickers

Graduated Measuring Devices

Include a set of markings (called graduations) on a linear or angular scale to which the object's feature of interest can be compared for measurement.

Altered Layer

Just below the surface is a layer of metal whose structure differs from that of the substrate.

Comparative Instruments

Make dimensional comparisons between two objects, such as a work part and a reference surface.

Nongraduated Measuring Devices

Possess no such scale and are used to make comparisons between dimensions or to transfer a dimension for measurement by a graduated device.

Roughness

Refers to the small, finely spaced deviations from the nominal surface that are determined by the material characteristics and the process that formed the surface.

Standard Test Surfaces

Sets of standard surface finish blocks are available, produced to specified roughness values. To estimate the roughness of a given test specimen, the surface is compared with the standard both visually and by the "fingernail test."

Precision Gage Blocks

Several grades of precision gage blocks are available, with closer tolerances for higher precision grades (master laboratory standard is the highest grade)

Surface Integrity

Study and control of this subsurface layer and any changes in it as a result of processing that may influence the performance of the finished part or product

Surface Texture

Surface roughness, designation of lay, and other measures provide superficial data on surface integrity.

Substrate

The bulk of the part, referred to as the substrate, has a grain structure that depends on previous processing of the metal; for example, the metal's substrate structure is affected by its chemical composition, the casting process originally used on the metal, and any deformation operations and heat treatments performed on the casting.

Precision

The degree of repeatability in the measurement process. Good precision means that random errors in the measurement are minimized.

Accuracy

The degree to which the measured value agrees with the true value of the quantity of interest.

Stylus Instruments

The disadvantage of the fingernail test is its subjectivity. Several stylus-type instruments are commercially available to measure surface roughness—similar to the fingernail test, but more scientific.

Surface Texture

The exterior of the part is a surface whose topography is anything but straight and smooth. In this highly magnified cross section, the surface has roughness, waviness, and flaws. It also possesses a pattern and/or direction resulting from the mechanical process that produced it.

Rule (Steel Rule)

The most basic of the graduated measuring devices.

Plug Gage

The most common limit gage for checking hole diameter.

Thread Gages

The plug is threaded for checking internal threads on parts

Tolerance

The total amount by which a specific dimension is permitted to vary - Unilateral Tolerance - Bilateral Tolerance - Limit Dimensions

Microstructural Examination

This involves standard metallographic techniques for preparing cross sections and obtaining photomicrographs for examination of microstructure in the surface layers compared with the substrate.

Cutoff Length

Used as a filter that separates the waviness in a measured surface from the roughness deviations.

Ring Gages

Used for checking cylindrical diameters; each gage is a ring whose opening is machined to one of the tolerance limits of the part diameter.

Outside Caliper

Used for measuring outside dimensions such as a diameter

Visual Examination

Visual examination can reveal various surface flaws such as cracks, craters, laps, and seams.

Surfaces

What one touches when holding an object such as a manufactured part

Inside Caliper

Used to measure the distance between two internal surfaces.

Residual Stress Profile

X-ray diffraction techniques can be employed to measure residual stresses in the surface layers of a part.

Lay

Predominant direction or pattern of the surface texture

Taper Gages

Consisting of a tapered plug for checking tapered holes