Measurement procedure and GD and t Rules

How to make Inspection standard with GD&T rules?

Some product are build with multiple components. Those components are assembled to make sub assembly, then these sub assembly are assemble with other components to make final assembly components.

If we have final part drawing, we have to make final part inspection standard. And for sub assembly components we have to make inspection standard for that component. Because to make many parts with same quality, we have to fix a standard to making. That standard is inspection standard. 

By referring inspection standards, we check parts & do assembly of parts. After this again check final component as per its inspection standard. So inspection standard is very important for sustaining quality product throughout its product life.  

Basically Inspection standard is measurement procedure of a part. Measurement procedure mean, how we will check the part/do the inspection of particular part. 

In this article, we will cover following topics:-

  1. What is inspection standard
  2. Why inspection standard required.
  3. What are requirements for making inspection standard?
  4. What is Datum and GD&T rules in Part drawing
  5. Measurement tools & their least counts
  6. Rules of GD&T for part inspection
  7. Conclusion

What is inspection standard of a part?

Well as we read, it is measurement procedure of checking a part as per required condition. Inspection standard cover details like
1. What to check, 
2. Where to check, 
3. By which instrument to be check, 
4. How many time to be check, 
5. From which reference Datum to be check 
6. In which direction to be check, 
7. Special control over some critical dimensions
8. Some GD&T explanation in standard
9. Mentions of reference standards for Quality and testing purpose
10. Contain drawing or customer requirement's notes

So all above mention points to be added in Inspection standard format for any part. 

We make inspection standard format for part inspection, that format could be for final part or for particular part. There could be multiple format for part inspection, but ultimate purpose is to cover all required inspection check points. But better inspection is that which contain all information. Because it is like "a Guide Book" for inspector to check the part. 

Because there could be different ideology or method for doing inspection of part by different-different people. But if we make single inspection standard for a part, then we are freezing a method of doing inspection. Well then you must have to gain knowledge of inspection. But before that you must have to know about drawing & its tolerance requirements. 

Why inspection standard required?

As explained, many people have their own method of inspection, so to make single method of checking of a particular part, we need inspection standard. Also we can not do judgment by merely checking some drawing dimensions. Inspection standard cover many information, we need to follow those requirement to check a part. 

Inspection standard is retained throughout product life. It will be used, when inspection will be required. Inspection standard is master for inspection, for inspection standard drawing is master. 

What are requirement for making Inspection standard?

Drawing understanding is first requirement, then reference standard. While making inspection standard we try to explain, what will be method of doing inspection. So for inspection of any part alignment is very important.  

Here drawing understanding mean, knowing GD&T drawing symbols and customer specific notes and views, Datums and other basic requirements.

So here Datum is first thing that need to understand:-

What is Datum?

A datum is a plane, a straight line, or a point that is used as a reference when processing a material or measuring the dimensions of a target.

According to ASME Y14.5-2009, definition of Datum is as below:-

A theoretically exact point, axis, line, plane, or combination thereof derived from theoretical datum feature (Symbol) simulator.

What is meaning of "derived from theoretical datum feature" in Datum definition
Before understanding theoretical datum feature, we have to first understand datum feature

What is Datum feature in drawing?

It is a feature of part that is identified with either a datum feature symbol or datum target symbol

If you not understand, it mean for any part, we have to identified datum in part drawing (Like Datum-A, B ,C Plane, surface, point or any circle line, etc.) from where dimensional measurement should be made. I show you datum feature in drawing with a picture (See below Picture).

Picture 1- Datum Feature in Drawing Explained

Datum feature could be of two types:-

1. Theoretical datum feature simulator
2. Physical Datum feature Simulator

Theoretical datum feature simulator mean theoretically perfect boundary used to established a datum feature from a specified datum feature. Means- A datum datum feature (A, B C line, plane) either from body center or from body surface or from center of Hole from where dimensional measurement is possible for any target datum datum feature. This center point and line is theoretical boundary used to measure dimension, rather part fixing is on other surface of part. 

Physical datum feature simulator mean physical boundary used to established a simulated datum from a specified datum feature. Mean a physical boundary, What it could be -Surface Plate, Fixture element, etc.

So this was little intro about Datum in drawing. This has to be identified to make any measurement procedure any part by CMM or by any measurement instrument. 

Apart from Datum feature there are other drawing feature that has also to be identified to make correct Inspection standard. Those feature could be GD&T symbols and drawing symbols. 

GD&T symbols list is as below:-
Picture-2: GD&T symbols List 

You can read details explanation of GD&T symbol and how to measure them by clicking this link -GD&T symbols in Drawing-How to measure them After drawing proper understanding, next thing is to decide part alignment. 

How to do Alignment of part for Inspection?

First we divide part in X, Y, Z coordinate in drawing, this could be done by drawing line by pencil on drawing. Next we Follow 3-2-1 Rule for alignment. 

We need 3 points to make a plane. 
We need 2 points to make a line.
We need 1 point that is intersection point, which will be origin. 

So target is to find origin. For that we have to find surface of part in drawing, or we can say we will start with surface. Generally it will be in Z axis. Surface is that datum feature on which part will be rest. 

Next is to make Axis. It is a line, meeting two points. While making axis, we have to remember that axis will the line which will divide the part in half, or we can say we are finding X or Y axis (Any One).

So far we have found 2 axis, one-Z axis and other X, Y. Now we have to found last axis, that will make Origin after intersecting others.

Here X, Y axis drawn by pencil on drawing paper could be theoretical datum feature. But we do this to make origin point. 
Lets say we make surface in Z axis and A line is passing from 2 points, and we have to draw a perpendicular line (or has to make) to intersect axis line on a point. That intersection point could be datum feature. 

See below picture for understanding-Basic part alignment 

Picture-3 : Part alignment procedure of part

In Picture what we see.. Part has been divided into 2 axis-X or Y. Now we found datum for part resting. Datum A is surface, for which we need 4 points. Next is to found axis passing over that surface. Datum B is axis passing from center of two circle of Locks. Now we have to draw a perpendicular line over that axis to make point.  We can draw by CAD system, but definition of that perpendicular line to be define properly in inspection standard. So here that line is center line from dimension 5, Between two planes of locks. This line will intersect Axis 1 on same surface, which will create point. This point is Origin of part. Origin in itself is Datum feature, from where all target dimension  to me measured. 

GD&T rules for Part inspection

GD&T rules mean, basic representation of  symbols of Datum & Tolerances in drawing.

1. Datum feature triangle may be Filled or Not filled
2. Datum feature surface could be on feature surface or on an extended line (See Picture-4)
3. Datum can be represent by small letter or Upper case letter or by special symbol like-#, * 
4. When datum is given on dimension then it is passing from center of the dimension (See Pic-5)
5. You can not offset datum from a dimension, if you that that origin will shift. 

Picture-4 : GD&T Rules for part inspection

Picture-5 : GD&T rules for part inspection.


 With this article we have explained "What is inspection standard or measurement standard, why inspection standard is required in industries, how to make perfect inspection standard. What thing to be remember while making inspection standard.

Also we have learned about basic definition of Datum and datum feature from ASME Y14.5.
We learned definition and there representation in drawing, what are rules to identified the datum in part drawing. 

And we learned about how to do alignment of part for inspection by CMM. How to identify the origin of part for inspection.  We also learned about GD&T rules of part inspection.

Hope this article will help people to understand basic requirement for measurement standard, if you have any concern or suggestion write on

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