Global Dimensioning & Tolerancing

By Niraj M Wanikar, Chief Editor, CNCTimes.com, 2018-06-30 19:50:00

What is Tolerance?

An acceptable amount of dimensional variation that will still allow an object to function correctly.

Few points to be known for Tolerances:

• No manufacturing process is perfect – tolerances define the acceptable limits of dimensions
• Tolerances should be based on the function of the part
• Often, the tolerances will affect the manufacturing process that is chosen
• Higher tolerances = higher cost
• Specify dimensions only to precision and tolerance necessary for the part to function properly
• Proper tolerancing ensures interchangeability – parts purchased from different vendors will function properly

Geometric Dimensioning & Tolerance

GD&T is a symbolic language used to specify the size, shape, form, orientation and location of features on a part.  It is a design tool that communicates design intent. It was created to ensure the proper assembly of mating parts, to improve quality, and to reduce cost. Geometric Dimensioning and Tolerancing (GD&T) is the way to accomplishing these objectives as defined by ASME (American Society of Mechanical Engineers) as Dimensioning and Tolerance ASME Y14.5M-1994 standard. Now, we also have an international standard that is ISO 1101.

Few points to be known for Tolerance:

• Applying tolerances to dimensions is acceptable for parts that are not mass produced or do not mate with other parts.
• Dimensional tolerance alone may not ensure that parts will function correctly, in more critical parts
• Geometric Dimensioning and Tolerancing (GD&T) augments dimensional tolerances to control important geometric features.

For example, if only tolerance is applied without considering dimensioning then the part may fail. Hence global dimensioning is very crucial.

The following figures are from Shigley’s Mechanical Engineering Design, 10^th edition, Budynas and Nisbett, McGraw Hill, 2015.

Figure 1

Fig1: Assume all the dimensions are in cm and we have the same tolerance (+/-0.01) in every single dimension, the result will be as shown in fig 2

Figure 2

Figure2: This part may meet all the dimensional tolerances of the previous figure but may not function correctly. We will witness deflection from everywhere. To rectify it, we can specify the deflection or the deformation with the GD&T symbols.

Also, the large “L” shaped figures in the figure 2 are known as Datum. It is basically used as a test jig, something that is defined as a perfect flat surface which can serve as a reference point for all the measurements.

Figure 3

Figure 3) There are several different kinds of geometric features to be considered during GD&T

1. Size
2. Location
3. Orientation
4. Form

Figure 4

Figure 4 Other than form controls, Datum is required for GD&T specification

We cannot say that the top surface is to be parallel without saying what it is parallel to – in this case the bottom surface is datum.

Figure 5

Figure 5 Datum designation on drawings. Three methods of designating a datum feature.

Figure 6

Figure 6 Features Control Frame

You can see various GD&T symbol applied for proper dimensioning.

We can generate the four geometric attributes of a feature as displayed in Figure 3 using the below table (Table 20 -1)

Why is GD&T so Important?

Geometric Dimensioning and Tolerancing (GD&T) was created to address the numerous issues that had been experienced throughout the years as associations endeavored to portray their part geometry. It is a "dialect" made out of images, used to productively and precisely convey geometric prerequisites for related highlights on components and assemblies. GD&T is vital to comprehend detailed engineering illustrations and has been effectively utilized by engineers, designers, and manufacturers in the automobile, aviation, electronics, and assembling ventures. It helps to specify and control the form, location, and introduction of the highlights of segments and fabricated parts.

The GD&T symbols for measurements on a section are characterized in connection to a datum—a hypothetically idealized point, line, or plane on a section that is utilized as a kind of the perspective point for tolerance calculations and dimensional estimations. The datum(s) on each part is viewed as the "zero point," and computations are worked starting from there and then to every other measurement. This is to guarantee the consistency of the part, giving manufacturing and measurement clarity to the design, production, and quality specialists. Furthermore, utilizing datums radically simplifies the design and specification procedures.

The currently used standards of ASME Y14.5M-2009 within the US and ISO 1101:2012 abroad have replaced previously complex and long descriptions with simple symbols. These symbols should be clearly and unanimously understood by everyone who reads and interprets an engineering drawing.

The Author: Avinash Pandey, MS Graduate in Mechanical Engineering & Management from University of Glasgow, UK, is a content writer at CNCTimes Pvt Ltd., the number one platform for channelizing resources. Having the audacity for learning about new technologies from the manufacturing sector, Avinash Pandey holds specialty in writing case studies, technical articles, technical blogs, and social media marketing content.

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