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Outline

Title
Abstract
Figures
Casting Methods
Other Expendable Mould Methods
Other Permanent Mould Methods
Casting Materials
Introduction to Cast Irons
Other Materials
Designing Castings
Design Intent
Background
Summary
Introduction
Conclusion and Recommended Further Actions
References

Parametric Modelling and Quality Verification of Castings

Profile image of Juhani OrkasJuhani Orkas

2018

Abstract

Parametric Modelling and Quality Verification of Castings Master programme Master's Programme in Mechanical Engineering Code ENG25 Thesis supervisor Prof. Juhani Orkas Thesis advisor(s) M.Sc. (Tech.

Figures (45)
By alloying iron with relatively large quantities of carbon and silicon, the composition is brought near the eutectic point to lower the melting point, improve fluency and allow the forming of graphite. Moreover, cooling rate and inoculation providing crystallisation nuclei also affect the amount of graphite, with slow cooling resulting in more graphite and less cementite. As opposed to graphite irons, white irons contain no free carbon as graphite, but the carbon is bound as carbides. [5] A microstructure consisting of lamellar layers of ferrite and cementite is called pearlite. The layers are oriented in the same direction within a grain, but the orientation varies from grain to grain. [3] Pear itic structure is achieved in austenitic transformation with a relatively low cooling rate. [5] | Even though cementite is hard and brittle, the greater proportion of ferrite makes pearlite still quite ductile. [4] Also bainite can be produced in the austenitic transfor- mation. Bainite is a strong steel microstructure consisting of ferrite and cementite, shaped as very fine needles or plates. Bainite requires faster cooling in order to prevent the austenite from diffusing into pearlite. [5][3] When the material is quenched into even lower tempera- tures, strong and hard martensite is formed, another allotrope of iron. Ledeburite is a eu- tectic mixture of austenite and cementite found in white irons. Ledeburitic microstructure is hard and brittle, and is formed with fast cooling and low silicon content. [4]
By alloying iron with relatively large quantities of carbon and silicon, the composition is brought near the eutectic point to lower the melting point, improve fluency and allow the forming of graphite. Moreover, cooling rate and inoculation providing crystallisation nuclei also affect the amount of graphite, with slow cooling resulting in more graphite and less cementite. As opposed to graphite irons, white irons contain no free carbon as graphite, but the carbon is bound as carbides. [5] A microstructure consisting of lamellar layers of ferrite and cementite is called pearlite. The layers are oriented in the same direction within a grain, but the orientation varies from grain to grain. [3] Pear itic structure is achieved in austenitic transformation with a relatively low cooling rate. [5] | Even though cementite is hard and brittle, the greater proportion of ferrite makes pearlite still quite ductile. [4] Also bainite can be produced in the austenitic transfor- mation. Bainite is a strong steel microstructure consisting of ferrite and cementite, shaped as very fine needles or plates. Bainite requires faster cooling in order to prevent the austenite from diffusing into pearlite. [5][3] When the material is quenched into even lower tempera- tures, strong and hard martensite is formed, another allotrope of iron. Ledeburite is a eu- tectic mixture of austenite and cementite found in white irons. Ledeburitic microstructure is hard and brittle, and is formed with fast cooling and low silicon content. [4]
Grey iron is typically used in low-cost applications with moderate mechanical load. How- ever, its other properties allow using it more widely. For its heat conductivity and lubricating properties, grey iron is a valid option for e.g. combustion engine components. Because of its vibration-damping ability, it is well suitable for machine bodies and frames. [5]
Grey iron is typically used in low-cost applications with moderate mechanical load. How- ever, its other properties allow using it more widely. For its heat conductivity and lubricating properties, grey iron is a valid option for e.g. combustion engine components. Because of its vibration-damping ability, it is well suitable for machine bodies and frames. [5]
Figure 4. Exemplary drawings according to ISO 8062-2. A) accumulation method with ISO 8062-3 general tolerances, b) multiple tolerancing method with general surface profile tol- erances. Symbols ® and Q indicate the final moulded and machined conditions of the cast- ing, respectively. [14] Without the envelope requirement, the form tolerance of machined part tryr is left out from he expected he equation. As seen from | depend on the form and dimensional tolerances o olerance depends on the nominal dimension of the casting. Therefore, initial values for the olerances trcr and tpcr must be chosen for the ca known based on a guess or the machined dimension. The result does not necessarily match range within which the first guess fal Equations (1) and (2), the nominal dimensions of the casting f the casting. Contradictorily, the casting culation before the nominal dimension is s, and another calculation round must be done, now with casting tolerances chosen based on the first result. As tolerance values are directly used in the calculation of casting nomina count only t he worst case of each tolerance. This dimensions, the outcome takes into ac- might lead to an unnecessary increase in the amount of material removed by machining. The effect can be diminished by assigning smaller tolerances. [14]
Figure 4. Exemplary drawings according to ISO 8062-2. A) accumulation method with ISO 8062-3 general tolerances, b) multiple tolerancing method with general surface profile tol- erances. Symbols ® and Q indicate the final moulded and machined conditions of the cast- ing, respectively. [14] Without the envelope requirement, the form tolerance of machined part tryr is left out from he expected he equation. As seen from | depend on the form and dimensional tolerances o olerance depends on the nominal dimension of the casting. Therefore, initial values for the olerances trcr and tpcr must be chosen for the ca known based on a guess or the machined dimension. The result does not necessarily match range within which the first guess fal Equations (1) and (2), the nominal dimensions of the casting f the casting. Contradictorily, the casting culation before the nominal dimension is s, and another calculation round must be done, now with casting tolerances chosen based on the first result. As tolerance values are directly used in the calculation of casting nomina count only t he worst case of each tolerance. This dimensions, the outcome takes into ac- might lead to an unnecessary increase in the amount of material removed by machining. The effect can be diminished by assigning smaller tolerances. [14]
Figure 5. Surface profile tolerance definition. A) drawing indication, b) interpretation. [16] Even though general surface profile tolerance is recommended in ISO 8062-2, the require- ments and instructions for its use are quite vague. Defining the datum system for surface profile tolerancing is only presented as an example instead of specifying the requirements for utilisation in casting design, e.g. the datum system is not properly defined. In addition, the calculation of nominal dimensions is specified only for ISO 8062-3 tolerances.
Figure 5. Surface profile tolerance definition. A) drawing indication, b) interpretation. [16] Even though general surface profile tolerance is recommended in ISO 8062-2, the require- ments and instructions for its use are quite vague. Defining the datum system for surface profile tolerancing is only presented as an example instead of specifying the requirements for utilisation in casting design, e.g. the datum system is not properly defined. In addition, the calculation of nominal dimensions is specified only for ISO 8062-3 tolerances.
As the datum system RST is placed on surfaces remaining unmachined, the machining should also be related to the datum system RST. When the casting nominal dimensions are calculated as in Figure 7 and Equation (3), the casting and machining datums are the same RST, and the measured casting surfaces are entirely within the tolerance zone, the final ma- chined condition is always achieved. If the machining tolerances are related to a different datum system, there is no guarantee that enough material exists for successful machining, and additional material is needed. [17]
As the datum system RST is placed on surfaces remaining unmachined, the machining should also be related to the datum system RST. When the casting nominal dimensions are calculated as in Figure 7 and Equation (3), the casting and machining datums are the same RST, and the measured casting surfaces are entirely within the tolerance zone, the final ma- chined condition is always achieved. If the machining tolerances are related to a different datum system, there is no guarantee that enough material exists for successful machining, and additional material is needed. [17]
Figure 9. Bridge CMM and an angled touch probe with a ruby stylus tip. [22] Coordinate measuring machines or CMMs are measuring devices used extensively in man- ufacturing industries for quality verification. CMMs measure the geometry of a physical object by coordinates of discrete poin position. The interface between the m. contact or proximity of the measured surface. Varying types of typical is a touch probe that triggers t probes exist, but he measurement when it obtains physical con the measured object. The machine measures and calculates the coordinates of t where the tip of the probe is in contact with the surface. The con usually a hard, wear-resistant and very accurate sphere at the tip of a stylus. Ruby is a prom- inently used material for stylus tips. Other types of probes include laser probes that require no contact with the measured surface , but measure the distance acting part of the s on the surfaces of the object in relation to a reference achine and the measured surface is a probe that senses he most act with he point probe is between the probe and the surface using e.g. laser triangulation [21]. Examples of a CMM and a touch probe are shown in Figure 9.
Figure 9. Bridge CMM and an angled touch probe with a ruby stylus tip. [22] Coordinate measuring machines or CMMs are measuring devices used extensively in man- ufacturing industries for quality verification. CMMs measure the geometry of a physical object by coordinates of discrete poin position. The interface between the m. contact or proximity of the measured surface. Varying types of typical is a touch probe that triggers t probes exist, but he measurement when it obtains physical con the measured object. The machine measures and calculates the coordinates of t where the tip of the probe is in contact with the surface. The con usually a hard, wear-resistant and very accurate sphere at the tip of a stylus. Ruby is a prom- inently used material for stylus tips. Other types of probes include laser probes that require no contact with the measured surface , but measure the distance acting part of the s on the surfaces of the object in relation to a reference achine and the measured surface is a probe that senses he most act with he point probe is between the probe and the surface using e.g. laser triangulation [21]. Examples of a CMM and a touch probe are shown in Figure 9.
Another commonly used CMM type is an articulated arm coordinate measuring machine (AACMM), shown in Figure 10. As the name suggests, the probe is mounted at the end of an arm consisting of several beams assembled together with rotational joints. Whereas bridge CMMs are more immobile measuring stations, AACMMs are often portable, and the probe is moved and contacted with the measured surface by hand. The contacting probe utilises similar techniques as any CMMs, but the coordinates are calculated based on the angles of the joints measured with rotary encoders. The angular positions of the axes are converted into Cartesian coordinates of the measured point with a series of coordinate transformations. [24]
Another commonly used CMM type is an articulated arm coordinate measuring machine (AACMM), shown in Figure 10. As the name suggests, the probe is mounted at the end of an arm consisting of several beams assembled together with rotational joints. Whereas bridge CMMs are more immobile measuring stations, AACMMs are often portable, and the probe is moved and contacted with the measured surface by hand. The contacting probe utilises similar techniques as any CMMs, but the coordinates are calculated based on the angles of the joints measured with rotary encoders. The angular positions of the axes are converted into Cartesian coordinates of the measured point with a series of coordinate transformations. [24]
Figure 13. Structured light imaging principle [44] and a fringe pattern projected on a Wheel rim [27]. Structured light technique is an active imaging method where a projector illuminates the object with a light pattern. The method is demonstrated in Figure 13. Regular light stripes are projected onto the object, but the camera sees them as distorted because of the 3D shape of the object. Since the orientation of the projector and the camera are known, the distance calculation again relies on triangulation. Each point on each stripe is captured into different position in the image sensor of the camera, whereu similar to that of laser triangulation. The difference han being scanned with a laser beam or laser stripe, using the stripe pattern. This makes the technique pon the point coordinate calculation is between the two methods is that rather the whole scene is illuminated at once fast and allows scanning even moving argets, but the projected patterns and methods used may slow done the data acquisition rate because of multiple shots of the scene or heavy calcu ation. Structured light scanner can save as many data points as the camera has pixels, but the resolution of the projector limits the number of stripes in the projected pattern.
Figure 13. Structured light imaging principle [44] and a fringe pattern projected on a Wheel rim [27]. Structured light technique is an active imaging method where a projector illuminates the object with a light pattern. The method is demonstrated in Figure 13. Regular light stripes are projected onto the object, but the camera sees them as distorted because of the 3D shape of the object. Since the orientation of the projector and the camera are known, the distance calculation again relies on triangulation. Each point on each stripe is captured into different position in the image sensor of the camera, whereu similar to that of laser triangulation. The difference han being scanned with a laser beam or laser stripe, using the stripe pattern. This makes the technique pon the point coordinate calculation is between the two methods is that rather the whole scene is illuminated at once fast and allows scanning even moving argets, but the projected patterns and methods used may slow done the data acquisition rate because of multiple shots of the scene or heavy calcu ation. Structured light scanner can save as many data points as the camera has pixels, but the resolution of the projector limits the number of stripes in the projected pattern.
Figure 14. Structured light projection patterns. A) binary, b) grey code, c) phase shift, d) segment pattern, e) colour, f) colour coded grid. Adapted from [44]. Other options for the projected pattern are shown in Figure 14 e) and f). In Figure 14 e), the wavelength of the projected light is varied across the pattern, resulting in a rainbow-coloured pattern. The stripe indexing is based on the different wavelengths, all visible at once. There- fore, only one shot is needed, and each point location can be calculated quickly with trian- gulation, making the system fast and suitable for also dynamic objects. [44] Figure 14 f) shows a colour-coded grid, where also horizontal locations can be identified with projected colours. Many other projecting methods exist based on e.g. coloured stripes, dot arrays or phase shifting with multiple frequencies. [44][46]
Figure 14. Structured light projection patterns. A) binary, b) grey code, c) phase shift, d) segment pattern, e) colour, f) colour coded grid. Adapted from [44]. Other options for the projected pattern are shown in Figure 14 e) and f). In Figure 14 e), the wavelength of the projected light is varied across the pattern, resulting in a rainbow-coloured pattern. The stripe indexing is based on the different wavelengths, all visible at once. There- fore, only one shot is needed, and each point location can be calculated quickly with trian- gulation, making the system fast and suitable for also dynamic objects. [44] Figure 14 f) shows a colour-coded grid, where also horizontal locations can be identified with projected colours. Many other projecting methods exist based on e.g. coloured stripes, dot arrays or phase shifting with multiple frequencies. [44][46]
to the parameter change according to the defined constraints, and subsequently, every feature defined after the affected feature in the model tree is regenerated and changed according to the pre-defined rules. As a result, the parametrised model delivers different configurations of the product with little effort when different initial values are assigned to the initial param- eters. The parameter values can be changed either inside the CAD software or in a PDM system to better integrate the product configuration creation to production control and order management.
to the parameter change according to the defined constraints, and subsequently, every feature defined after the affected feature in the model tree is regenerated and changed according to the pre-defined rules. As a result, the parametrised model delivers different configurations of the product with little effort when different initial values are assigned to the initial param- eters. The parameter values can be changed either inside the CAD software or in a PDM system to better integrate the product configuration creation to production control and order management.
Figure 17. Design intent reaffirmed with 3D annotations. [61] pointing to the relevant features in the model, clarifying the proper alteration convention. [61] An example of conveying design intent using annotation is shown in Figure 17.
Figure 17. Design intent reaffirmed with 3D annotations. [61] pointing to the relevant features in the model, clarifying the proper alteration convention. [61] An example of conveying design intent using annotation is shown in Figure 17.
problem reverts to the standardisation of modelling procedures considering the ease of edit- ing, reusability, utilisation of parametrisation and the needs of the company. The modelling methods determine how well the model can be parametrically modified. Therefore, compa- nies often establish their own modelling standards to suit their specific needs. Numerous different standardised methods and procedures have been established [59] some of which have been even patented [58].
problem reverts to the standardisation of modelling procedures considering the ease of edit- ing, reusability, utilisation of parametrisation and the needs of the company. The modelling methods determine how well the model can be parametrically modified. Therefore, compa- nies often establish their own modelling standards to suit their specific needs. Numerous different standardised methods and procedures have been established [59] some of which have been even patented [58].
Figure 19. NMX hoisting machine and its main components. 1: rear body, 2: front body, 3: rotor and traction sheave, 4: brakes. A traction sheave of an NMX hoisting machine was chosen for the tolerance model case study. An NMX hoisting machine and its main components are presented in Figure 19, and the traction sheave in more detail in Figure 20. A simplified casting drawing of the traction heave is presented in Appendix A. The traction sheave is the wheel that drives the ropes onnected to the car and the counterweight. The traction sheave also acts as a rotor for the ectric machine. Permanent magnets are installed onto the traction sheave, and the rear body ouses the coils. The casting material is ductile iron. Considering the material and the size f the traction sheave, the most suitable manufacturing method is conventional sand casting with machine moulding. The traction sheave was chosen for the case study because of its criticality to the whole hoisting machine operation, but also for its relatively simple geome- try. The axial symmetry of the casting makes it a suitable test piece for the parametrisation. Compared to for example the front and rear bodies, the benefits of a simpler shape are obvi- ous. aQaon onpoa
Figure 19. NMX hoisting machine and its main components. 1: rear body, 2: front body, 3: rotor and traction sheave, 4: brakes. A traction sheave of an NMX hoisting machine was chosen for the tolerance model case study. An NMX hoisting machine and its main components are presented in Figure 19, and the traction sheave in more detail in Figure 20. A simplified casting drawing of the traction heave is presented in Appendix A. The traction sheave is the wheel that drives the ropes onnected to the car and the counterweight. The traction sheave also acts as a rotor for the ectric machine. Permanent magnets are installed onto the traction sheave, and the rear body ouses the coils. The casting material is ductile iron. Considering the material and the size f the traction sheave, the most suitable manufacturing method is conventional sand casting with machine moulding. The traction sheave was chosen for the case study because of its criticality to the whole hoisting machine operation, but also for its relatively simple geome- try. The axial symmetry of the casting makes it a suitable test piece for the parametrisation. Compared to for example the front and rear bodies, the benefits of a simpler shape are obvi- ous. aQaon onpoa
Figure 20. Traction sheave with functional surfaces indicated. 1: rope grooves, 2: brake surface, 3: bearing housings, 4: magnet surface, 5: possible fixing planes for machining.
Figure 20. Traction sheave with functional surfaces indicated. 1: rope grooves, 2: brake surface, 3: bearing housings, 4: magnet surface, 5: possible fixing planes for machining.
The tolerance models are created as two Family Table instances of the generic part as shown in Figure 21. The dimension of the offset feature is selected as the only Family Table param- eter to be changed between model instances. The offset value is set to half the surface profile tolerance and to positive for one instance and negative for the other. The dimension must be set contain an absolute value to avoid changing the direction of the feature each time the model is regenerated as Creo does as default. Now the model has two solid models created as Family Table instances, representing the surfaces between which the real measured sur- face must fit. The comparison between the nominal and the tolerance models is shown in Figure 22.
The tolerance models are created as two Family Table instances of the generic part as shown in Figure 21. The dimension of the offset feature is selected as the only Family Table param- eter to be changed between model instances. The offset value is set to half the surface profile tolerance and to positive for one instance and negative for the other. The dimension must be set contain an absolute value to avoid changing the direction of the feature each time the model is regenerated as Creo does as default. Now the model has two solid models created as Family Table instances, representing the surfaces between which the real measured sur- face must fit. The comparison between the nominal and the tolerance models is shown in Figure 22.
Figure 23. Top left: the end of the model tree. The bordering round is added after the offset features. The last two extrudes are the text markings. Top right: the relations for round radii when using individually indicated surface profile tolerances. Bottom: tolerance parameters in Family Table.
Figure 23. Top left: the end of the model tree. The bordering round is added after the offset features. The last two extrudes are the text markings. Top right: the relations for round radii when using individually indicated surface profile tolerances. Bottom: tolerance parameters in Family Table.
Figure 24. Tolerance of the 18 mm dimension replaced with an individual surface profile tolerance tighter than the general surface profile tolerance. Left: all original radii. Right: radii changed by offset value. model and inner round as it should. More individual surface profile tolerances could be de- fined similarly as long as the bordering round features are taken into account as the ones in Figure 23. A downside of defining round radii in relations is that the nominal radii cannot be modified directly in the model or by editing the feature in the tree. Instead, the nominal dimension is hidden in the relations. Creo notifies the user when trying to modify a dimen- sion defined in the relations window, but it is easily missed by a designer other than the creator of the model.
Figure 24. Tolerance of the 18 mm dimension replaced with an individual surface profile tolerance tighter than the general surface profile tolerance. Left: all original radii. Right: radii changed by offset value. model and inner round as it should. More individual surface profile tolerances could be de- fined similarly as long as the bordering round features are taken into account as the ones in Figure 23. A downside of defining round radii in relations is that the nominal radii cannot be modified directly in the model or by editing the feature in the tree. Instead, the nominal dimension is hidden in the relations. Creo notifies the user when trying to modify a dimen- sion defined in the relations window, but it is easily missed by a designer other than the creator of the model.
The generic (nominal) model has some requirements when it comes to wall thickness toler- ances. Often castings, especially prototypes, are modelled using the final machined part as a basis. Casting features are then modelled on top of that, for example RMAs are added to the nominal machined condition as seen in Figure 25 a). The figure shows that none of the driv- ing dimensions corresponds to actual wall thicknesses. Therefore, the dimensions shown cannot be toleranced as they do not represent the final nominal wall thickness of the casting. In this case, the final wall thicknesses and the base dimensions are within the same range in the tolerance table [17, p. 8] and would result in the same tolerance, but this generalisation cannot be made in every casting. In conclusion, to be able to include wall thickness and complete cylinder tolerances to the model, the casting must be re-modelled with the dimen- sions driving the final shape instead of a base shape.
The generic (nominal) model has some requirements when it comes to wall thickness toler- ances. Often castings, especially prototypes, are modelled using the final machined part as a basis. Casting features are then modelled on top of that, for example RMAs are added to the nominal machined condition as seen in Figure 25 a). The figure shows that none of the driv- ing dimensions corresponds to actual wall thicknesses. Therefore, the dimensions shown cannot be toleranced as they do not represent the final nominal wall thickness of the casting. In this case, the final wall thicknesses and the base dimensions are within the same range in the tolerance table [17, p. 8] and would result in the same tolerance, but this generalisation cannot be made in every casting. In conclusion, to be able to include wall thickness and complete cylinder tolerances to the model, the casting must be re-modelled with the dimen- sions driving the final shape instead of a base shape.
The drafts cause some problems when implementing the wall thickness tolerances. Accord- ing to ISO 8062-4, drafts add material to the nominal shape unless otherwise stated [17, p. 7]. Thus, the nominal dimension to be parametrised is usually the dimension driving the geometry before the draft feature is added to the model. However, in Creo a split draft (part- ing plane in the middle of the drafted surface) causes material to be removed from the base shape. The comparison is shown in Figure 26. The nominal wall thickness is the narrowest part of the demo piece in the figure. As the dimension drives the thickest part of the wall instead of the nominal wall thickness, the dimension cannot be parametrised for wall thick- ness tolerancing. To avoid this problem, the drafts must be done individually or using other features so that the dimensions correspond directly to the nominal wall thickness. For exam- ple, in the test part in Figure 25 b), the drafts are included in the base shape defined with a Revolve feature. However, creating the drafts with other tools may increase the workload of modelling when the case is not as simple.
The drafts cause some problems when implementing the wall thickness tolerances. Accord- ing to ISO 8062-4, drafts add material to the nominal shape unless otherwise stated [17, p. 7]. Thus, the nominal dimension to be parametrised is usually the dimension driving the geometry before the draft feature is added to the model. However, in Creo a split draft (part- ing plane in the middle of the drafted surface) causes material to be removed from the base shape. The comparison is shown in Figure 26. The nominal wall thickness is the narrowest part of the demo piece in the figure. As the dimension drives the thickest part of the wall instead of the nominal wall thickness, the dimension cannot be parametrised for wall thick- ness tolerancing. To avoid this problem, the drafts must be done individually or using other features so that the dimensions correspond directly to the nominal wall thickness. For exam- ple, in the test part in Figure 25 b), the drafts are included in the base shape defined with a Revolve feature. However, creating the drafts with other tools may increase the workload of modelling when the case is not as simple.
The Excel file used in the calculations is presented in Appendix B. The first sheet shows a template to which the picked dimensions and parameters (IDs for different version numbers) are copied from the Family Table temporary Excel file. Additionally, the desired tolerance grade S is set on this sheet. The second sheet contains the tolerance table from ISO 8062-4. Command Calculate Dimension Limits executes a VBA macro that goes through every di- mension, checks the tolerancing range in which the dimension is, checks the tolerance ac- cording to the selected grade and finally calculates the limit values for the dimension. The VBA code is also presented in Appendix B. Now the tolerance values are calculated, and they can be copied back to the Family Table temporary file. By saving the file, the changes are saved to the Family Table, thus enabling the minimum and maximum dimensions to be set to the Family Table instances. The resulting tolerance models are compared to the nom- inal model in Figure 27.
The Excel file used in the calculations is presented in Appendix B. The first sheet shows a template to which the picked dimensions and parameters (IDs for different version numbers) are copied from the Family Table temporary Excel file. Additionally, the desired tolerance grade S is set on this sheet. The second sheet contains the tolerance table from ISO 8062-4. Command Calculate Dimension Limits executes a VBA macro that goes through every di- mension, checks the tolerancing range in which the dimension is, checks the tolerance ac- cording to the selected grade and finally calculates the limit values for the dimension. The VBA code is also presented in Appendix B. Now the tolerance values are calculated, and they can be copied back to the Family Table temporary file. By saving the file, the changes are saved to the Family Table, thus enabling the minimum and maximum dimensions to be set to the Family Table instances. The resulting tolerance models are compared to the nom- inal model in Figure 27.
Although the wall thickness tolerance models can be generated with moderate amount of work, the applicability of the tolerance models remains questionable. A model similar to the wall thickness demo part might be applicable for strength analysis, but it does not represent minimum or maximum material conditions of the casting which would be needed in order to evaluate the mass range. For example, a casting includes most material when both the surface profile tolerance and the wall thickness tolerance are at their maximum limit so that the casting is largest and wall thickness smallest possible. On the other hand, the machined part includes the least material when the surface profile tolerance is at its maximum, because more material is removed by machining.
Although the wall thickness tolerance models can be generated with moderate amount of work, the applicability of the tolerance models remains questionable. A model similar to the wall thickness demo part might be applicable for strength analysis, but it does not represent minimum or maximum material conditions of the casting which would be needed in order to evaluate the mass range. For example, a casting includes most material when both the surface profile tolerance and the wall thickness tolerance are at their maximum limit so that the casting is largest and wall thickness smallest possible. On the other hand, the machined part includes the least material when the surface profile tolerance is at its maximum, because more material is removed by machining.
In addition, some features might cause regeneration failures because of out-of-range dimen. sions. For example, the chamfer shown in Figure 29 fails with the minimum tolerance mode because the size of the chamfer exceeds the dimensional difference between the machinec surface and the cast surface in the minimum state. This failure can be solved by simply re- ducing the size of the chamfer. A better way would be to replace the chamfer with a revolve feature with references to the cast and machined surfaces instead of the chamfered edge This way, the chamfer only has an angular dimension instead of a depth dimension, and the size of the newly created chamfer changes in relation to the surfaces.
In addition, some features might cause regeneration failures because of out-of-range dimen. sions. For example, the chamfer shown in Figure 29 fails with the minimum tolerance mode because the size of the chamfer exceeds the dimensional difference between the machinec surface and the cast surface in the minimum state. This failure can be solved by simply re- ducing the size of the chamfer. A better way would be to replace the chamfer with a revolve feature with references to the cast and machined surfaces instead of the chamfered edge This way, the chamfer only has an angular dimension instead of a depth dimension, and the size of the newly created chamfer changes in relation to the surfaces.
Figure 30. Family Table of machining assembly. Different instances of the casting compo- nent are used for different instances of the machining assembly. The only component in the mac moved as assembly cuts. In addi hining assembly is the casting from which material is re- ion to the generic, two Family Table instances are created for the assembly. In Family Table, the instance of the casting can be changed using Replace Using Family Member command . The command lets the user choose which instance of the component is used in each instance of the assembly. [67] This comes in handy if the ma- chined part tolerance limit mode instances like the previously cons s are wanted to be automatically updateable Family Table tructed surface profile tolerance models. Figure 30 presents the Family Table of the machining assembly with machining tolerances and the casting in- stances. Only the brake surface dimensions are shown here for demonstration. Here again the references of the dimensions must be assigned to correspond to the toleranced dimen- sions of the machining drawing. An excerpt from traction sheave machining drawing is pre- sented in Figure 31 a) showing the dimensional tolerances. Figure 31 b) shows the defining sketch, and Figure 31 c) the comparison between the tolerance models.
Figure 30. Family Table of machining assembly. Different instances of the casting compo- nent are used for different instances of the machining assembly. The only component in the mac moved as assembly cuts. In addi hining assembly is the casting from which material is re- ion to the generic, two Family Table instances are created for the assembly. In Family Table, the instance of the casting can be changed using Replace Using Family Member command . The command lets the user choose which instance of the component is used in each instance of the assembly. [67] This comes in handy if the ma- chined part tolerance limit mode instances like the previously cons s are wanted to be automatically updateable Family Table tructed surface profile tolerance models. Figure 30 presents the Family Table of the machining assembly with machining tolerances and the casting in- stances. Only the brake surface dimensions are shown here for demonstration. Here again the references of the dimensions must be assigned to correspond to the toleranced dimen- sions of the machining drawing. An excerpt from traction sheave machining drawing is pre- sented in Figure 31 a) showing the dimensional tolerances. Figure 31 b) shows the defining sketch, and Figure 31 c) the comparison between the tolerance models.
In Figure 32 e), the situation is more difficult to work around. There is no actual flaw in the modelling technique as there is no other simple way to create the rounds shown. Still, the offset feature fails at the vertices marked with red according to failure diagnostics. A simple, yet not the most elegant solution, is to move the failing rounds to the end of the model tree ifter the offset feature and defining the radii similarly to the ones in the individual surface ‘olerance models. The radius of the round must be again added to the Family Table or set to follow the offset value in the relations windows as described in Chapter 6.2.2. The failing offset features can also be seen as a modelling quality indicator. The testing on the bodies suggested that the regeneration failures correspond somewhat to the casting de- sign flaws. For example, a casting model should not have very small round radii, small edges or small surfaces, since they are impossible to be duplicated to the pattern and subsequently to the casting. If a feature in a model has dimensions close to the tolerance value, it is ques- tionable whether the model should include that shape in the first place since the small shape can be lost within the allowed variation. Sometimes round radii below the practical minimum are used in CAD models if the feature fails with larger radii. Instead, the geometry should be modified to allow the desired round radius since the reason for the failure is in the base geometry, even if it is not very eminent. In any case, the supplier has to modify rounds too small before the pattern manufacturing, resulting in a casting whose rounds do not match those of the CAD model. Not all regeneration failures were caused by design flaws, but if surface profile tolerance models are going to be used, the designer must consider it through- out the whole modelling process and avoid the mentioned issues.
In Figure 32 e), the situation is more difficult to work around. There is no actual flaw in the modelling technique as there is no other simple way to create the rounds shown. Still, the offset feature fails at the vertices marked with red according to failure diagnostics. A simple, yet not the most elegant solution, is to move the failing rounds to the end of the model tree ifter the offset feature and defining the radii similarly to the ones in the individual surface ‘olerance models. The radius of the round must be again added to the Family Table or set to follow the offset value in the relations windows as described in Chapter 6.2.2. The failing offset features can also be seen as a modelling quality indicator. The testing on the bodies suggested that the regeneration failures correspond somewhat to the casting de- sign flaws. For example, a casting model should not have very small round radii, small edges or small surfaces, since they are impossible to be duplicated to the pattern and subsequently to the casting. If a feature in a model has dimensions close to the tolerance value, it is ques- tionable whether the model should include that shape in the first place since the small shape can be lost within the allowed variation. Sometimes round radii below the practical minimum are used in CAD models if the feature fails with larger radii. Instead, the geometry should be modified to allow the desired round radius since the reason for the failure is in the base geometry, even if it is not very eminent. In any case, the supplier has to modify rounds too small before the pattern manufacturing, resulting in a casting whose rounds do not match those of the CAD model. Not all regeneration failures were caused by design flaws, but if surface profile tolerance models are going to be used, the designer must consider it through- out the whole modelling process and avoid the mentioned issues.
he physical casting. If the base shape is wanted to be preserved anyhow, the different por- ions of the nominal dimension of the casting (base, machining and casting tolerances, RMA) can be shown in the base shape sketch as construction lines instead of defining separate features for each. The same could be also achieved with a sketched skeleton model. Figure 33 shows a demonstration where the contributions to the casting nominal dimension are il- ustrated with construction lines. The nominal dimensions are based on the machining di- mensions according to ISO 8062-4 as presented in Figure 7 and Equation (3). Each of the contributors are visible without affecting the actual nominal wall thickness to be para- metrised as opposed to modelling style in Figure 25 a). This way, the model tree is cleaner, clearer and easier for the next modeller to master, and the model is ready for wall thickness tolerancing if needed while the components of the nominal dimensions are still visible. RMA and surface profile tolerance are chosen according to ISO 8062-4 recommendations. RMA grade E and surface profile tolerance grade P8 were used for calculating the nominal casting dimensions. As for existing CAD models, the wall thickness tolerance models may require quite a lot of re-modelling. If any wall thicknesses or complete cylinder sizes are modelled in multiple steps with multiple features as in Figure 25 a), the base shape must be modelled again from the beginning. Furthermore, not only the base shape would be re-modelled, but every child feature referencing the re-modelled shapes and finally the assemblies referencing the re- modelled surfaces. The amount of work required is proportional to the size and complexity of the part. Whilst new models constructed from scratch can be modelled to support wall thickness tolerancing by following the stated guidelines, the workload of converting existing models outweighs the benefits. If needed, weakest states of the models can still be manually constructed for the calculations of single parts as have been done this far.
he physical casting. If the base shape is wanted to be preserved anyhow, the different por- ions of the nominal dimension of the casting (base, machining and casting tolerances, RMA) can be shown in the base shape sketch as construction lines instead of defining separate features for each. The same could be also achieved with a sketched skeleton model. Figure 33 shows a demonstration where the contributions to the casting nominal dimension are il- ustrated with construction lines. The nominal dimensions are based on the machining di- mensions according to ISO 8062-4 as presented in Figure 7 and Equation (3). Each of the contributors are visible without affecting the actual nominal wall thickness to be para- metrised as opposed to modelling style in Figure 25 a). This way, the model tree is cleaner, clearer and easier for the next modeller to master, and the model is ready for wall thickness tolerancing if needed while the components of the nominal dimensions are still visible. RMA and surface profile tolerance are chosen according to ISO 8062-4 recommendations. RMA grade E and surface profile tolerance grade P8 were used for calculating the nominal casting dimensions. As for existing CAD models, the wall thickness tolerance models may require quite a lot of re-modelling. If any wall thicknesses or complete cylinder sizes are modelled in multiple steps with multiple features as in Figure 25 a), the base shape must be modelled again from the beginning. Furthermore, not only the base shape would be re-modelled, but every child feature referencing the re-modelled shapes and finally the assemblies referencing the re- modelled surfaces. The amount of work required is proportional to the size and complexity of the part. Whilst new models constructed from scratch can be modelled to support wall thickness tolerancing by following the stated guidelines, the workload of converting existing models outweighs the benefits. If needed, weakest states of the models can still be manually constructed for the calculations of single parts as have been done this far.
The weakness of best fit alignment and the effect of the fixing for machining is demonstrated in Figure 34 with a simplified workpiece. Figure 34 a) shows the casting in its nominal state with the lathe jaw fixing surfaces. Figure 34 b) represents a measured casting that fulfils the surface profile tolerance, but when fixed to the lathe, it sets askew (Figure 34 c), now leaving some parts of the casting outside of the tolerance zone. With the traction sheave, the issue could also be worked around by assigning smaller, individual tolerances on the fixing planes. However, it is not practical to assign tighter tolerances to surfaces with no functionality in the actual operation of the traction sheave. This becomes more evident when the fixing sur- faces are large and flat planes or the workpiece can be fixed in several different positions. Furthermore, the tighter individual tolerance for the fixing planes should be separately cal- culated based on the other shape limits.
The weakness of best fit alignment and the effect of the fixing for machining is demonstrated in Figure 34 with a simplified workpiece. Figure 34 a) shows the casting in its nominal state with the lathe jaw fixing surfaces. Figure 34 b) represents a measured casting that fulfils the surface profile tolerance, but when fixed to the lathe, it sets askew (Figure 34 c), now leaving some parts of the casting outside of the tolerance zone. With the traction sheave, the issue could also be worked around by assigning smaller, individual tolerances on the fixing planes. However, it is not practical to assign tighter tolerances to surfaces with no functionality in the actual operation of the traction sheave. This becomes more evident when the fixing sur- faces are large and flat planes or the workpiece can be fixed in several different positions. Furthermore, the tighter individual tolerance for the fixing planes should be separately cal- culated based on the other shape limits.
Figure 35. Alignment effect on the measurement results. A) and b) show the error with best fit, c) and d) the error when comparing to the surface profile tolerance models. When using geometric, RPS or local best fit alignment, the utilisation of the tolerance models becomes different. The measured 3D model must be aligned by its RST datums to its nomi- nal location within the surface profile tolerance so that the tolerance is equally divided to both sides of the reference datum. The tolerance models constructed earlier contain only the minimum or maximum condition without the nominal data included in the same instance. Therefore, the utilisation requires additional steps. The features used as datums or used to define datum targets must be left to their nominal state to align the measured model to the nominal location. Figure 35 c) shows why the surfaces of the tolerance model cannot be used for alignment, and Figure 35 d) demonstrates how the surface profile tolerance models should be constructed in order to use the tolerance models in measurement. The surfaces related to the RST datums are deselected and excluded from the offset feature to leave them to the nominal position. However, now the surface profile tolerance of the datum features is not included in the model, but must be separately inspected during the measurement.
Figure 35. Alignment effect on the measurement results. A) and b) show the error with best fit, c) and d) the error when comparing to the surface profile tolerance models. When using geometric, RPS or local best fit alignment, the utilisation of the tolerance models becomes different. The measured 3D model must be aligned by its RST datums to its nomi- nal location within the surface profile tolerance so that the tolerance is equally divided to both sides of the reference datum. The tolerance models constructed earlier contain only the minimum or maximum condition without the nominal data included in the same instance. Therefore, the utilisation requires additional steps. The features used as datums or used to define datum targets must be left to their nominal state to align the measured model to the nominal location. Figure 35 c) shows why the surfaces of the tolerance model cannot be used for alignment, and Figure 35 d) demonstrates how the surface profile tolerance models should be constructed in order to use the tolerance models in measurement. The surfaces related to the RST datums are deselected and excluded from the offset feature to leave them to the nominal position. However, now the surface profile tolerance of the datum features is not included in the model, but must be separately inspected during the measurement.
Figure 36. A flowchart depicting the casting design and measurement process. The combination of digital 3D product definition, ISO 8062-4 and 3D scanning as the pri- mary measurement method sets the framework for casting design and quality verification. The overall procedure and workflow is demonstrated in Figure 36. Instead of coping with outdated tools and standards, the proposed process is more up-to-date and compliant with the methods that are already often used despite the lack of formally defined guidelines.
Figure 36. A flowchart depicting the casting design and measurement process. The combination of digital 3D product definition, ISO 8062-4 and 3D scanning as the pri- mary measurement method sets the framework for casting design and quality verification. The overall procedure and workflow is demonstrated in Figure 36. Instead of coping with outdated tools and standards, the proposed process is more up-to-date and compliant with the methods that are already often used despite the lack of formally defined guidelines.
Appendix B. Wall Thickness Tolerance Calculator Second sheet with wall thickness tolerance table. Private Sub CommandButtonl_Click()
Appendix B. Wall Thickness Tolerance Calculator Second sheet with wall thickness tolerance table. Private Sub CommandButtonl_Click()

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