Datum reference frame Position and shape tolerances ... · PDF filePosition and shape...
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Šimon Kovář Datum reference frame Position and shape tolerances Tolerance analysis
Transcript of Datum reference frame Position and shape tolerances ... · PDF filePosition and shape...
Šimon Kovář
Datum reference framePosition and shape tolerances
Tolerance analysis
Datum reference frames are typically for 3D. A typicaldatum reference frame is made up of three planes. Forexample, the three planes could be one "face side" andtwo "datum edges". These three planes are marked A, Band C, where A is the face side, B is the first datum edge,and C is the second datum edge. In this case, the datumreference frame is A/B/C. A/B/C is shown at the end offeature control frame to show from where themeasurement is taken.
Datum reference frame
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The engineer selects A/B/C based on the dimensionalfunction of the part. Typically, a part is required to fit withother parts. The functional datums are chosen based onhow the part attaches.Typically, the functional datums are not used tomanufacture the part. The manufacturing datums aretypically different from the functional datums to save cost,improve process speed, and repeatability. A toleranceanalysis may be needed in many cases to convert betweenthe functional datums and the manufacturing datums.
Datum reference frame
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There are typically 6 degrees of freedom that need to beconsidered by the engineer before choosing which feature isA, B,or C. For this example, A is the primary datum, B is thesecondary, and C is the tertiary datum. The primary datumcontrols the most degrees of freedom. The tertiary datumcontrols the least degrees of freedom. For this example, of ablock of wood, Datum A controls 3 degrees of freedom, Bcontrols 2 degrees of freedom, and C controls 1 degree offreedom. 3+2+1 = 6, all 6 degrees of freedom are considered.
Datum reference frame
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Datum reference frame
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Datum reference frame
C
A
B
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Datum reference frame - example
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Datum reference frame
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Datum reference frameConversion of Position (Cylindrical) Tolerance Zones to/from Coordinate Tolerance Zones
tolerance zone tolerance zone
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Position and shape tolerance
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Position and shape tolerance
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Position and shape tolerance
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Position and shape tolerance
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Position and shape tolerance
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Position and shape tolerance
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Geometric Characteristic Symbols
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An example of the application of maximum material requirement to coaxiality is illustrated in the figure below
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An example of the application of minimummaterial requirement
The feature control frame indicates that the 0.8 tolerance applies whenthe hole is at its least material condition, or at size 13.5. At size 12.5, theavailable geometric tolerance would be 0.8 “stated tolerance” + (13.5-12.5 or 1.0 “bonus” tolerance) = 1.8 mm.
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Geometric Characteristic Symbols
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Tolerance Zone
If the zone is identified as a diameter, the
zone is a cylinder.
If there is no zone description, it is
parallel to the surface or a uniform
boundary in the shape of the desired
form.
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Positional tolerancing of holes
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Positional tolerancing of holes
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Profile tolerance
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Profile tolerance
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Concentricity & Symmetry Tolerances
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Angularity & Perpendicularity
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Parallelism & Straightness
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Tolerance analysisIs the general term for activities related to the study of accumulated variation inmechanical parts and assemblies. Its methods may be used on other types ofsystems subject to accumulated variation, such as mechanical and electricalsystems. Engineers analyze tolerances for the purpose of evaluating geometricdimensioning and tolerancing.The choice of method of calculation of tolerances and limit deviations ofdimensional chain components affects manufacturing accuracy and assemblyinterchangeability of components. Therefore, economy of production andoperation depends on it. To solve tolerance relations in dimensional chains,engineering practice uses three basic methods:
• arithmetic method of calculation WC• statistical method of calculation RSS• method of group interchangeability
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Other tolerance analysis are
• Analysis of a dimensional chain deformed as a result of temperaturechange.• Extended statistic analysis of dimensional chain using the "6 Sigma„
method.• Tolerance analysis of a dimensional chain during selective assembly
including optimization of the number of assembled products.
All solved tasks enable work with standardized tolerance values, bothin designing and in optimization of the dimensional chain.
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Theory
A linear dimensional chain is a set of independent parallel dimensionswhich continue each other to create a geometrically closed circuit. Theycan be dimensions specifying the mutual position of components on onepart (Fig. 1) or dimensions of several parts in an assembly unit (Fig. 2).
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TheoryA dimensional chain consists of separate partial components (input dimensions) andends with a closed component (resulting dimension). Partial components (A, B, C,…) aredimensions either directly dimensioned in the drawing or following from previousmanufacturing, possibly assembly operations. The closed component (Z) in the givenchain represents the resulting manufacturing or assembly dimension, which is the resultof combining partial dimensions as a scaled manufacturing dimension, possibly assemblyclearance or interference of a component. The size, tolerance and limit deviations of theresulting dimension depend directly on the size and tolerance of partial dimensions.Depending on how the change of partial component affects the change of the closedcomponent, two types of components are distinguished in dimensional chains:
- increasing components - partial components, the increase of which results in anincrease of the closed component
- decreasing components - partial components, the increase of which results in adecrease of the closed component
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Theory
When solving tolerance relations in dimensional chains, two types of problemsoccur:
• Tolerance analysis - direct tasks, control. Using known limit deviations of all partialcomponents, the limit deviation of the closed component is set. Direct tasks arenambiguous in calculation and are usually used for checking components andassembly units manufactured according to the specific drawing.
• Tolerance synthesis - indirect tasks, designing. Using known limit deviations of aclosed component given by the functional demands, limit deviations of partial components are designed. Indirect tasks are solved when designing functional groups and assemblies.
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Tolerance analysisWorst-caseWorst-case tolerance analysis is the traditional type of tolerancecalculation. The individual variables are placed at their tolerance limits inorder to make the measurement as large or as small as possible. Thismodel predicts the maximum expected variation of the measurement.Designing to worst-case tolerance requirements guarantees 100 % of theparts will assemble and function properly, regardless of the actualcomponent variation. The major drawback is that the worst-case modeloften requires very tight individual component tolerances. The obviousresult is expensive manufacturing and inspection processes and/or highscrap rates. Worst-case tolerancing is often required by the customer forcritical mechanical interfaces and spare part replacement interfaces.
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Worst-case – base example
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Tolerance analysis
Statistical variation RSS (Root Sum Squares)The statistical variation analysis model takes advantage of the principlesof statistics to relax the component tolerances without sacrificing quality.Each component’s variation is modeled as a statistical distribution andthese distributions are summed to predict the distribution of theassembly measurement. Thus, statistical variation analysis predicts adistribution that describes the assembly variation, not the extremevalues of that variation. This analysis model provides increased designflexibility by allowing the designer to design to any quality level, not just100 percent.
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Tolerance analysisRSS (Root Sum Squares) method
This method of calculation is a traditional as well as the mostwidespread method of statistical calculation of dimensional chains. TheRSS method is based on the assumption that individual partialcomponents are manufactured with the level of process capability(quality) 3.
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6 Sigma method(selective assembly)
In case of large production volumes, themean value of the process characteristicshifts in the course of time due to theinfluence of various factors (erroneousset-up, wear of tools and jigs, temperaturechanges, etc.). A shift of 1.5 from theideal value is typical. In case oftraditionally approached processes with3 level of capability, that represents anincrease of the off-size product ratio toapprox. 67000 per one million produced.
In general engineering, the manufacturing process was traditionally considered satisfactorily efficienton level 3. That means an estimated 2700 rejected products per one million produced. Althoughsuch portion of off-size products seems very good at first sight, it is considered ever more and moreinsufficient in some spheres of production. Besides, it is almost impossible to keep the mean value ofthe process characteristic curve exactly in the middle of the tolerance field in the long term.
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Group interchangeability method(selective assembly)
The selective assembly method is used in mass and large-lotproduction of precise products which do not require workinginterchangeability of components inside the product. The assembly ofthe product is preceded by sorting of individual components intotolerance subsets. Manufacturing dimensions of components may beprescribed with a larger tolerance. The narrowed resulting dimensionaltolerance is achieved by the functional matching (combination) ofsorted subsets.
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Group interchangeability method(selective assembly)
The selective assembly method is a very effective method of solvingdimensional chains, enabling a substantial increase inmanufacturing tolerances of partial components and thus asignificant reduction in manufacturing costs. On the other hand,application of this method places increased demands on productassembly. Operating costs will also increase, as it is usuallynecessary to replace the whole assembled component in case ofwear or damage of a partial component.
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http://nvlpubs.nist.gov/nistpubs/jres/104/4/html/j44mac.htm#c1
http://www.mitcalc.com/doc/tolanalysis1d/help/cz/tolanalysis1dtxt.htm
http://www.tcdcinc.com/media/NADCA%20GD&T.pdf
http://www.tec-ease.com/gdt-terms.php
https://books.google.cz/books?id=tesKAAAAQBAJ&pg=PA502&lpg=PA502&dq=datum+reference+symbol&source=bl&ots=MP-wuCczrI&sig=AkmBHRXbA7wGsbNKDkBIwl3K_Sc&hl=cs&sa=X&ei=0HsJVd_tGInAPIj8gbgM&ved=0CD0Q6AEwAw#v=onepage&q=datum%20reference%20symbol&f=false
http://nvlpubs.nist.gov/nistpubs/jres/104/4/html/j44mac.htm#c1
http://www.meadinfo.org/2009/05/gd-datum-better-insight-asme-y145.html
http://www.qualitydigest.com/inside/metrology-column/establishing-datum-reference-frames.html
http://www.roymech.co.uk/Useful_Tables/Drawing/draw_geom_notes.html
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https://in.misumi-ec.com/contents/tech/press/23.html
