Manufacturing Processes Chap 35 Metrology Metrology Definition The

Manufacturing Processes Chap. 35 - Metrology

Metrology • Definition: – “The measurement of dimensions”. • Relevance: – Dimensions are measured to ensure that a part is manufactured consistently and within the specified range of accuracy.

Inspection Objective: • To provide feedback information on the actual size of the part, with respect to (wrt) the specified size in engineering drawing. • Traditionally measurements are taken after a part or component has been manufactured. • New trend is for “in-process” inspection, taking measurements while the part is being produced.

Standard Measuring Temperature • Instruments are typically calibrated at 20 deg C / 68 deg F. • One should try to take measurements at this temperature to favor accuracy. • This is the standard for precision measuring work. If you are measuring accuracy greater that 0. 0001”, you usually measure in a controlled environment (ISO regulated).

Allowance vs Tolerance • Needed when you want to fit two mating parts. • Allowance: Intentional difference in dimensions between mating parts. (~ Smallest exterior fitting part – Largest interior fitting part) (smallest hole – largest shaft) - Determines the tightest fit between the parts. Allowance can be specified as: - A clearance: largest shaft is smaller than smallest hole. - An interference: hole is smaller that shaft.

Allowance vs Tolerance • Tolerance: Undesirable but permissible deviation from a desired dimension. • Reason: No part can be made exactly to a specified dimension, except by chance. • Such level of exactness is not necessary or economical. • It is necessary to allow deviation from theoretical or nominal value. • Deviation must be controlled so parts will function well together.

Allowance vs Tolerance • Relevance: Tolerances impact the proper functioning and manufacturing cost of a part. The smaller the tolerance, the higher the manufacturing cost. Important only when a part is to be assembled or mated with another part. Free/non-functional surfaces do not need close tolerances.

Dimensional Tolerances Can specify bilateral, lateral or limits. • Bilateral: 2. 000” 0. 002” • Unilateral: 2. 000” + 0. 000” - 0. 004” • Limits: 2. 002” 1. 998” 2. 004” 2. 000”

Fits • Fits are categorized in classes, 1 through 8, and are specified according to the application. – Loose, Free, Medium, Snug, Wringing, Tight, Medium, Heavy Force and Shrink Fit • For example: – loose implies a large allowance where accuracy is not essential. – snug means zero allowance, no motion desired, tightest achievable manual fit. – shrink fit: large negative allowance, used for permanent shrink on steel members.

Geometric Tolerances • Maximum allowable deviation of a form or a position from a perfect geometry (as implied by an engineering drawing). • Tolerance represents the diameter or width of a zone required for part accuracy. – Form tolerances: Flatness, straightness, roundness, cylindricity. – Profile: Line, Surface – Orientation: Angularity, Perpendicularity, Parallelism – Location: Position, Concentricity

Datums • Reference entities from which tolerances are specified or stated. They can be a point, an axis, a plane or surface. • Up to 3 datum surfaces can be used to specify tolerance. • Ex.

Datum Reference examples • Straightness: indicates the limits of how much a surface or axis can bow wrt a straight line. • Flatness: The planar surface must lies between two parallel planes 0. 50” apart. • Perpendicularity: (Vertical) plane must be perp. To the reference within 0. 5” • Circularity Cylindricity: – circular feature must be within a tolerance defined by two concentric circles cylinders. • Profile: acceptable deviation of an outline of an object from that specified.

Key Terms: Accuracy: degree of agreement between measured dimension and its true magnitude. Precision: degree to which an instrument gives a repeated measurement. Resolution: smallest dimension that can be read on an instrument. Rule of 10: “An instrument should be 10 times more precise than the dimensional tolerances of the part being measured”: ~ gage capability.

Factors for selecting a proper measuring instrument: • Gage capability: – (the gage must be 10 times more precise than the tolerance being measured). • Linearity: – is calibration accurate over entire measuring range? • Repeatability: – can I take the same reading over and over a standard? • Stability: – is calibration stable over time? How sensitive is it to temperature, humidity? • Sensitivity / resolution: – the smallest difference in dimensions the instrument can detect. • Magnification: – the more accurate the device, the greater the magnif. factor it should have.

Factors for selecting a proper measuring instrument: • Size and type of part or features to be measured • Environmental conditions • Required Operator Skills • Cost of Equipment • Speed

Factors that contribute to deviation of dimensions: • Static/Dynamic deflections fluctuating forces. due to vibrations and • Variations in properties and dimensions of incoming material. • Distortion due to temperature changes. • Tool wear. • Human error.

Length Standards in Industry • Gage Blocks – Provide industry with linear standards of high accuracy. – Used in everyday manufacturing. – Conceived by Carl Johansson in 1900. – Are highly precise, individual square, round or rectangular blocks of various sizes. – Can be assembled to achieve different lengths. – Flat surfaces are ground to a mirror finish.

Length Standards in Industry • Gage Blocks – Have two very flat and parallel surfaces at a specified distance apart. Flatness / parallelism within. 00002”. – Commonly used as accurate reference lengths. – Are heat treated to relieve internal stresses and minimize dimensional change. – Can assemble by sliding one past another with hand pressure. Can build any desired dimension.

Gages

Line Graduated Instruments Linear – Direct Reading Steel Rules/Scales: for making linear measurements; accuracy up to 0. 040”. Vernier Calipers: for measuring inside and outside lengths; Accuracy up to 0. 001”. Also come with digital readouts: less subject to human errors. Micrometers: for measuring thickness, inside or outside dimensions of parts. Accuracy up to 0. 0001”. Digital mikes can be hooked up to a PC for statistical process control.

Line Graduated Instruments

Line Graduated Instruments Linear – Indirect Reading • Calipers/ Dividers: used to transfer the measured size to a direct –reading instrument like a rule. Has limited accuracy.

Angle Measurement • Surface Plate: – A horizontal slab usually made of cast iron or natural stone (granite). – Used for its low thermal expansion, resistance to corrosion and being non-magnetic. • Angle Gage Blocks: – can be assembled in various combinations and are used similarly to a sine bar.

Angle Measurement

Angle Measurement Bevel Protractor: Place blades of protractor against part. Combination Square: Used for 45 and 90 degree angles. Sine Bar: Place part on an inclined bar and adjust angle via gage blacks on a surface plate. Use dial indicator to scan the surface of the part.

Angle Measurement

Comparative Length - Dial Indicators: mechanical device that converts linear displacement of a pointer to a rotation of an indicator on a circular dial. Accuracy up to. 00004”. - Electronic Gages: senses motion of contact pointer through changes in resistance of a strain gage. - Advantages: ease of operation, rapid response, digital readout, reduced possibility of human error.

Comparative Length

Non-Contact Instruments - Laser Scan Micrometer: Used for rotating, vibrating, high temperature or delicate parts. Resolution up to. 000005”.

Measurement of Geometric Features • Straightness: – can use a straightedge, dial indicators, transits, or laser beams. • Flatness and Perpendicularity: – can measured via a surface plate and a dial indicator. • Roundness: – the deviation from true roundness (a perfect circle). Critical for proper functioning of rotating shafts, pistons, etc.

Measurement of Geometric Features - Full Indicator Movement Method: - Place round part on V-Block, rotate the part while an indicator touches the surface. Rotate the part 1 full turn. Difference between max and min reading is the TIR (total indicator reading). • Profile: - Measure via a profile gage or template or dial indicators. • Threads: - Measure with thread plug gages, screw-pitch gages, snap gages. • Contours: - Optical Comparators are used to check profiles on a screen to which an image is projected.

Coordinate Measuring Machines • Consists of a surface plate and a bridge to which a ram is attached. A probe is fixed at the end of the ram. • Machine can be programmed or taught to move to different locations and take specific measurements. • It is a high-speed measurement instrument, with accuracy up to. 00001”. • Machine at MRC has accuracy of 0. 00015”.

Gages • Fixed Gages: indicate whether part too large is asmall to orcompared to an established dimension. Do not measure actual dimensions. • Plug Gages: typically used for holes. Has two sides: a GO and a NO-GO side is smaller. GO side slides into a hole smaller that the gage diam. NO-GO side will not go into the hole. • Ring gages: used for shafts or similar round parts. • Snap gages: used to measure external dimensions. Have adjustable gaging surfaces that can be set to create a GO NO-GO gage.

Optical Instruments • Used to measure surfaces that are too delicate or small for contact inspection instruments. - Microscopes: used to measure very fine details on small workpieces. Toolmaker’s Microscope can read up to. 0001”. - Fiberscopes and Boroscopes: used when surfaces are inaccessible to the instrument. Used to inspect engine turbine blades without disassembly. - SEM: Magnification up to 100, 000 X. Excellent detail can be seen.

Optical Instruments