Dimensions and Tolerances Archives

Dual Dimensioning and ASME Y14.5M-1994

This entry is part 2 of 8 in the series Dimensions and Tolerances

Dual dimensioning is the drafting practice of using multiple units of measure in a dimension in the same direction of a feature. SolidWorks and many other CAD programs support dual dimensioning. This support is usually a little quirky. It’s actually not the fault of the CAD application. At one point, it was a surprize to me (and often is to others too) that no current drafting standard actually supports dual dimensioning. In retrospect, this makes perfect sense.

My experience is with ASME Y14.5M-1994. When invoking ASME Y14.5M-1994 (or even ANSI Y14.5M-1982), one will find that rules regarding dual dimensions do not exist. ANSI Y14.5M-1982 does mention in its appendix that support for dual dimension no longer exists in the standard. This is apparently because it was mentioned in a previous version. That said, dual dimensioning has never really ever been allowed by any incarnation of Y14.5. This is because of very specific wording under the standard’s Fundamental Rules. The wording may vary between versions, but carries the same meaning in all versions. In ASME Y14.5M, that wording is as such in 1.4(d), “Dimensions shall be selected and arranged to suit the function and mating relationship of a part and shall not be subject to more than one interpretation.” (Support for dual dimensions in pre-1982 versions was a mistake that was likely political in nature.)

General practice in the use of dual dimensions is that they are of equal importance to the primary dimension. This creates issues in that it allows for more than one interpretation of the dimension. It is nearly impossible for nominals and tolerance ranges to be identical between units of measure. This means that the dual dimension tolerance range is usually resized to fit within the tolerance range of the primary unit of measure. This creates a situation where the dimension has more than one interpretation, which is specifically prohibited by 1.4(d). The conclusion that can be drawn from this is that dual dimensions are actually not allowed by ASME Y14.5M-1994. This is the hard argument against the use of dual dimensions. I could end this article right here. However, I will also explore the soft arguments against their use.

ASME Y14.100-2004 paragraph 4.32.3 uses soft language to discourage the practice of converting inch to metric and vise verse (“should not be used”). This is known as soft conversion. This is not an outright prohibition against dual dimensioning by itself. However, the practice of soft conversion is integral to using dual dimensions. With this practice discouraged, dual dimensioning is also discouraged.

ASME Y14.5M-1994 defines a reference dimension as such,

“A dimension usually without tolerance, used for information purposes only. A reference dim is a repeat of a dimension or is derived from other values shown on the drawing or on related drawings. It is considered auxiliary information and does not govern production or inspection operations.”

By definition of reference dimensions, dual dimensions must be treated as reference dimensions. However, anyone who uses them knows this is generally not their intent. As generally intended, dual dimensions are disallowed unless they are considered reference only.

The final soft argument is gleamed in the wording of ASME Y14.5M-1994 paragraph 1.5. This paragraph assumes dual dimensions are not in use. For example it begins one paragraph as so, “Where some inch dimensions are shown on a millimeter-dimensioned drawing…”. It never then says “Where many inch dims are used on a metric drawing….” This is not a specific exclusion, but should be noted for its wording. It does allow for the use of both inch and metric units on the same drawing, but not multiple values of dimensions for the same features.

With all of these arguments aside, CAD applications do attempt to accommodate users who feel they need this capability. However, if used, caution must be exercised. Handling of dual dimensions by CAD (and common practice) can create confusion on a drawing, particularly if the software assumes values for the dual dimensions and its tolerances.

In the effort to avoid issues and violations of the standards, it is my opinion that if dual dimensions are used, they should be noted as for reference only on the drawing. This can be accomplished by adding a note similar to “DUAL DIMENSIONS IN BRACKETS ARE FOR REFERENCE ONLY.” This avoids problems caused by multiple interpretations for dimensions. Of course, over use of reference dimensions is also discouraged by ASME Y14.5M-1994. But hey, who’s it hurting?

For SolidWorks, dual dimensions on a drawing may be employed by going to Tools>Options>Document Properties>Detailing and checking Dual dimensions display. Also at that location is the choice to display the dual dimension on top, bottom, left or right of the primary dimension. These are SolidWorks 2007 instructions (other versions of SolidWorks should be similar).

I did make a sample SolidWorks macro that will turn on dual dimensions for a drawing and automatically set them to display on the bottom (default is top). This example macro can be downloaded here. It can be modified to use any settings as default.

For the record, this article was inspired by multiple posts on various SolidWorks related forums over the past few months such as these at SW Forums, eng-tips.com, and Pro/E discussion at eng-tips.com.

Dimensioning of Slots in SOLIDWORKS for ASME Y14.5

This entry is part 4 of 8 in the series Dimensions and Tolerances

Ever since the additions of the slot sketch tool for 2009 and the Hole Wizard Slot for 2014, SOLIDWORKS almost seems like a whole new software for the those who design machined parts. Adding these tools were long overdue. Additionally, SOLIDWORKS supports the standard methods for dimensioning slots when they are created by using these tools.

ASME Y14.5M-1994 paragraph 1.8.10 and figure 1-35 provide three methods for the dimensioning of slots, with no stipulation regarding which is preferred for particular scenarios. (Note: all three methods require the insertion of a non-dimensioned “2X R” note pointing at one of the slot’s end radii.)

In one fashion or another, SOLIDWORKS supports all three methods, though it does have a default for both simple slots and arc slots. For brevity, this article will only cover simple slots.

The first slot dimensioning method (a) provides the width and the distance between the end radii center points.

Dimensioning Method (a)

Method (a)

The second method (b) is the easiest and simplest to dimension. Simply state width and overall length, and use an arrow to point to the slot’s object line. Though originally reserved for punching operations, ASME Y14.5M-1994 (and later versions) allows for the use of this method on any simple slot. When using Hole Callout to dimension a slot in SOLIDWORKS 2009 or later, this is the type of dimension that is inserted.

Dimensioning Method (b)

Method (b)

The third method (c) provides the width and overall length of the slot in linear dimensions. This method is preferred if the slot has positional tolerances that use the boundary method (see ASME Y14.5M-1994 figure 5-47).

Dimensioning Method (c)

Method (c)

For all of the above methods, add the “2X R” separately by using Smart Dimension tool.

Side note: of the three choices, the ASME board almost left out (a) and (b). The original release draft of ASME Y14.5M-(1994) only shows method (c) in figure 1-35.

It’s All Over!

When “All Over” is applied to a Profile of a Surface, it pretty much defines the entire shape of a part in every direction.

ASME Y14.5M-2009 has been out for a little while now (after almost a year’s delay). There are significant improvements and clarifications. One addition in particular caught my attention, the ALL OVER symbol. When applied to a Profile of a Surface, it pretty much defines the entire shape of a part in every direction (not just ALL AROUND which applies to the profile of a surface along a particular plane).

The symbol is either a double circle at the vertex of the associated bent leader, or the words ALL OVER placed immediately below the feature control frame.

The symbol indicates that a profile tolerance or other specification shall apply all over the three-dimensional profile of a part. It is applied as “unless otherwise specified” to allow for other existing dimensions and tolerances to take precedence.

The advantage of using this symbol is that it provides control of surfaces over an entire part without regard to part orientation, thus allowing us to directly reference the CAD model as basic and fully controlled, while still detailing critical dimensions and tolerances. This may help companies better parts where they rely on the CAD model to provide complete specification. In fact, where a CAD model is declared basic, companies may be able to effectively place the Profile of a Surface FCF with the ALL OVER symbol right into their drawing title blocks along side other tolerancing information.

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.

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.

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.