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:

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.

Integrally Cast Lettering & Marking

One of the primary benefits provided by the Investment Casting process is that part detail can very often be furnished at negligible additional cost.  Engineers frequently take advantage of this “free” capability by incorporating cast lettering and marking into their part designs.  Marking can be almost anything from letters, numbers, logos, trademarks or be simply decorative.

There are basically two types of cast marking, “Integral” or “Applied”.  Integral-marking is embedded into a pattern mold so that the identical marking appears on each part cast.  Applied-marking such as that made by laser, vibro-etch or ink stamping is applied to the finished product and used most often to identify castings with information that changes part-to-part or lot-to-lot such as serial numbers or heat treat lot data.  We will devote our attention here to integral-marking.

Integral-marking, being permanently cut into the pattern mold, is somewhat difficult to alter.  Although there are methods by which to integrally cast variable information this does add an element of cost to the casting and so is better suited for one of the applied-marking methods.

Integral-marking can be either raised or depressed on the casting surface but given a choice foundries tend to prefer that marking be raised.  The reason for this preference is that positive features on a part are the inverse in the pattern mold.  Thus a depressed part marking will be raised on the surface of the mold so will be very susceptible to damage from handling of the mold.  A designer desiring a depressed integral-marking (thus raised in the pattern mold) should consider designing the depressed lettering to be on a “raised pad” with the pad height being taller than the marking is deep.  In this manner, in the pattern mold, the “pad” will be deeper in the mold than the lettering is high so the lettering be somewhat more protected from damage when the mold is being handled.

Though commonplace, whether integral-marking be raised or depressed, there are other challenges that can be mitigated with good design.  With investment casting it is important that integral-marking be sufficiently substantial in height and stroke-width that the process will robustly reproduce them.  If marking is too shallow it will not be legible on the cast part or if too deep the integral-marking may suffer from metal penetration so, for example, the loops of a letter “B” fill with metal to then more resemble a letter “D”.

O’Fallon Casting suggests that the raised height or depth of the integral-marking be .010 - .015” from the casting surface depending on the choice of characters or fonts.  Characters and forts will cast optimally without any sharp edges or corners.

Incorporating Integral-marking into designs is one way to take advantage of the investment casting’s ability to provide part detail at minimal expense.  If you have any question if an integral-marking should be included in your casting design, please consult your O’Fallon Casting Sales Engineer.

Surface Texture of Investment Castings

The Investment Casting Institute’s “The Investment Casting Handbook” states that a 125 RMS surface texture is typical (for steel castings) and that no other casting process produces a finer surface finish than does investment casting.  Although “The Investment Casting Handbook” says Typical, a 125 RMS is commonly cited by the industry and a 125 RMS Max has become a common callout on many investment casting drawings.  As the surface texture provided by O’Fallon Casting is generally much better than a 125 RMS Max requirement it is not considered to be an issue at Contract Review.  However, every few years a profilometer inspection of an as-cast surface texture will find and reject some area of a part for being in excess of a 125 RMS Max drawing requirement.

Because of these occasional incidents I argue against placing a surface texture requirement on a casting drawing unless the texture is important to the function of the part.  My thoughts are echoed in ANSI/ASME B46.1 Appendix B on the Control & Production of Surface Texture.  Appendix B1 states that:  “Surface characteristics should not be controlled on a drawing…” as “Unnecessary restrictions may increase production costs…”.  Since the surface texture of a casting is a function of a given foundry process, the options available to improve the native surface texture will entail some nature of secondary operation that will add expense to the cost of the product. 

ANSI/ASME B46.1 Appendix B reinforces this notion by further stating that “Surface texture is the result of the processing method…”  In effect a Sand Casting will have a sand cast surface texture while a Die Casting will have a die cast surface texture and an Investment Casting an investment casting surface texture.
An accompanying Figure (B1) in ANSI/ASME B46.1 shows the normal ranges of surface texture by processing method which for Investment Castings is 60 – 200 RMS.  Designers should consider 60 – 200RMS as an appropriate range of variation when specifying a surface texture requirement and also deciding if a given surface should be machined to a finer finish or remain as-cast.

The occasional rejection of castings to a 125 RMS Max requirement is also in part explained by ANSI/ASME B46.1 where it states that “Castings are characterized by random distribution of nondirectional deviations from the nominal surface.” 
 ANSI/ASME B46.1 further states that “Surface characteristics of castings should never be considered on the same basis as machined surfaces.”  An inspection of an as-cast surface texture with a profilometer is in itself an inappropriate method for a foundry product.  The inspection of Surface texture as-cast surfaces should instead be performed by visual comparison to a standard.  GAR Electoforming (http://garelectroforming.com) manufactures a “C-9 Cast Microfinish Comparator” that can be used as a standard for visual and tactual comparison to a cast surface consistent with ASNI/ASME B46.1.  The use of this type of Comparator is highly recommended for the evaluation of an as-cast surface texture. 

Overall it is recommended that a Surface Texture should only be specified on a drawing when it is important to the function of the end product.  The surface texture of a casting will be representative of its manufacturing process and it should be inspected visually to an appropriate surface texture gage.

In-Process Weld Rework

Castings are susceptible to having small discontinuities such as inclusions, voids, cracks, etc. that are cosmetically unappealing or potentially even a cause of end product failure.  In-process weld rework of castings, sometimes also referred to as “cosmetic weld repair”, is a routine and commonplace foundry activity that is used to mend such casting discontinuities.  A casting that has been welded and blended will be dimensionally, physically, chemically and metallurgically compliant to drawing requirements.

Foundry customers derive both direct and indirect benefit from weld rework as it helps make castings more affordable and improves foundry on-time delivery.  Foundries factor a casting-yield in their production runs which is an allowance for some quantity of scrap.  A poorer than predicted yield of castings will jeopardize a customer delivery, burden the foundry with the cost for the unanticipated scrap, and also the additional expense for a re-run of parts.  In-process weld rework provides a foundry with an opportunity to salvage parts that might otherwise be scrap and so protect the customer delivery.  Foundries won’t perform unnecessary amounts of weld rework as the cost can easily exceed the value of a casting.  In effect weld rework stabilizes foundry yields which, in turn, improves the delivery and the overall affordability of a casting.

The practice for in-process weld rework is well established and presents no deleterious risk to the quality of a casting.  A properly performed weld, because of its rapid solidification, is likely stronger than the base metal of the casting.*   If you think about it prosaically, the welding of a casting is essentially a “local re-liquefaction” of something that was formerly all liquid.

For the heat treatable alloys to be metallurgically compliant the weld rework of castings must be performed prior to any heat treatment.  Post treatment welding will destroy the effect of the heat treatment and so is often called “green welding”.  Green welding should only be conducted with the express written permission of the customer.

 Weld rework of commercial castings is generally performed by foundries without reservation.  Military and aerospace drawings, however, are likely to contain an AMS or ASTM material specification that will prohibit weld rework of castings without first having written customer authorization.  Drawing notes are frequently used to convey the required permission to the foundry.  In instances where an in-process weld authorization is not automatically being flowed a foundry should identify this at contract review issue and formally request a rework authorization from its customer.

Welding of castings should be referred to as an in-process weld “rework” instead of “repair”.  In aerospace and perhaps in other industries as well, the word “Repair” alludes to a very specific and closely controlled activity.  Unfortunately, the foundry industry does often identify the procedure as a “Weld Repair”.  In aerospace vernacular an in-process weld rework being performed prior to heat treatment it is not considered to be a “Repair” and use of the term should be discouraged.

The most commonly referenced specification for the weld rework of investment castings is AMS2694.  Titled “In-Process Welding of Castings” the “Purpose” of AMS 2694 is to define “the requirements for in-process correction of foundry discontinuities by manual welding of castings.”  A discontinuity is further defined as a casting nonconformance such as a crack, damage, hot tear, cold shut, shrinkage, porosity, gas hole, inclusion, etc.  Weld rework may not be used to correct dimensional discrepancies, discontinuities found during machining, or to fabricate portions of castings unless specifically authorized by a customer.  AMS2694 requires that weld rework must be performed by qualified welders and that the welds must be blended and inspected to the requirements of the drawing.  Residual linear indications are never acceptable.

In-process weld rework is a routine foundry operation that enhances the affordability and delivery of cast products.  If you have any questions concerns regarding weld rework please contact your O’Fallon Casting Sales Engineer.

*There are a number of studies that attest to this fact.  See Paper presented by Authors Gerald Gagel, Daniel Hoefert, Joseph Hirvela, & Randy Oehrlein at AFS CastExpo 2013 on the “Effect of Weld Repair on Static and Dynamic Tensile Properties of E357-T6 Sand Castings” and concludes that the researchers found that “…repair welding had no detrimental effects on tensile or fatigue properties.”.