Straightness Flatness Roundness symbols in GD&T

Straightness, Flatness & Roundness symbols in GD&T

In GD&T there more than 14 GD&T symbols. See below picture contains list of GD&T symbols which are mostly used in Engineering drawings. 

GD&T Symbol List

But here in this article you will learn about 3 symbols 
1. Straightness 
2. Flatness
3. Roundness

1. Straightness

According to ASME Y14.5-2009 definition of straightness is as below

Definition:- Straightness is a condition where an element of a surface, or derived median line, is a straight line. A straightness tolerance specifies a tolerance zone within which the considered element of a surface or derived median line must lie. A straightness tolerance is applied in the view where the elements to be controlled are represented by a straight line. 

From definition- an element of a surface mean edge of part, Bottom face, side face, etc. Or derived median line mean- Exact center line of part. 

Symbol of Straightness is shown in below picture:-

Straightness Symbol in GD&T

There are two types a) Surface straightness b) Axis straightness.

Surface Straightness:- The standard form of straightness is a 2-Dimensional tolerance that is used to ensure that a part is uniform across a surface or feature. It is defined as the variance of the surface within a specified line on that surface.

Axis straightness:- Axis straightness is actually a 3D tolerance that constrains the center axis of the part preventing it from bending or twisting too far.

Types of Straightness in GD&T

Measurement of straightness in GD&T

To ensure that a part or feature is axially straight, a cylinder gauge is used to determine if the part fits in its total envelope at MMC. This is both a control of the diameter and of the axial straightness. The ID of the cylinder gauge represents the maximum virtual condition of the part.

Relation between Straightness and flatness

Well not direct relation, each has its different meaning. Straightness is related to variation on a straight line. While flatness is variation across a plane (over a surface). 

By below formula we calculate Gauge cylinder I,D by using straightness symbol. 

Gauge Cylinder ID = Max Ø part (MMC) + Straightness Tolerance

2. Flatness

According to ASME Y14.5-2009 definition of Flatness is as below

Definition:- Flatness is the condition of a surface or derived median plane having all elements in one plane. A flatness tolerance specifies a tolerance zone defined by two parallel planes within which the surface or derived median plane must lie.

Symbol of Flatness is shown in below picture

Flatness Symbol in GD&T 

Relation between Datum & Flatness symbol can be understand with the below picture- There is no relation between Datum feature and Flatness. 

Datum and Flatness relation

Measurement of flatness

Flatness is can be measured using a height gauge run across the surface of the part if only the reference feature is held parallel. One thing you have to check, amount of variation across the surface, should not go beyond the flatness tolerance.

Remember what mistake we do while doing measurement for flatness, we put the part on surface plate & then we check with respect to fixed reference datum by sliding height gauge over the surface of part. Well this was the method of checking parallelism, as we fix bottom as reference feature.  Modern CMM (Faro-Arm, Mitutoyo CMM) can be used for Flatness measurement. Difference between measurement of flatness & parallelism can be understood by below picture.

Basically we check flatness where we have to find out amount of waviness, variation in surface after tightening. Below is drawing of a part where we have to check flatness, think how we can do it?

Flatness in Drawing Explained

Relation between Flatness & Parallelism

Flatness is checked over a surface of an object without any datum, while parallelism is checked on surface of an object with respect to datum.

As I have explained there is no relation with any datum feature. So we can check flatness by moving/sliding dial gauge probe, CMM probe all round of the surface, then we will see amount of variation, if that variation is more than 50 micron, then it is NG.

For parallelism first we will check height with respect to datum, and then we check amount of variation within observation with respect to datum.

Note:- Flatness & Roughness is decided on the basis of required surface finishing.

3. Roundness (Circularity)

According to ASME Y14.5-2009 definition of Roundness is as below

A circularity tolerance specifies a tolerance zone bounded by two concentric circles within which each circular element of the surface must lie. 

This graphical symbol describes how close an object is true circle at each cross section of a round object. It is also called roundness. It is not depended upon datum.

Roundness Symbol in GD&T

Measurement of roundness

We use puppy dial gauge to check the circularity over a round object. So for that we fix the object in rotating jig, attaché probe of puppy dial on object and check how much variation is object in dial gauge, when we rotate the object around the central axis. The circumference of each cross-section of the outer diameter shall be contained between two co-planar concentric circles 0.03 apart.  

This is required where we have to check roundness of bearing, shaft, etc. Attribute gauge which could be use to check roundness:- Pin Plug gauge (Simply-Pin gauge).

Roundness Measurement method

Relation between circularity and cylindricity

Circularity is concerned with individual measurement around the surface in one circle. While in cylindricity we check the roundness on entire stack.

Here cylindricity = circularity + straightness

In next article you can read about Cylindricity, Surface profile & Profile of line. 

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