Don’t you just love Wednesdays? Monday is a distant memory, Friday is just in sight and of course…Standards Wednesday. If you have never heard of Standards Wednesday…where have you been? On Wednesdays, I try my best to break down different standards and specifications that impact our daily life as engineers, designers and drafters. For the past few weeks we have been taking a detailed look at ASME Y14.5-1994, and this week is no different. Today we will get knee deep into Section 2.7, Limits of Size; try not to get any on your good shirt.

What are Limits of Size?

Limits of Size refers to individual features of size and how much their form and size may vary. Of course, this applies unless it specified differently elsewhere on the drawing, but we will get to that later.

Individual Feature of Size (Rule#1)

When the size of a feature is toleranced, with conventional methods (bilateral, unilateral and limits, etc…) ,the limits of size define the maximum variation of Form and Size for the feature. Imagine a zone created by the upper and lower limits of the dimension tolerance, all variations of the feature must lie within this zone through out the length of the feature.

Variations of Size

If you were to create a cross section of the feature, at any random point, the entire section must fall within the zone described above. No matter how many times you make that cross section, each section must lie within that zone.

Variations of Form (Envelope Principle)

I think most everybody knows that the size of the feature is controlled by tolerance, but did you know that  the form is also affected. First thing you should know is, when the feature is at MMC the form MUST be geometrically perfect. MMC, or Maximum Material Condition, refers to the side of the tolerance zone that allows for the most amount of actual physical material. For example, if you have a rod, the upper limit of the tolerance has the most material, or MMC. That’s easy but what is the MMC of a hole? Well, the smaller the hole the more material in the part; so the lower limit of the tolerance of a hole is the MMC. Why does the feature have to be geometrically perfect at MMC? If a rod is the biggest it can get; it must be perfectly straight or it will violate the tolerance zone.

As a feature size moves away from the MMC condition, the form no longer needs to be perfect; as long as it still lies within the tolerance zone. Look at this figure… (The one below, not mine!)

The first figure shows how the feature is dimensioned on the print (1.000+/-.250). The next two figures show the tolerance zone created, shown in blue. You can see from these two figures, the finished part can be dramatically different then the perfect form; as long as it lies within the described zone. Heck, you can have a round hole drawn but the tolerance zone is large enough…you could get a square.

Exceptions to the Rule

As with almost all rules in life, there are exceptions to rule #1. There are times when the form of the feature is not controlled.

Exception #1

Form control based on Limits of Size does not apply to raw materials such as bar stock, rod stock, and other items that are made to other governing standards that specify straightness, flatness, and other characteristics. Un-machined surfaces are to be considered controlled by the other standards unless you apply separate geometric tolerances to these surfaces.

Exception #2

Form control based on Limits of Size does not apply to parts that have a Free State Condition that is different then the constrained state. The Free State Condition refers to the state of a part when all constraints that are applied during the manufacturing process are removed.

Perfect Form at MMC not Required?

Sometimes you just don’t care if a part is a little “off” when it is at MMC. Perhaps you have a soft metal tube that may not be exactly straight and it would be too costly to try to control the straightness. You can just add the states “PERFECT FORM AT MMC NOT REQUIRED”. Your machinist will thank you.

It So Hard To Say Good-bye…

Well…that does it! I am outta here… my brain hurts more to come shortly. Keep those great questions coming.

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