GD & T Basics

Geometrical Tolerances

Geometrical tolerances control the geometry in drawing. Perfect geometry or close geometrical relations improve functionality.

There are following geometrical tolerances which are classified according to their use in different situations.

To control the form of geometry

1-Straightness  2-Flatness  3-Cylinder city  4-Circularity

To control orientation of geometry

5-Parallelism  6-Perpendicularity  7-Angularity

To control the profile of geometry

8-Profile of a surface  9-Profile of a line

To control the run out

10-Circular run out  11-Total Run out

To control the location of geometry

12-Position  13-Concentricity  14-Symmetry 

Let us now see definition, where to use and how to assign each type.

Form controlling geometric tolerances‘

These tolerances ask us to maintain the outline of a profile. They have no

relation with any center or any datum. They are defined by them selves only.

1-Straightness

Definition

In the image shown here the straightness error is of 0.05 mm.

A kind of bent profile is not acceptable if a diff. in between 

two extreme bend points is 0.05 mm.

How to show this in drawing

This is a mold tool ejector pin and it's straight ness along the diameter

2.00 mm  Is needed in between 0 to 0.02. plus or minus.

How to measure it.

1-A fixture manufactured with a very high precision will give exact straightness value. Ensure that pin does not swivel during testing.

2-Start the dial indicator from one side and move it towards other end slowly.

Note the deviation of indicator. Calculate the diff between min. and max. values.

3-A fixture will give wrong result if it is manufactured at precision

less than 0.02 mm

2-Flatness

Definition

It defines the straightness of flat surface.

In the first image, the surface area covered by length and width of

rectangular top surface is having bend, twist or irregularity of 0.05 mm.

How to show this in drawing

These are shown in a box and leader placed on the face where

flatness in critical to quality.

How to measure it

Place the plate on surface plate. Move dial probe on whole surface area. Note the highest and lowest reading.

Another method is applying a blue paste on one of the plates, mate them as to be mated in real time, if all the blue gets transferred on second plate, then plates are flat enough. How ever this method will

give result as “Yes” or “No” only.

Straightness is applicable to straight profile and flatness is applicable to planar surface.

Both the terms govern the bent or twisting occurring during normal

usage.

Is Straightness/Flatness related to s/f finish?

No a s/f with bad finish will have irregularity as shown in last image.

So separate instruction shall be given to sensitise the manufacturer

about surface finish.