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GD&T: An Internationally Approved Standard.

September 26, 2012
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This standard gives the guidelines on the rules and principles of creating (mechanical) engineering drawings not only in the mechanical field but also in the electronic engineering field, rather in any engineering field.

This is the basic and very important subject and the success of the application depends on how well you master in this subject, it is easy but little bit complicated to understand.

Since the world is technologically advancing worldwide, it has become necessary to standardize the use and application of this standard to enable the user from any part of the world would understand the drawings or models created using the guidelines of the standard. The reason for standardization is because to make a unique communication media within the industries; no matter where the drawings are created and the location of the manufacturing takes place may also be different. So it is necessary to have a common communication media throughout the world and that gives the opportunity for the engineers learn and understand the guidelines on how the drawings are created before release for production.

There are two elements inherent in this standard namely: Geometry and Dimensioning and Tolerancing. Standard ASME-Y14.5M-1994 is titled as Dimensioning and Tolerancing

Geometry: Those who are taken the Plane Geometry, Solid Geometry and Analytical Geometry in their curriculum, they may find the whole concept of G D & T easily without much effort. Again, in any new subject there is a learning curve or time to understand the concept and apply them in the practical field. Design engineers give shape to their imagination by creating drawing of the object they are visualizing for a customer according to their requirement. Manufacturing engineer gives physical shape to that product and the quality verifies whether it meets the customer requirement. Therefore design engineer has full authority to creating something from the customer specification and the manufacturing engineer has the authority for process design or to make the product from the design drawing, quality control inspection has the authority to verify the product and create the report whether the product meets or does not meet the drawing requirement. The authority of a quality assurance engineer is the combination of all of these above and he/she can even has the authority to get the engineering drawing changed if proven that the design does not meet the requirement. A good engineering drawing should have all the views such as: front elevation (view) plan (top view) side views (end views) and sometimes the designer has to cut certain portion of the object and project it some area in the drawing to explain some of the hidden spots, cavities, holes etc. are normally not visible from the three views (front elevation, plan and side view) to others to understand it

Dimensioning & Tolerancing.

Creating an engineering drawing on a paper or model is not sufficient, the hardest part is to give proper dimension to the objected created. Tolerance conditions are based on the calculations based on several factors, material, and method, environment life of the part, safety, reliability, quality and fit form function of mating part. Economical factor is a big factor that most of the design engineers ignore while dimensioning the object. Variability reduction is one of the objectives of the designer with that in mind they cannot tighten the tolerances beyond Producibility measure and at the same time they cannot make the tolerances too loose.

Types of Tolerances:

There are several terms and symbols used in this standard those are also very important and should be understood its meaning and how to verify those in the real world by inspection

Commonly used terms are given below without the symbols

  • At Maximum Material Condition
  • At Least Material Condition
  • Projected Tolerance Zone
  • Free State
  • Tangent Plane
  • Diameter
  • Spherical Plane
  • Radius
  • Spherical Radius
  • Controlled Radius
  • Reference
  • Arc Length
  • Statistical Tolerance and
  • Between

 

Deeper you understand the standard, more you will enjoy using this tool and you may be able to identify the deficiencies in the engineering drawing language when you evaluate them closely during problem solving process. A perfect engineering drawing will eliminate several problems down the line or in the assembly and functioning of the assembly or end product. Eighty percent of the product or process related problems are caused by twenty percent of the engineering drawing issues.  To get the maximum bang for the buck you should find a matured and experienced instructor in this field to walk you through the process until you understand the concept. Each of these tolerances mentioned above has definition and explanation in the real field of its application; this can be found in the ASME-Y14.5M – 1994 Standard. I recommend getting a copy of this from the distributors of the standard and learn at your leisure.

Tolerances of Location

This is bit complicated portion of the standard related to the definition and application of feature control frames, true position with maximum material condition and least material condition concept. Mr. Bill Tandler has given an excellent presentation on Datums, Feature control frames and compound feature control frames in the June and July issue of the Quality Magazine. I recommend going over his articles to get an idea of the definition and application of those terms.

It is difficult to work with the given engineering data such as true position, maximum material condition, least material condition, basic dimension etc. To convert the given true position tolerance into a working format or working form by using a simple mathematical calculation into a linear tolerance value and it becomes easy for the manufacturing engineer for the planning and programing the CNC machine to make the part. After making the parts, it is recommended to evaluate the first produced part one hundred percent to make sure that part does indeed meet the engineering drawing requirement. Any abnormalities or the features do not meet the requirement it should be documented and corrected before continuing the production. Inspection data from the manufactured part can be converted into true position tolerance to verify the accuracy to the drawing requirement. An accurately planned and executed manufacturing process will always deliver the right product every time. Process monitoring is very important to assure all the key elements or the variables are controlled that may have influence with: man, machine, method, material, measurement and environment.

 

maximum material condition

 

Above demonstration shows how the maximum material condition (MMC) changes according to the growth of the given hole diameter (0.260 +/- 0.004) and reaches to the point of least material condition (LMC)

Basic dimension (0.875) is a dimension contained in a rectangular format shown in the engineering drawing. That means, the mating part has a feature that requires 0.875 spaces in between for the next assembly without any tolerance. We know the fact that without any tolerance manufacturing would be very difficult and therefore the designer has allowed a true position tolerance for the hole to compensate or accommodate the feature with 0.875 dimension.

MMC – Maximum Material Condition: for a hole = the smallest size of the hole within the given specification, see the table above.

LMC – Least Material Condition: for a hole = largest size of the hole within the given specification, see the table above.

MMC for a shaft is the largest outside diameter of the shaft within the given specification.

LMC for a shaft is the smallest outside diameter of the shaft within the given specification.

Example: A shaft of 0.750 +/- 0.005

MMC of the shaft = 0.755 (Maximum diameter)

LMC of the shaft = 0.745 (Least diameter).

Other forms of dimensions used on the engineering drawing are the (1) unilateral tolerance, (2) bilateral tolerance and (3) range tolerance.

Example: (1) Unilateral tolerance: 0.750 + 0.000 / - 0.005 and 0.750 + 0.005 / - 0.000

Example: (2) Bilateral tolerance: 0.750 + / - 0.005

Example: (3) Range tolerance: 0.750 / 0.760 (Generally shown from small number to large number)

It is necessary to convert the given engineering data or information into a working format or working form by using a simple mathematical formula. For example: True Position tolerance into linear dimensional tolerance, that makes the manufacturing engineer in planning the manufacturing process and from the dimensional inspection data of the part, can be converted into true position tolerance to make sure the results are meeting the engineering drawing requirements. See the example below at Fig: 1 and the demonstration in the copy of excel data sheet followed the Fig: 1.

 

Figure One GDT

 

REQUIREMENT. Fig 1

INSPECTION RESULTS

 

X Axis

Y Axis

X Axis

Y Axis

Hole Dia

1

0.375

1.375

0.3795

1.3705

0.260

2

0.375

0.375

0.3765

0.372

0.257

3

2.025

1.375

2.020

1.379

0.263

4

2.025

0.375

2.0285

0.3705

0.2565

 

CALCULATE THE ACTUAL HOLE LOCATION

YOUR INPUT

X Axis

Y Axis

Actual Hole Location

Guide

-0.0045

0.0045

0.013

0.010

-0.0015

0.003

0.007

0.007

0.005

-0.004

0.013

0.013

-0.0035

0.0045

0.0114

0.007

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