ASME Standards Archives - Page 5 of 9

Datum Changes in ASME Y14.5-2009

Under ASME Y14.5-2009, Maximum Material Condition (MMC) can now apply to datums that are features of size and also surfaces. The 94 standard would only allow MMC on datums that were features of size and NOT surfaces.

The following is posted about datum changes with the permission of the author, David DeLong, who is a ASME GD&T Professional (GDTP) at Quality Management Services, Inc.

Datum Changes to ASME Y14.5 – 2009

A feature of size is a hole or pin of any shape and also a width. In most cases in GD&T, the holes or pins are most important to assembly and are used a great deal as secondary and tertiary datums. Usually, the perimeter of a non-cylindrical part is not functionally important. There are certain cases where there may be a partial hole or cutout that is used in assembly and could now be referenced as a datum.

Maximum Material Boundary

The Maximum Material Boundary (MMB) is a new term used in the 2009 standard and replaces the terms “Maximum Material Condition” and also “Virtual Condition Size” when referring to a datums referenced with the maximum material condition symbol.

In certain cases, MMB is the maximum material size while in other situations, it is the virtual condition size. It depends upon whether the datum is a primary, secondary or tertiary datum.

Let’s review the MMB for datum G in the above example.

If datum G was referenced as a primary datum, the MMB would be the MMC size of the hole which would be the smallest allowable size of the 12 mm hole which is 11.6 mm. It does not make any difference whether or not the feature actually has a virtual condition size as shown, the MMB is still 11.6 mm..

In our example, datum G is referenced at MMC as a secondary datum so the MMB is 12 – 0.4 – 0.2 = 11.4 mm which is the virtual condition size of the hole. If the secondary datum did not have a virtual condition size, it would default to its maximum material condition size of 11.6.

Datum H Reviewed

If datum H was referenced as a primary datum, the MMB would be its maximum material condition size or smallest allowable size – 8.6 mm.

If datum H was referenced as a secondary datum, the MMB would be its virtual condition size but, in our situation, we have two (2) virtual condition sizes.

The positional tolerance shown would give us a virtual condition diametrical tolerance zone size of 9 – 0.4 (MMC) – 0.3 (perpendicularity) = 8.3 mm.

We also have a refinement of the positional tolerance with a perpendicularity requirement. In this situation, we have a virtual condition size of 9 – 0.4 (MMC) – 0.2 (perpendicularity) = 8.4 mm.

So, if datum H was referenced as a secondary datum, one would use the perpendicularity refinement resulting in a MMB of 9 – 0.4 – 0.2 (perpendicularity) = 8.4 mm.

In our situation, datum H is a tertiary datum and only used for orienting (anti-rotation) the part about datum G so that we are able to confirm all the dimensions. In our situation, we will use the MMB of 9 – 0.4 – 0.3 (positional) = 8.3 mm which includes the positional tolerances rather than its refinement of a perpendicular tolerance.

Here we have 4 holes of 8 +/- 0.3 mm. The feature control frame reflects a positional tolera

nce of a diametrical tolerance zone of 0.25 mm beyond the MMC referencing primary datum A (usually the mounting surface), secondary datum G at MMC (12 mm hole) and tertiary datum H also at MMC (9 mm hole).

We have already discussed that fact that the MMB changes depending upon whether it is a primary, secondary or tertiary datum. If there is any doubt about the MMB, one can reflect the actual MMB size in the feature control frame as shown above using brackets about the MMB size. This method can also be used if MMB size differs from the calculated size.

Let’s say we wanted the MMB size of datum H to be its refinement size of 8.4. One would then replace the 8.3 in the feature control frame with the refined size of 8.4 and that superseded the calculated MMB size.

For further details, please see the full article at Datums 2009.

Point Location (Virtual Sharp)

Point Locations by another name, such VIrtual Sharps

The names for dimensioning methods within ASME Y14.5 often do not match the common names. For example, what most of us call ordinate dimensioning is officially labelled as rectangular coordinate dimensioning. This can make information about certain dimensioning methods hard to find within the standard. One dimensioning method that is particularly difficult to find is point location. A point location is where a point is located by the intersection of extension lines only. The method is known by so many other names.

theoretical sharp corner
theoretical corner
theoretical sharp
apex
intersect
intersection
intersection point
imaginary point
virtual sharp
and likely others as well

The SOLIDWORKS application uses the term virtual sharp. SOLIDWORKS offers a list of options for the delineation of virtual sharps (i.e., point locations). These options are found at Tools pulldown>Options...>Document Properties tab>Dimensions heading>Virtual Sharps subheading. The only method supported by ASME Y14.5-2018 is the use of intersecting extension lines from two surfaces; so called witness in SOLIDWORKS.

The standard does not require any other identifier or labelling. Yet many of us do feel compelled to add some sort of label to the dimension, using one of the above terms or their initials. A label does add clarity, particularly when the scale of a view makes display of a point location hard to read.

I covered this topic once before from a slightly different perspective in this article: Virtual Sharps. That article includes instructions on how to create a virtual sharp in SOLIDWORKS drawings.

Engineering related societies

The following is a list of professional societies related to the mechanical engineering field. These groups either have some sort of certification process or are responsible for the control of various commonly used standards. If other societies should be added to this list, please feel free to comment with their information, or email me directly.

American Society of Mechanical Engineers (ASME)
ASTM International (formerly American Society for Testing and Materials)
International Organization for Standardation (ISO)
National Society of Professional Engineers (NSPE)
Society of Plastics Engineers (SPE)
SAE International (formerly Society of Automotive Engineers)
Society of the Plastics Industry (SPI)

Controlled Radius

It’s been many years since ASME Y14.5M-1994 introduced the controlled radius symbol. Yet, we will still frequent find individuals in the industry who have never seen the symbol, nor know what it is. The symbol is CR.

It’s been many years since ASME Y14.5M-1994 introduced the controlled radius symbol. Yet, we will still frequent find individuals in the industry who have never seen the symbol, nor know what it is. The symbol is CR. Really, a controlled radius is actually just a radius that is a fair curve, with no reversals. I’ve not read ASME Y14.5-1982 in a very long time, but I believe this is actually similar to the original definition of a plain ol’ radius from the older standard.

Since ASME Y14.5M-1994, a simple radius has no fair or reversal limitation. As long as the arc of the radius feature’s profile falls within the tolerance zone, it is considered acceptable. These are represented by R.

So much time has gone by since the introduction of CR, I am left wondering why so many people have never seen it. The reason CR was created, as it seems, was to allow engineers to specify a radius without the need for it to be fair or non-reversed. This is good for breaking edges or filling corners. A CR would be more useful when fit and/or function is important, such as guiding features. In this way, the added expense of a creating a fair and non-reversed curve would only be employed when it is necessary for function.

Interpretation of Limits (ASME)

Some might look at the limits of a tolerance zone as non-absolute, but is that correct? ASME standards tell a different story for Interpretation of Limits.

When reading tolerances on engineering drawings, one of the finer points that comes up during Quality inspection is how to interpret tolerance limits. Some might look at the limits of a tolerance zone as non-absolute.

In other words, if a feature measures 14.004, but the upper limit specified on the drawing is 14.00, then one might be inclined to accept the part because 14.004 can be rounded to 14.00. However, according to ASME Y14.5-2009 (and any earlier versions), this is false reasoning.

All limits are absolute. Dimensional limits, regardless of the number of decimal places, are used as if they were continued with zeros.

The example given is similar to this: 12.2 means 12.20…0 (zero to infinity).

So, with that clear statement, interpretation of limits is always absolute. A measurement of 14.004 is a nonconforming part if the upper limit is 14.00. This is important, as it eliminates ambiguity and the opportunity to fudge with the numbers in a way that can affect quality and even product definition over time.

Drawings represent final product

One comment I’ve seen about ASME suggests that it is geared towards fully detailing product definition. One trap that rookie designers and engineers will often fall into is over-specifying their parts by placing manufacturing process information on the drawing.

The new designer may do this because maybe a machine shop made the part wrong and was trying to work the rookie’s inexperience to weasel out of their responsibility. Maybe someone in Quality Control was confused by a drawing because they don’t have adequate blueprint reading skills, so they come to the new designer to ask that more information be spelled out on the drawing (when it is already fully specified). These are just a couple of examples. Often, new designers don’t know why manufacturing processes are not included on drawings, nor even that there exists standards that forbid it.

ASME Y14.5-2009 (and previous versions) states:

1.4(d)The drawing should define a part without specifying manufacturing methods. …However, in those instances where manufacturing, processing, quality assurance, or environmental information is essential to the definition of engineering requirements, it shall be specified on the drawing or in a document referenced on the drawing.

It is usually pretty obvious when manufacturing methods are necessary to the engineering requirements, even to the individuals new to the field. Unless one is in particular industries, manufacturing methods are almost never required. A drawing should fully detail the final product without over specification.

ASME Y14.5-2009 adds as an example:

Thus, only the diameter of a hole is given without indicating whether it is to be drilled, reamed, punched, or made by any other operation.

The manufacturer is responsible to provide a final product that complies with the drawing regardless to the processes they use. It is still important for designers to know the processes that will most likely be employed, so they know that the product is economically manufacturable. This does not mean that they should unnecessarily limit the manufacturer to particular processes.