Standard Investment Casting Linear Tolerances

Relative to other foundry processes Investment Casting is a very precise method to produce a Near-Net shape cast product.  This capability stems both from the repeatability of the process and that a draft angle need not be added to cast surfaces.  As with all manufacturing processes there will exist some part-to-part variation that needs to be accommodated by a dimensional tolerance.

The standard design guidelines for the industry have been established by the Investment Casting Institute.  This includes standards for linear tolerance as published by the ICI in the Investment Casting Handbook.  Their recommendations for linear tolerances are shown in the following table:

LINEAR TOLERANCE
	DIMENSIONS	NORMAL	PREMIIUM
	up to 1"	± .010"	±.005
	up to 2"	± .013"	±.010
	up to 3"	± .016"	±.013
	up to 4"	± .019"	±.015
	up to 5"	± .022"	±.017
	up to 6"	± .025"	±.020
	up to 7"	± .028"	±.022
	up to 8"	± .031"	±.024
	up to 9"	± .034"	±.026
	up to 10"	± .037"	±.028
> 10" allow ±.005" per inch

To correctly apply the tolerances from this table it is first important to have a clear understanding of what constitutes a linear dimension.  If we first understand that cubic inch is a volume of 1” x 1” x 1” and then that a square inch is an area of 1” x 1”  then it follows that a linear inch a length defined as the distance between two points (1”).

Therefore, any dimension to a feature that can be measured with a ruler or a pair of calipers should have applied a linear tolerance from the table.

However, properly applying the table can be confusing for Designers when tolerancing non-linear features.  Castings, for example, are frequently measured from a datum structure that is established by datum points.  When measuring a casting from datum points we no longer have a linear point-to-point measurement but a non-linear point-to-plane dimension and the size and shape (flatness & perpendicularity) of the plane must now be taken into account.

Take the instance of a table top.  If we were to measure the thickness of the table top with a pair of calipers we then have a linear dimension to which a linear tolerance should be applied.  However, if we instead to measure the height of the table top from the floor and run an indicator across the area of table top we would expect to receive a much wider measurement variation because the table top is not perfectly flat nor square to the floor and the amount of variation would be relative to the size of the table top.

When measuring a casting from a datum structure the size of the feature and the distance from the datum both need to be taken into account in the tolerance.  So in addition to the linear tolerance per the table an allowance for .005”/inch of tolerance must be provided to account for variations flatness, perpendicularity and parallelism of the casting.

As a general rule of thumb Engineers should consider the farthest feature that being measured from the 0,0,0 point of the three perpendicular datums.  If the longest measurement was for example 10” the tolerance from the table for that linear segment would be ±.037” or an equivalent profile tolerance of .074” of the part from the datums.

For instances where it is necessary that a design include a tolerance of a feature that is tighter than the industry standard it is important consult with a foundry to assure that the desired capability is manufacturable.  Even if the foundry feedback is negative or perhaps finds that additional tooling will be required, it is always better to engineer a manufacturable design than to deal with the cost and consequences of one that isn’t.

For further information contact O’Fallon Casting.

To purchase a copy of the Investment Casting Handbook contact the Investment Casting Institute:

Investment Casting Institute
136 Summit Avenue
Montvale, NJ   07645-1720

www.investmentcasting.org

Casting Conversions

I was looking at an Investment Casting Institute publication entitled Case Studies and Applications which elaborates on the substantial savings realized when fabricated assemblies are converted to One-Piece castings.  In that Investment Casting is such an effective manufacturing method for realizing this type of savings one wonders why companies don’t place greater emphasis on achieving them.  Perhaps their not knowing a good place to look to find a casting conversion candidate is the answer.

The Investment Casting process provides two main cost benefits to its customers.  First is its Near-Net-Shape capability that stems from the dimensional precision of the process and that no draft angle is required.  Secondly the Investment Casting process provides an ability to combine multiple pieces into a One-Piece structure with few design constraints and without a major impact to cost. With easily machined alloys, such as aluminum, the One-Piece advantage takes on a greater significance than does Near-Net-Shape. 

Considering this, then the most ready opportunities to realize a significant savings from a conversion to an aluminum investment casting is to find situations where components are being assembled together either with a fastener, braze or weld.  Weldments and Dip Brazed assemblies are obvious candidates to becoming Investment Castings.

The direct savings from the elimination of an assembly operation can be substantial but the indirect savings from a reduction of Part Count can be even larger.  One-Piece cast structures also lower the risk of assembly point failures with the additional benefit of the Near-Net-Shape precision of the investment casting process and potential for component weight reduction.

Investment Castings present a cost effective method for the manufacture of complex and near-net shapes.  Replacing fabricated structures with One-Piece castings is a good starting point to achieve substantial savings.

O'Fallon Casting Publishes Design Guide

O’Fallon Casting has published its first “Investment Casting Design Guide” which is now available for free download on the OFC Website (www.ofalloncasting.com/design guide/). 

Investment Casting is a cost effect method for the manufacture of complex, near-net-shape, products but some specialized expertise is required for their design. The Design Guide is a natural offshoot of O’Fallon Casting’s “IC-101” class on the basics for the design of Investment Casting and addresses that need for information of best design practices.

Engineers prefer Investment Casting as it provides them with more freedom and is the most straightforward foundry process for which to design.  The Guide provides recommendations for allowances and other considerations that are generally necessary for the successful design of an investment cast product.  Basic information on topics such as linear tolerance, fillet radii allowance, and castable hole sizes, etc. are included in the guide.

The Design Guide does not supplant the need for Concurrent Engineering with a foundry.  Castings are cost effective in combining features into One-Piece casting designs that reduce the Part Count of an assembly and consultation with a foundry remains the best way to achieve the most affordable configuration of a cast product. 

OFC hopes that the information in the O’Fallon Casting “Investment Casting Design Guide” will assist Engineers take better advantage of the strengths of the investment casting process to design more affordable products for their customers.

The Design Engineer's Role in Casting Procurment

It is a well-known axiom that 70% - 90% of the cost of a product is the result of design decisions.  Although we might debate as to the preciseness of this estimation the fact remains that design is key element to the manufacturability, availability, reliability and overall affordability of any manufactured product.

Castings are inherently a cost effective method to manufacture shapes that reduces the part-count of an assembly and therefore improves the affordability of that product.  However, casting design does require some specialized expertise and appreciation of the strength and weaknesses of the foundry process.

Any poorly designed product as a consequence will incur unnecessary or unexpected costs.  For example, the cost of poor product manufacturability might be reflected in late deliveries, short shipments or frequent rejections.  These types of design related issues will drive a rise of both Inventory and Administrative costs.

Less obvious is the cost of a missed opportunity where the product might have been manufactured via a superior method.  Poor design choices can threaten both the reputation and profitability of an enterprise.

Good casting design accomplishes two things.  First the combining multiple parts into a one-piece cast structure will reduce the amount of assembly required to manufacture a product.  Such Part-Count Reduction favorably impacts not only the direct manufacturing cost but also reduces inventory and administrative costs.  Secondly, because of the Near-Net-Shape capability, castings reduce the cost for secondary machining.  Investment Casting is an effective foundry process by which to take advantage of both capabilities.

The Designing of Investment Castings is a relatively straightforward process as there are few constraints on configuration and it requires no consideration of a draft angle.  Therefore, anytime two separate members need be assembled together presents an opportunity to combine them into a 1-piece cast structure.  Obvious candidates for conversion to casting are dip-brazed or welded assemblies.

Admittedly there are a few casting design “Rules of Thumb” that do need to be taken into consideration and knowledge of those aspects will help to avoid any unintended “Designed–In” costs.  O’Fallon Casting readily assists its customers with several services to help enhance the value of their casting designs.  First O’Fallon Casting provides its customers a Concurrent Engineering Service to critique their casting designs and offer recommendations for improvement.  Secondly, OFC offers to conduct a 3-Hour, IC-101, class on the basics of Investment Casting design at Customer facilities.  Thirdly, O’Fallon Casting also offers a 3-Day, IC-201, class held at O’Fallon Casting for customers desiring an in-depth exposure to Investment Casting.

Castings provide an opportunity to enhance the affordability of customer designed products.  O’Fallon Casting wants to assist its customers in making the most effective use of castings in their products.

For more information please call your OFC Sales Engineer.

AFS Metalcasting Supply Chain & Design Summits

O’Fallon Casting had the pleasure of addressing both the American Foundry Society’s Metalcasting Supply Chain Summit and Metalcasting Design Summit held in Chicago on February 3rd and 4th.  The AFS held these two Summits as a vehicle to update users of cast metal products as to the State of the Industry and to learn of innovation within the industry.  The two summits also provided forums for Industry Experts to interact with the Supply Chain and Engineering Professionals in roundtable discussions of pertinent issues such as Lead Times, Casting Conversions, and Specifications.

At the Metalcasting Supply Chain Summit O’Fallon Casting presented a paper entitled:  “Cost Factors of Aluminum Investment Castings”.  This presentation provided an overview of the Investment Casting process, summed the major issues as regard to cost and suggested ways that users might optimize the value in their cast metal products.

At the Metalcasting Design Summit O’Fallon Casting presented a paper entitled:  “The Role of the Design Engineer in the Supply Chain”.  The paper argued that 70% – 90% of Product Cost is the result of Design Decisions and demonstrated how effective casting design can reduce  the Part Count and thus the cost of secondary machining and assembly operations in their engineered products.  This point was reinforced with an analysis of the exquisite casting design of O’Fallon Casting’s AFS/MCDP “2013-Casting of the Year”.