Straightness, Flatness & Roundness symbols in GD&T
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
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
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|
|Datum and Flatness relation|
Measurement of flatness
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
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.
Note:- Flatness & Roughness is decided on the basis of required surface finishing.
3. Roundness (Circularity)
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
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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