School of EngineeringDepartment of Mechanic EngineeringLaboratory of Fabrication Process
Practice 1:Metrology
Theoretical Framework1
Size (measurable)
Attribute of a phenomenon, body or substance, which it is susceptible of being distinguished qualitatively and determined quantitatively.
NOTE:
1)The word size concern to a magnitude in general way or in a particular way.
EXAMPLE:
a)Magnitude in general way: length, time, mass,temperature, electric resistance, concentration in a substance;
b)Magnitude in particular way: length of a determined stick,electric resistance in a determined tread, concentration of ethanol in a solution of wine.
2)The magnitudes that can be classified each other in order, are called magnitudes of the same nature.
3)Magnitudes of the same nature are in the next groups: work, heat, energy, thickness, circumference, and wavelength.
4)Symbols of the magnitudes are defined in ISO 31 standard.
Basic Magnitude
Any magnitude in a system that is acceptedas an agreement of bothparts is independent for any.
EXAMPLE: Magnitude of length, mass and time generally used as basic magnitude in the mechanic area.
NOTE: the basic magnitudes from the System International of units (SI) are in the table of the note 1.12.
1 Source: Centro Español de Metrología (CEM) Alfar, 2, 28760 - Tres Cantos – Madrid Telephone:
+34918074700
Dimension of a Magnitude
Represented from a system of magnitude as the product of the power factor, which represent the basic magnitude in that system.
EXAMPLES:
a)If we have a system with basic units of length, mass and time, represented as L, M, T each one, the Strong dimension is LMT-2
b)In the same system of magnitudes, ML-3 is the dimension of mass concentration, and the mass density
NOTE:
1)The factor representing a basic magnitude is called the dimension from that basic magnitude.
2)For the dimension’s algebra peculiarity review ISO 31-0.
Unit (measure)
Defined magnitude, which is compared with other magnitudes from the same nature, so they can be expressedquantitatively.
NOTE:
1)Units of measure must be assigned according with their names and symbols.
2)Units of magnitudes with same dimension can have the same name and symbol, even if they are not from the same nature.
International System of Units,SI
System adopted and recommended for the General Conference of Weight and Measure (CGPM).
NOTE:
SI nowadays base in seven basic units:
Magnitude / Name / Symbol / Magnitude / Name / SymbolLength / Meter / M / Thermodynamic Temperature / Kelvin / K
Mass / Kilogram / Kg / Quantity of substance / Mol / mol
Time / Second / S / Luminous Intensity / Candle / cd
Electric current / Ampere / A
Measure
Set of operations, which determine a values magnitude
NOTE: The operation development can be automatically.
Metrology
Science of measure or dimension.
NOTE:Metrology includes every aspect, theoretical and practical, from the measure, no matter the nature or doubt, any field of science and technology.
Doubt of measure
Parameter associated with the final result, values dispersal that might be attributed to the measure.2
NOTE:
1)The parameter can be, for example, a standard deviation or semi width from it with a level of trust determined.
2)Doubt of measure includes some components that can be evaluated from the statistical distribution of some measures, which can be characterized with the experimental standard deviation. The other components, which also can be characterized by standard deviation, are evaluated with the prediction based on experience or any other information.
3)We understand that the total from the measure is the best estimation from the measure value and with the doubt components, even the ones that come from systematic effects, like corrections and patterns of reference, add in the dispersion of the standard deviation.
Tools / Equipment of measure
Gadget, which purpose is take measures, it can be used alone or with some other tools.
Transducer of measure
Device that receive a signal and show a measure according with the law used in that moment.
EXAMPLE:
a)thermocouple
b)current transformer
c)strain gauge
d)pH electrode
2 This definition comes from theGuía para la Expresión de la Incertidumbre de Medida where you can find more detail information (watch 2.2.4 and the annex D[10]).
Measure Equipment display
Equipment, which shows a rate.
EXAMPLE:
a)Indicator analog voltmeter.
b)Digital Frequency meter.
Instrument of measure recorder
Instrument of measure that register a rate.
EXAMPLE:
a)barograph
b)thermoluminescent dosimeter
c)Recording spectrometer
NOTE:
1)The record (display) can be analogic (continuous line or discontinuous) or digital.
2)The values of different magnitudes can be register at the same time.
3)A register instrument also can display information.
General Notes
1)In general, as you raise the hierarchy of the measure, you are demand to explain how you get that average total and the doubt of the answer. However, at any level, from the commercial activities and rules of market, through the industrial engineering, to the primary national laboratories and the “Buro Internacional de Pesos y Medidad” (BIPM), all the information required to re-evaluate the measure process must be available to the one who might need it.
2)Every day a lot of measures are made without explain the doubt rate. So many measures are made with instruments that need periodical calibration or legal inspection. If those instruments are update with the calibrations or any other document that required, you are able to get the necessary information in order to know the doubt rate of the tools from those documents.
3)When you say the final measure and the doubt rate, you should give more information than the required. So as an advise you can do:
a)Describe clearly the used method in order to get the final average and the doubt rate, starting with the experimental observations and the input data.
b)Make a list of all the components of the doubt rate, and record the way you evaluate those components.
c)Show the resultant analyze methodology in a way you can follow easily it step by step, and in case you need it you will be able to repeat that step, getting the same answer.
d)Give all the information, adjustments and constant data used at the experiment with all the sources.
4)Always is important to make yourself the next question: Have you gave enough and clear information, so in the future the experiment can be review and refresh if there appears new information or data?
TOLERANCE AND SIZING
The basic purpose of any manufacturing process is to produce pieces in a determined size and shape. All the required information including size and shape in order to get a good and quality product is in the technical drawing from the part.
However, is impossible to get a measure equal in all the made pieces.Any process no matter how simply it is will have differences.When you have together all these size differences plus the doubt of the own measure in order to get a good design and performance in the product, is necessarily consider a gap in the nominal dimensionspecified in the drawing. This gap consider in the design of the drawing is called Tolerance.
In general you can suppose that with a small tolerance you probable will have a good performance in the piece. However, you can also suppose that with bigger tolerances the piece will be cheaper and easier to fabricate.That’s why the Design engineer is responsible of defining an acceptable tolerance for each dimension in a part, component or assembly and every dimension must have a small tolerance enoughto get a proper performance of the product and wide enough so that the fabrication process will be competitive in the market.
INSTRUMENT OF MEDITION IN THE LABORATORY
Scale
This is the most basic instrument of measure. The scale is use to take measures alone or it can be combined with a compass, with precisionof 1/64” (≈0.015” o 1.4mm).
The appropriate use of the scale combined with other measure instrumentslike inside or outside compass, inside gauge, flex meter, together they are the basis of the dimensional measure.
The correct use of the scale must be learned with practice.
Compass
Before instruments like Vernier were discovered, people use compasses and rules to take measures (inside, outside, dividers, hermaphrodite). For example to take a measure of an outside diameter, you first use the compass to take the dimension, put the piece between the ends of the compass and then putthe compass over the rule to read the measure. Another way to proceed with the compass is using the rule and put the tips of the compass in a specific dimension and then you can start roughing, when the compass get a cross the piece it has the measure you want.
Compassesformeasure
Vernier
Nowadays this is one of the most mechanical instruments used to take measures in the industry. Today are analogue and digital version. It is a lineal instrument; you can take inside, outside measure or deepness. The precision of the Vernier have to be with the graduation of scales, the design of the guides to move it, and the parallelism and perpendicularity of the jaws. All these details also have to be with the quality and stability from the material and the process of fabrication. And finally the most important factor to determine the precision is the ability of the operator that will take the measure.
Analogic VernierDigital Vernier
Micrometer
The micrometer is also a measure tool very convenient and it is used to measure objects with high precision, using data with hundredths of millimeters (0.01mm) or thousands of inches (0.001”).
Micrometer is an instrument that works with two end points, which get closer througha millimeter screw of high precision. Nowadays there is a version analogic and another digital. It is use for outside measures; there are versions of flat faces (cylinders), tips (thinmeasures), plates (to get the measure between gear theet) and some other special versions.The maximum length that the micrometer give us is limited not like the Vernier, but it also give you a higher precision. Usually the length of a micrometer is of 25 mm, the smallest are from 0 to 25 mm, the next one is from 25 to 50 mm and so on.
Telescopic gauges
A telescopic gauge is an indirect measure instrument. You must introduce the top of the gauge (closed) inside a circumference or any space and then you open the top of the gauge and the tips will touch the inside walls from the figure you are measuring. Then you just take out the gauge and then you can use a Vernier or a micrometer to measure the space of the gauge head that will be equal to the inside space in the figure. You must lock the top of the gauge before you take it out from the space you are measuring, in order to have a good measure.
Appropriated use of the telescopic gauges.
Telescopic gauge
First you choose the correct gauge according with the diameter of the piece you want to measure. You close the top of the gauge, until the tips of the head are completely inside and then you lock the top of the gauge spinning a screw until it is fit enough to hold the tips in the top contracted.
The gauge is opened loosing slowly the adjusting screw, always avoiding a sudden shock between the walls of the piece and the gauge.
Put the gauge in the center of the piece, holding the opposite side, they you open the gauge until it touch both walls, always being careful of putting the gauge in the middle of the piece.
Adjust the screw so the top of the gauge will keep the size measured and then take it out the piece without modifying that size.
Use a Vernier or a micrometer in order to measure the size took with the gauge.
COORDINATE MEASURING MACHINE
There is a hugevariety of coordinate measuring machines (CMM) around the world. There are too many machines of general use, but there are also other machines made to achieve specific works in special components or for defined industries. These kinds of machines are common in the automotive, aerospace, molds, tools, die industries, etc.
Every CMM has a settle table with a mobile structure which holds a tip. The tip is a device of contact that forms part of the measure tool and it is made of a precision globe (or even with a pointed tapered shape), it is overlap in a handle connected mechanically with a position sensor, able to detect coordinates with precision in the space.
An essential piece of the CMM is the tip because through this one the surface that will be measure is touched, in order to measure a dimension in a flat surface, the tip must touch the surface.
These tip or contact detectors are known as TTPs (Touch Trigger Probes).These are located in an assembly of springs with three poles and three pairs of balls. The three pairs of balls and the three rollers, that at the same time are connected to make an electrical circuit, can be easily take out of balance with any move at the tip. As is shown in the next image, if you need to measure X, then the tip is move until it is touching the face A. So then the CMM determine the 3D position of that point. After that you need to move the sensor to the point B and touch it, so the dimension of Xwill be the difference between the two points and also with the diameter of the tip. There are so many kinds of tips shapes that can be use in order to make easy measure parts hard to achieve.
The precision of this kind of measuresdepend on factors like the kind of tip, the CMM, the controller and the software that process the information, also the quality of the measure made. The precision of this kind of machines is of 0.01 mm (≈0.0005”) and the repeatability of these machines are from 0.02 mm (≈0.001”).
Besides that this machines are easy to handle, this measuretools can be operate quickly.
Goals
1)The student will understand the security rules relevant for this practice.
2)The student will recognize the way that the measure must be done, and the conventional measures tools.
3)The student will know most frequently measure tools used and conventional in factories.
4)The student will distinguish how to understand the measures and the doubt associated with it.
5)The student will use the conventional measure tools (scale, Vernier, Micrometer) in order to get the dimensions of a simple piece.
6)The student will learn abilities, and limitations of measures instruments like the optical comparator and the CMM.
SECURITY
To use the basic measure tools is necessary to practice the next advises.
WARNING! / REASONDo not apply excessive force in the measure tools / This can make a permanent deformation in the tool.
Do not take out any measure tool from the metrology area. / This area has controlled temperature and taking out the instrument from this environment can make wrong measures because of thermal dilatation.
Clean the piece and the contact area between the piece and the measure tool. / Get a right measure.
Keep always a clean measure tool. / Do not get the tool out of calibration and then do not lose precision.
To operate the coordinate machine you must follow beside the last warnings, the next advises.
WARNING! / REASONUse an air pressure of 3 bar (Check out the manometer behind the machine). / To achieve stability and make precision measures.
Do not hit the tip of the CMM. / The tips are made of beautiful materials (jewels like ruby) and they are delicate, brittle and expensive.
Do not make any sudden movement with the axis of the CMM. / Hitting and excessive force in the machine will take out of balance the machine.
WARNING! / REASON
Work with the machine only with regulated electrical power supply. / Avoid any oscillation of voltage and any possible electric shock because may damage the transducer in the machine.
Keep fresh environment while the machine is working. / The machine has electronic devices highly sensitives to heat.
The user SHOULD NOT lean on any part of the CMM. Beside, SHOULD NOT get up the table or the machine any other piece than the one that will be measured, write on the marble, or any other action like these. / Any mechanical deformation will take out the calibration of the machine.
MATERIAL, TOOLS AND EQUIPMENT
1)Material
a)Pieces for manual measures
b)Pieces for coordinate measures.
2)Tools
a)Scale, inside and outside compass
b)Vernier
c)Micrometer
3)Equipment
d)CMM Poli with tip and centering sphere.