Blind holes and pockets are common features of investment casting designs. They serve to lighten the part, provide designed clearances and are examples of the type of detail that can often be provided by the investment casting process at negligible cost to a customer.
For any feature to be castable, it is necessary for an IC foundry to build a minimum number layers of ceramic shell in or about it. The ratio between the diameter of a hole and its depth determines if the ceramic shell will fully form. By definition blind holes & pockets are open from only one end and are more difficult to build layers of ceramic than are through-holes that are open from two ends. As blind holes are more difficult it follows that the castable ratio between the diameter and depth is different than it is for through holes (see the OFC Blog on Through Holes for more information).
The greater the diameter of the blind hole the greater the depth that it can be cast. For example a Ø.12” diameter blind hole can be cast .12” in depth (1x) whereas a Ø.50” diameter blind hole can be cast .75” in depth (x1½). O’Fallon Casting publishes an Investment Casting Design Guide that is available for free download on its website. The Design Guide contains a table that delineates the castable sizes of blind holes.
All blind holes & pockets must have a fillet radius around the base of the feature. A fillet is critical as a sharp corner can potentially cause a failure in the casting process to produce the feature (see the OFC Blog on Fillet radii for more information). A minimum fillet radius of .06”R is recommended or a full spherical radius with Ø.12” or smaller diameter blind holes.
Investment casting wax patterns are produced with injection molds and blind holes and pockets are “positive” features in a mold. As the injected pattern wax cools and contracts the pattern will tend to adhere to those positives. A small amount of draft angle on pocket walls can sometimes help to facilitate the removal of the wax pattern from the mold. Although a draft angle is not generally required for investment casting designs an allowance to tool blind pockets with up to a 1° of draft may benefit the manufacturability of the part.
Working within the recommendations in the Design Guide will help to prevent cast features from becoming a cost drivers. Should it be necessary to design a feature outside those recommendations we suggest that you consult a foundry to determine if there is any impact on part manufacturability and of any cost implications. It is very likely that a foundry can suggest changes that will improve a part’s castability without impacting its cost.
If you have any questions regarding the casting of Blind Holes & Pockets, contact your O’Fallon Casting Sales Engineer.
Tuesday, 3 May 2016
Monday, 14 December 2015
Investment Casting “Through-holes”
A common question asked by Design Engineers in concurrent engineering is whether a particular hole in a part is castable? To answer that question requires an analysis of the four dimensional components that constitute the description of the hole: diameter, length, diametrical tolerance and locational tolerance.
First, I should be very clear that the casting of holes is a “good thing”. Holes are the type of part detail that can generally be provided by the investment casting process at negligible cost to a customer. Cast holes can also be of benefit to the foundry as they enhance the dimensional stability of a casting.
Straight through-holes are more easily cast than are blind holes. For the purposes of this essay we’ll confine our attention to straight holes that are open from both ends. I shall address the special circumstances of casting blind holes and internal cores in other blogs.
Roundness: Let’s begin with an understanding that the form, or shape, of a cast hole is never “perfect” (but neither is a machined hole) and will vary slightly in size and shape along its length. This loss of perfection is induced by the contraction of the hot metal as it cools in the casting. As a result the diameter of a cast hole can be pulled out of shape as areas around it solidify and become smaller. A second phenomenon concerns holes cast through a thin thickness of metal tending to nonfill unevenly around the diameter. The measurement of the location of a cast hole can sometimes be difficult as any loss of accuracy in determining the centerline of an imperfect shape also diminishes the accuracy of the measurement.
Diametrical Tolerance: The tolerance on the diameter of a hole must conform to Standard Investment Casting tolerances (approximately ±.005 inch/inch) as do all cast features. Additional tolerance will required dependent on the length of hole being cast.
Diameter to Length Ratio: To reproduce a feature it is necessary for investment casting to build a minimum number layers of ceramic shell in or about it. In the case where a hole diameter is too small relative to its length the ceramic will be unable to fully form and the shell will fail when cast with metal. The ratio between the diameter and length of the hole will determine if the feature is castable.
The smaller the diameter of a through-hole the shorter the length that can be cast. For example a hole diameter of Ø.06” can be cast up to two diameters in length where a Ø.25” hole can be cast up to five diameters in length.
In instances where the diameter to length ratio is too small there is an alternative option to produce the hole from a preformed ceramic core. Preformed ceramic core add expense to the casting but are a very viable option for the manufacture of fine detail that can’t be otherwise be produced directly by the investment casting process. For more information about ceramic cores please see a related blog on the casting of blind holes and internal cores.
Locational Tolerance: The locational tolerance for a hole is more often an issue than is its form tolerance. The locational tolerance of holes must be in accordance with Standard Investment Casting Tolerances (approximately ±.005 inch/inch). As locationally important holes can be machined to a much higher tolerance cast through-holes are most often used for less critical features such as fluid flow passages, wire feeds or for ventilation.
A Designer can, however, sometimes mitigate a locational tolerance with some creative dimensioning. For example, cast holes might require less tolerance if dimensioned to nearby “local” features rather than directly to the datums.
O’Fallon Casting publishes an Investment Casting Design Guide that is available for free download on its website. The Design Guide contains tables that delineate the castable sizes of holes. Please download the Design Guide and in the event that your requirements are inside of the industry recommendations contact your O’Fallon Casting Sales Engineer to verify if a design is manufacturable.
First, I should be very clear that the casting of holes is a “good thing”. Holes are the type of part detail that can generally be provided by the investment casting process at negligible cost to a customer. Cast holes can also be of benefit to the foundry as they enhance the dimensional stability of a casting.
Straight through-holes are more easily cast than are blind holes. For the purposes of this essay we’ll confine our attention to straight holes that are open from both ends. I shall address the special circumstances of casting blind holes and internal cores in other blogs.
Roundness: Let’s begin with an understanding that the form, or shape, of a cast hole is never “perfect” (but neither is a machined hole) and will vary slightly in size and shape along its length. This loss of perfection is induced by the contraction of the hot metal as it cools in the casting. As a result the diameter of a cast hole can be pulled out of shape as areas around it solidify and become smaller. A second phenomenon concerns holes cast through a thin thickness of metal tending to nonfill unevenly around the diameter. The measurement of the location of a cast hole can sometimes be difficult as any loss of accuracy in determining the centerline of an imperfect shape also diminishes the accuracy of the measurement.
Diametrical Tolerance: The tolerance on the diameter of a hole must conform to Standard Investment Casting tolerances (approximately ±.005 inch/inch) as do all cast features. Additional tolerance will required dependent on the length of hole being cast.
Diameter to Length Ratio: To reproduce a feature it is necessary for investment casting to build a minimum number layers of ceramic shell in or about it. In the case where a hole diameter is too small relative to its length the ceramic will be unable to fully form and the shell will fail when cast with metal. The ratio between the diameter and length of the hole will determine if the feature is castable.
The smaller the diameter of a through-hole the shorter the length that can be cast. For example a hole diameter of Ø.06” can be cast up to two diameters in length where a Ø.25” hole can be cast up to five diameters in length.
In instances where the diameter to length ratio is too small there is an alternative option to produce the hole from a preformed ceramic core. Preformed ceramic core add expense to the casting but are a very viable option for the manufacture of fine detail that can’t be otherwise be produced directly by the investment casting process. For more information about ceramic cores please see a related blog on the casting of blind holes and internal cores.
Locational Tolerance: The locational tolerance for a hole is more often an issue than is its form tolerance. The locational tolerance of holes must be in accordance with Standard Investment Casting Tolerances (approximately ±.005 inch/inch). As locationally important holes can be machined to a much higher tolerance cast through-holes are most often used for less critical features such as fluid flow passages, wire feeds or for ventilation.
A Designer can, however, sometimes mitigate a locational tolerance with some creative dimensioning. For example, cast holes might require less tolerance if dimensioned to nearby “local” features rather than directly to the datums.
O’Fallon Casting publishes an Investment Casting Design Guide that is available for free download on its website. The Design Guide contains tables that delineate the castable sizes of holes. Please download the Design Guide and in the event that your requirements are inside of the industry recommendations contact your O’Fallon Casting Sales Engineer to verify if a design is manufacturable.
Friday, 11 December 2015
Model Based Definition – Advantages & Disadvantages
Prior to the widespread integration of CAD systems in the 1980’s part drawings were filled with leader lines to features delineated by dimensions for size and location with independent tolerances. CAD systems have made the designing of parts both better and quicker and have a positive impact downstream from product design. Also known as Digital Product Definition (DPD) the MBD revolution in design has had both good and bad repercussions for the foundry industry.
O’Fallon Casting has been at the industry forefront in adopting technologies that support Model Based Definition designs. OFC can tool, manufacture and inspect its manufactured castings to a customer solid model without the need of a 2D drawing.
OFC Customers have varying degrees of MBD implementation. Some provide a solid model with a fully dimensioned drawings others with partially dimensioned and sometimes with drawings that contain no dimensions whatsoever. O’Fallon Casting has found that limited dimension drawings, especially for larger sizes of castings, are an effective way for an engineer to convey information regarding the important features of a part design.
Model Based Definition has helped reduce the cycle time from receipt of a design to the shipment of product. Toolmaking has become faster and more accurate as has the inspection of first articles. The First Pass Yield of MBD parts is better than it is for castings from tooling manufactured from fully dimensioned drawings.
The manufacture of Rapid Prototyping has greatly expanded with the advent of CAD solid model files. As the use of rapid prototypes has increased the materials, processes and equipment to produce them has also improved. Additive manufactured patterns from SLA, SLS or Voxeljet technologies are used to produce 1-off quantities or even low rates of production directly from a customer CAD file. (It should be noted, however, to successfully manufacture rapid prototypes the supplied CAD solid models must be complete with fillet radii; a detail that is frequently omitted by engineers because of the modeling time required and size of the resultant files.)
Laser scanning inspection of castings to their Solid Models is both less expensive and more informative as compared to the normal inspection of a fully dimensioned drawing. Traditionally the inspection of a part surface to a dimensioned drawing is made by the measurement of a series of points along the surface. By comparison a laser scan accumulates millions of points per square inch and so is capable of detecting small variations in the surface that would remain undetected by a traditional “point” methods. The wealth of data from a laser scan makes the dimensional analysis of a casting better and aids the ability of an engineer to discern in-process deviations from tooling errors.
The variety of available CAD formats can be challenging to a supply chain especially among smaller subcontractors. Parasolid (.x_t) and Step (.stp) file formats are generally well supported across the industry. Iges (.igs) files can be universally read but can be difficult to use as they seem to be very prone to file errors. Formats other than Parasolid, Step or Iges are likely to be translated to a locally readable format at some point along the supply chain. The use of other than the generally excepted CAD formats can also serve to limit the number of available subcontractors and may increase the cost for services.
More the case in the past, but still true today, Document and Data control of MPD CAD files has been found to be “difficult”. Part drawings and associated CAD files can sometimes go out of “sync” with each other and obsolete revision files have been mistakenly distributed. As a result of bad experiences some customers have become reticent to supply their CAD files for fear they are not up-to-date.
Tooling subcontractors, however, have become dependent on their receiving a customer solid model for their design and manufacture of tooling. Without a supplied customer CAD file it often becomes necessary for a toolmaker to first create their own Solid Model of the part. This extra step adds time and expense to the procurement of the tool and any latent errors found in toolmaker developed CAD file can also contribute to delays in product deliveries.
Model based definition products have become the norm in the investment casting industry. MBD possesses advantages in speed and accuracy of a designed product that far outweigh the challenges to downstream users of the files.
O’Fallon Casting is very familiar with the manufacture of nonferrous investment castings to their CAD solid models. If you have any questions regarding Model Based Definition please contact your O’Fallon Casting Sales Engineer.
O’Fallon Casting has been at the industry forefront in adopting technologies that support Model Based Definition designs. OFC can tool, manufacture and inspect its manufactured castings to a customer solid model without the need of a 2D drawing.
OFC Customers have varying degrees of MBD implementation. Some provide a solid model with a fully dimensioned drawings others with partially dimensioned and sometimes with drawings that contain no dimensions whatsoever. O’Fallon Casting has found that limited dimension drawings, especially for larger sizes of castings, are an effective way for an engineer to convey information regarding the important features of a part design.
Model Based Definition has helped reduce the cycle time from receipt of a design to the shipment of product. Toolmaking has become faster and more accurate as has the inspection of first articles. The First Pass Yield of MBD parts is better than it is for castings from tooling manufactured from fully dimensioned drawings.
The manufacture of Rapid Prototyping has greatly expanded with the advent of CAD solid model files. As the use of rapid prototypes has increased the materials, processes and equipment to produce them has also improved. Additive manufactured patterns from SLA, SLS or Voxeljet technologies are used to produce 1-off quantities or even low rates of production directly from a customer CAD file. (It should be noted, however, to successfully manufacture rapid prototypes the supplied CAD solid models must be complete with fillet radii; a detail that is frequently omitted by engineers because of the modeling time required and size of the resultant files.)
Laser scanning inspection of castings to their Solid Models is both less expensive and more informative as compared to the normal inspection of a fully dimensioned drawing. Traditionally the inspection of a part surface to a dimensioned drawing is made by the measurement of a series of points along the surface. By comparison a laser scan accumulates millions of points per square inch and so is capable of detecting small variations in the surface that would remain undetected by a traditional “point” methods. The wealth of data from a laser scan makes the dimensional analysis of a casting better and aids the ability of an engineer to discern in-process deviations from tooling errors.
The variety of available CAD formats can be challenging to a supply chain especially among smaller subcontractors. Parasolid (.x_t) and Step (.stp) file formats are generally well supported across the industry. Iges (.igs) files can be universally read but can be difficult to use as they seem to be very prone to file errors. Formats other than Parasolid, Step or Iges are likely to be translated to a locally readable format at some point along the supply chain. The use of other than the generally excepted CAD formats can also serve to limit the number of available subcontractors and may increase the cost for services.
More the case in the past, but still true today, Document and Data control of MPD CAD files has been found to be “difficult”. Part drawings and associated CAD files can sometimes go out of “sync” with each other and obsolete revision files have been mistakenly distributed. As a result of bad experiences some customers have become reticent to supply their CAD files for fear they are not up-to-date.
Tooling subcontractors, however, have become dependent on their receiving a customer solid model for their design and manufacture of tooling. Without a supplied customer CAD file it often becomes necessary for a toolmaker to first create their own Solid Model of the part. This extra step adds time and expense to the procurement of the tool and any latent errors found in toolmaker developed CAD file can also contribute to delays in product deliveries.
Model based definition products have become the norm in the investment casting industry. MBD possesses advantages in speed and accuracy of a designed product that far outweigh the challenges to downstream users of the files.
O’Fallon Casting is very familiar with the manufacture of nonferrous investment castings to their CAD solid models. If you have any questions regarding Model Based Definition please contact your O’Fallon Casting Sales Engineer.
Wednesday, 9 December 2015
Laser Engraved Part Marking
O’Fallon Casting provides its customers with a better alternative and consistently legible option for the application of variable part marking information on castings: Laser Engraved Part Marking. This OFC technology eliminates customer complaints and rejections of castings for poor legibility of manually applied vibro-peened or ink stamped part marking.
O’Fallon Casting’s Laser Engraved Part Marking provides a .0001 - .0003” deep permanent marking that eliminates the variability of manually applied marking and replaces traditional techniques such as ink stamping, vibro-peening or electrochemical etching. Although not intended to replace “personal” ink stamps, such as Inspection Stamps, Laser Engraved Part Marking is a versatile technology for marking a cast surface with variable product information such as Melt & Heat Treat lot numbers, Part Serialization, or even a 2D Matrix barcode.
Laser Engraved Part Marking is recognized in standard marking specifications MIL-STD-130 (Table II) and AS478 (15A).
To eliminate a needless cause of Customer Dissatisfaction, O’Fallon Casting encourages Engineers to specify Laser Engraved Part Marking as a part marking option for the marking of variable information on their castings.
For more information about Laser Engraved Part Marking contact your O’Fallon Casting Sales Engineer.
O’Fallon Casting’s Laser Engraved Part Marking provides a .0001 - .0003” deep permanent marking that eliminates the variability of manually applied marking and replaces traditional techniques such as ink stamping, vibro-peening or electrochemical etching. Although not intended to replace “personal” ink stamps, such as Inspection Stamps, Laser Engraved Part Marking is a versatile technology for marking a cast surface with variable product information such as Melt & Heat Treat lot numbers, Part Serialization, or even a 2D Matrix barcode.
Laser Engraved Part Marking is recognized in standard marking specifications MIL-STD-130 (Table II) and AS478 (15A).
To eliminate a needless cause of Customer Dissatisfaction, O’Fallon Casting encourages Engineers to specify Laser Engraved Part Marking as a part marking option for the marking of variable information on their castings.
For more information about Laser Engraved Part Marking contact your O’Fallon Casting Sales Engineer.
Thursday, 19 November 2015
Geometric Dimensioning and Tolerancing of Castings
There is nothing really new about Geometric Dimensioning and Tolerancing so I was surprised by a question an engineer asked recently concerning GD&T. I had just finished speaking to a group of engineers about investment casting design and the first question asked was to express my opinion of “GD&T casting drawings”. Admittedly confused I lamely answered with something akin to “I’m good with it” and felt fortunate that there was no follow-up question on the same subject.
Truthfully, I am so accustomed to working with Geometric Dimensioning & Tolerancing (ASME Y14.5) that I likely haven’t given it much thought for a couple of decades. Now having revisited the subject I believe my initial response the GD&T questions wasn’t too bad after all because I am “good with it” but didn’t provide much insight. So, in my opinion GD&T is the better method for an engineer to define part requirements than was the old Plus / Minus system. It is, however, a more difficult system to learn and interpret and as a consequence there seem to be greater numbers of errors made with GD&T.
I also believe that GD&T paved the way for dimensionless drawings that have created a much larger quandary for designers of cast products. (This, perhaps, with a little clarification, might have been the true subject of the question being asked of me?) However, for the purposes of this essay I’ll confine my attention to addressing Geometric Dimensioning and Tolerancing of a fully dimensioned casting drawing.
If you’ve worked with castings you likely realize that dimensional variation accumulates in the longer dimensions. The Investment Casting Institute has published a Linear Tolerance Table* that translates roughly to ±.005 inch of tolerance per inch of length. Therefore, a dimension of 2-inches would require a ±.010 tolerance and a 4-inch dimension ±.020. It is important to remember that a linear dimension is the distance between two points such as can be measured with a pair of Vernier calipers.
Let’s compare the two methods as to how one would dimension and tolerance three coplanar holes on a surface. One point of clarification: The following example considers a casting in an ideal state and so is presumed to be perfectly flat and square. As already discussed a linear dimension is the distance between two points and not a point to a (datum) plane. In the “real world” tolerance consideration is 3-dimensional (-X-, -Y-, -Z-) and must also include the size and shape of the datum plane.
In a Plus / Minus drawing the dimensions to three coplanar cast holes might be tolerance as follows:
In GD&T drawing the locational dimensions become Basic and the feature (hole) is designated with a True Position (feature location) tolerance linked to a Maximum Material Condition:
GD&T True Positions have circular tolerance zones where the tolerance zones in a Plus / Minus system are rectangular, so the GD&T system does provide a “smidge” of additional tolerance. Otherwise these two systems are pretty much equivalent.
Note, however, that a ±.010 locational tolerance is roughly equivalent to a true position of .028. Rookie engineers sometimes make the mistake of presuming that a .020 inch (±.010) of a Plus / Minus tolerance zone is the same size as .020 of True Position geometric tolerance where in actuality there quite a bit of difference. Geometric dimensions originate from the nexus or 0,0 point of the -X- and -Y- datums (it’s Pythagorean’s value of c in the a2 + b2 = c2) where Plus / Minus systems dimensions are measured along an axis perpendicular to the datum plane (a & b).
It is readily apparent from the above tables that the dimensions furthest from the datums require significantly more tolerance in either system. This emphasizes the point that datum selection is an important consideration for the Tolerancing of a part regardless of the system being used. I can suggest that one way to help mitigate the stack up of tolerance is to centralize the datum structure.
Let’s revisit the previous scenario but this time with the nexus of -X- and -Y- datums better centralized and being coincident with the centerlines of Hole A
In the Plus / Minus system dimensions and tolerances for the three holes become:
In the GD&T system the dimensions and tolerances become:
As you can see, the effect of centralizing the two datums is to average the cumulative deviation bilaterally, in two directions, instead of unilaterally and shifting all of the tolerance requirements to the “far end”. The centralized –X- and –Y- datums should create a more acceptable tolerance condition for the designer.
The ±.005 inch/inch linear tolerance was established by the Investment Casting Institute as a guideline to assist companies in the design manufacturable castings for its member foundries. O’Fallon Casting and many other Institute members manufacture castings to tolerances tighter than those being recommended by the ICI. We suggest that in instances where a designer is being driven to work inside of the ICI recommended tolerances, that they confer with a foundry to confirm their capability.
The use of Geometric Dimensioning and Tolerancing on a fully dimensioned drawing should not present an issue to a foundry. In any dimensioning system the size of the part and the location of the datum planes will have a direct impact on the amount of tolerance necessary for a manufacturable and affordable casting. Centralized datums will prevent the stack-up of part variation from manifesting itself in one side of the casting by averaging the deviations in two directions.
If you have any questions regarding Geometric Dimensioning and Tolerancing of castings, please contact your O’Fallon Casting Sales Engineer.
*The Investment Casting Handbook published by the Investment Casting Institute, copyright 1997.
Truthfully, I am so accustomed to working with Geometric Dimensioning & Tolerancing (ASME Y14.5) that I likely haven’t given it much thought for a couple of decades. Now having revisited the subject I believe my initial response the GD&T questions wasn’t too bad after all because I am “good with it” but didn’t provide much insight. So, in my opinion GD&T is the better method for an engineer to define part requirements than was the old Plus / Minus system. It is, however, a more difficult system to learn and interpret and as a consequence there seem to be greater numbers of errors made with GD&T.
I also believe that GD&T paved the way for dimensionless drawings that have created a much larger quandary for designers of cast products. (This, perhaps, with a little clarification, might have been the true subject of the question being asked of me?) However, for the purposes of this essay I’ll confine my attention to addressing Geometric Dimensioning and Tolerancing of a fully dimensioned casting drawing.
If you’ve worked with castings you likely realize that dimensional variation accumulates in the longer dimensions. The Investment Casting Institute has published a Linear Tolerance Table* that translates roughly to ±.005 inch of tolerance per inch of length. Therefore, a dimension of 2-inches would require a ±.010 tolerance and a 4-inch dimension ±.020. It is important to remember that a linear dimension is the distance between two points such as can be measured with a pair of Vernier calipers.
Let’s compare the two methods as to how one would dimension and tolerance three coplanar holes on a surface. One point of clarification: The following example considers a casting in an ideal state and so is presumed to be perfectly flat and square. As already discussed a linear dimension is the distance between two points and not a point to a (datum) plane. In the “real world” tolerance consideration is 3-dimensional (-X-, -Y-, -Z-) and must also include the size and shape of the datum plane.
In a Plus / Minus drawing the dimensions to three coplanar cast holes might be tolerance as follows:
| Hole A: | Ø.50 ±.005 | 5.00 ±.025 from -X- | 5.00 ±.025 from -Y- |
| Hole B: | Ø.50 ±.005 | 2.00 ±.010 from -X- | 2.00 ±.010 from -Y- |
| Hole C: | Ø.50 ±.005 | 10.00 ±.050 from -X- | 10.00 ±.050 from -Y- |
In GD&T drawing the locational dimensions become Basic and the feature (hole) is designated with a True Position (feature location) tolerance linked to a Maximum Material Condition:
| Hole A: | Ø.50 ±.005 | 5.00 Basic from -X- | 5.00 Basic from -Y- | 070 True Position MMC |
| Hole B: | Ø.50 ±.005 | 2.00 Basic from -X- | 2.00 Basic from -Y- | 028 True Position MMC |
| Hole C: | Ø.50 ±.005 | 10.00 Basic from -X- | 10.00 Basic from -Y- | 140 True Position MMC |
GD&T True Positions have circular tolerance zones where the tolerance zones in a Plus / Minus system are rectangular, so the GD&T system does provide a “smidge” of additional tolerance. Otherwise these two systems are pretty much equivalent.
Note, however, that a ±.010 locational tolerance is roughly equivalent to a true position of .028. Rookie engineers sometimes make the mistake of presuming that a .020 inch (±.010) of a Plus / Minus tolerance zone is the same size as .020 of True Position geometric tolerance where in actuality there quite a bit of difference. Geometric dimensions originate from the nexus or 0,0 point of the -X- and -Y- datums (it’s Pythagorean’s value of c in the a2 + b2 = c2) where Plus / Minus systems dimensions are measured along an axis perpendicular to the datum plane (a & b).
It is readily apparent from the above tables that the dimensions furthest from the datums require significantly more tolerance in either system. This emphasizes the point that datum selection is an important consideration for the Tolerancing of a part regardless of the system being used. I can suggest that one way to help mitigate the stack up of tolerance is to centralize the datum structure.
Let’s revisit the previous scenario but this time with the nexus of -X- and -Y- datums better centralized and being coincident with the centerlines of Hole A
In the Plus / Minus system dimensions and tolerances for the three holes become:
| Hole A: | Ø.50 ±.005 | 0.00 ±.000 from -X- | 0.00 ±.000 from -Y- |
| Hole B: | Ø.50 ±.005 | 3.00 ±.015 from -X- | 3.00 ±.015 from -Y- |
| Hole C: | Ø.50 ±.005 | 5.00 ±.025 from -X- | 5.00 ±.025 from -Y- |
In the GD&T system the dimensions and tolerances become:
| Hole A: | Ø.50 ±.005 | 0.00 Basic from -X- | 0.00 Basic from -Y- | ..000 True Position MMC |
| Hole B: | Ø.50 ±.005 | 3.00 Basic from -X- | 3.00 Basic from -Y- | .042 True Position MMC |
| Hole C: | Ø.50 ±.005 | 5.00 Basic from -X- | 5.00 Basic from -Y- | .070 True Position MMC |
As you can see, the effect of centralizing the two datums is to average the cumulative deviation bilaterally, in two directions, instead of unilaterally and shifting all of the tolerance requirements to the “far end”. The centralized –X- and –Y- datums should create a more acceptable tolerance condition for the designer.
The ±.005 inch/inch linear tolerance was established by the Investment Casting Institute as a guideline to assist companies in the design manufacturable castings for its member foundries. O’Fallon Casting and many other Institute members manufacture castings to tolerances tighter than those being recommended by the ICI. We suggest that in instances where a designer is being driven to work inside of the ICI recommended tolerances, that they confer with a foundry to confirm their capability.
The use of Geometric Dimensioning and Tolerancing on a fully dimensioned drawing should not present an issue to a foundry. In any dimensioning system the size of the part and the location of the datum planes will have a direct impact on the amount of tolerance necessary for a manufacturable and affordable casting. Centralized datums will prevent the stack-up of part variation from manifesting itself in one side of the casting by averaging the deviations in two directions.
If you have any questions regarding Geometric Dimensioning and Tolerancing of castings, please contact your O’Fallon Casting Sales Engineer.
*The Investment Casting Handbook published by the Investment Casting Institute, copyright 1997.
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