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05 A - DIMENSIONS, SURFACES, AND THEIR MEASUREMENT


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Dimensions, tolerances, surfaces
Attributes specified by the product designer and determined by the manufacturing processes used to make the parts and products. Without proper dimensions, tolerances, and surface finish the part would be impossible to assemble or even use. PARTS DON'T MATCH IN ASSEMBLY WITHOUT PROPER DIMENSIONS AND TOLERANCES.
Dimension
Linear or angular size of a component on it's drawing.
Tolerance
A total amount by which a given dimension is allowed to vary.
Measurement
Procedure in which an unknown quantity is compared with a known standard. PROVIDES VALUE WITHIN BOUNDS OF ACCURACY AND PRECISION.
Accuracy
The degree to which the measured value agrees with the true value of the quantity of interest. It is a measurement procedure that is absent of systematic errors. >>> CALIBRATED CALLIPER ACCURATELY MEASURES METICULOUS MILLIMETRES
Precision
Degree of repeatability in measuring process. Good precision means high repeatability and minimal degree of random errors, such as human error, set-up errors, gradual wear, misalignment. >>> RANDOM IDIOTS POOP ON PRECISION
Precision gage blocks
Standard blocks to calibrate measuring instruments. Have high precession which depends on the price. >>> SMOOTH, STANDARD BLOCKS SET STRAIGHT MIGHTY MEASURING MICROMETERS
Graduated measuring devices
Device with markings (graduations), on a linear or angular scale >>> SLIDE CALIPERS, RULERS.
Comparative instruments
Provide dimensional comparisons between two objects, such as a work part and a reference surface, delivering the magnitude and the direction of deviation. >>> DIAL INDICATORS MEASURES ROTATING OFFSET
Fixed gages
PHYSICAL REPLICA OF ASSESSED PART, such as GO - NO-GO gages, plug gages, limit gauges. Used to speed up measurements in production and calibrating measuring devices.
Surface
CONSISTS OF SURFACE TEXTURE, AN ALTERED LAYER, AND SUBSTRATE, the altered layer is the layer affected by the environment wear and tear, corrosion, work hardening, and the substrate is the deeper layer of the surface, a layer which is affected by previous processing of the metal - heat treatment, casting, etc. >>> SURFACE TEXTURE ON THE ALTERED LAYER LAZILY LIE ON THE SUBSTRATE.
Nominal surface
The ideal part surface represented on an engineering drawing.
Importance of surfaces
A GOOD SURFACE MAKES A GOOD PART, due to aesthetics, safety, friction and wear, effect of surface on mechanical and physical properties, mating of components in assembly, and electrical contacts.
Surface integrity
STUDY AND CONTROL OF THE SURFACE LAYER - TEXTURE, ALTERED LAYER, SUBSTRATE AND ANY CHANGES TO IT. Alterations to surface layer could occur during processing or manufacturing: forging, press working, extruding, heat treating, cracks, work hardening. >>> HARD PROCESSING RAPES THE GENTLE SURFACE LAYER.
Surface texture
Repetitive or/and random deviations from a nominal surface. Defined by four features: [1] Roughness - small and tightly packed deviations from nominal surface due to nature of material or manufacturing process [2] Waviness - deviations of larger spacing due to work deflection, vibration, heat treatment. [3] Lay - dominant direction of pattern on surface. [4] Flaws - irregularities such as cracks, scratches, and inclusions. >>> ROUGH, lazy WAVES LAY FLAWS surfaces.
Surface finish
SMOOTHNESS AND QUALITY OF A SURFACE: smooth and shiny like a mirror for N2; rough and course like sand paper for N12. Based on surface roughness (Ra), which is the average of deviations in a certain length. Can vary depending on texture features, such as lay and the waviness, thus, sometimes should be measured on short spans of surface. >>> SMOOTH MIRROR AVERAGELY DEVIATES DEFECTIVE DEFECTS.
Measurement of surface roughness
Measured in three ways: [1] Visual test - comparison with standard surfaces. [2] Stylus instruments - electronic device with a claw that you drag on a surface. [3] Optical technique - shine a light or laser and measure reflectiveness and are very fast. >>> LAZY EYE SPOTS SLOW DIAMOND CLAW IN FAST FLASH OF LASER LIGHT.
Effect of manufacturing process
Manufacturing process determines the surface tolerance >>> N12 FOR CASTING; N7 FOR MILLING; N2 FOR BUFFING.
Evaluation of surface integrity
Surface integrity evaluation techiques can be difficult to do and sometimes even destructive, they include some of the following ones: [1] Surface texture - roughness, lay, and other measures provide some data on integrity. [2] Visual examination - discover cracks, laps, seams with your eyes and some fluorescent lighting. [3] Microstructural examination - metallographic examination of surface cross section. [4] Micro-hardness profile - hardness profile of cross section: give hardness vs. depth from the surface. [6] Residual stress profile - measure of residual stress by X-ray diffraction. >>>SURFACE TEXTURE VISUAL AND MICRO-STRUCTURAL examiner looses HARDNESS PROFILE due TO X-RAYS RESIDUAL STRESS