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.

Limits and fits

Limits and fits

Dimensional tolerances are decided according to type of required fits. The type of required fits decides the limits.

Following are types of fits.

Clearance Fit-Clearance always exists in between maximum allowed shaft diameter and minimum allowed hole diameter.

Take example of shaft and bushing. In worst case shaft dimension will be 59.99 and bushing ID will be 60.01.Clearance of 0.02 mm. 

Interference Fit-Interference always exists in between minimum allowed shaft diameter and maximum allowed hole diameter.

Take example of shaft and bushing. In worst case shaft dimension will be 60.01 and bushing ID will be 59.99.Interference of 0.02 mm. 

Transition Fit- Close Interference or close clearance always exists in between shaft and hole diameter.

This will be clear from this example .Following extreme conditions are possible here.

1-Shaft diameter 60.01mm and hole diameter 60.02mm-close clearance of 0.01 mm.

2-Shaft diameter 60.01 mm and hole diameter 60.04mm-close clearance of 0.03 mm.

3-Shaft diameter 60.05 and hole diameter 60.02 mm-close interference of 0.03 mm.

4-Shaft diameter 60.05 and hole diameter 60.04 mm-close interference of 0.01 mm.

CLEARANCE FIT – TRANSITION FIT – INTERFERENCE FIT-This is sequence for assembly efforts. Clearance fit will need minimum efforts, transition will need medium and interference type fit will need maximum efforts. But on the other hand  alignment accuracy level increases from clearance type to interference type of fit.

We designers should be smart enough  to decide type of fit according to functionality of part. There are many but following are some points ,we shall keep in mind while deciding type of fit.

1-For how many times dismantling of parts is needed in total life of assembly.

2-What is role of part in assembly, is it going to move, revolve etc.etc.

3-What is expected accuracy level of assembly.

4-Ether assembly allows me lubrication or not.

5-Kind of disassembly needed-destructive or non destructive.

6-Beyond that what is raw material of mating parts.

Types of dimensional Tolerance

Types of dimensional tolerances.

There are three types of dimensional tolerances. At this stage let us not think about codes like H7 and g6.

1.Bilateral 

2.Unilateral

3.Basic

1.In bilateral tolerance dimension is allowed to vary on plus as well as minus side of ideal condition.. 

2.In Unilateral tolerance dimension is allowed to vary on only positive or negative side of ideal condition

3.In Basic tolerances dimension is shown without tolerance and outlined by a rectangle. Allowed variation is shown in terms of geometric symbols.

What is basic dimension-Is it something with zero tolerance or something with open tolerance?

Both the statements are wrong because practically it is impossible to achieve dimension with zero tolerance and outlined box says that it is special kind of dimension, so open tolerance shall not be applicable at any time.

Observe that two drawings are same unless in first dim.20.00 is shown as basic and in second drawings it’s not shown. The second one  will produce component with more errors.

If I follow second drawing, I will assume that dim 40.00 as well as 20.00 can be manufactured with an error of 0.2 per side according to note, while maintaining geometric tolerance given to angle 27 degree. Angle 27 should maintain it’s angularity within 0.1 degrees. Is this possible without controlling height 20.00 mm?. Definitely It is possible as we can adjust dimension 40.00 mm accordingly.

What if I have to minimise errors?-Ok,

First drawing is telling me clearly that I have to control height 20.00 mm while controlling angle 27 within geometric tolerance.

Ok why I have been not given dim 20.00 mm with some close tolerance like 0.02 mm per side?

No it’s not proper way because angle is controlled by intersection point A and location of point A can not have tolerances mentioned two times. After maintaining 27 degrees within 0.1 with datum B and dim at 40.20,operator shall work on dim 20.00 to make it more accurate, as he has material in hand i.e. dimension 40.20 can be reduced up to 39.80

Giving a close tolerance to dim 20.00 is bad idea as operator may get confused between 2 tolerances.

Types of Tolerances

Types and Applications of tolerances in manufacturing drawings

Dear Reader,

Days have gone when mating parts were manufactured with physical reference of each other. In modern designs parts are manufactured

with specific tolerance to each dimension and hence interchangeability is obtained.

There are only two types of tolerances.

1. Dimensional tolerances.

2. Geometrical tolerances.

1.Dimensional tolerances.

Name it self states that this type of tolerance is related to dimensions of a feature.

Example: Length in mm shown as 20.00±0.02 and Diameter shown as

10-0.02/-0.04 Means manufacturer has been given freedom of 20µ on each side

for length ,he may produce a part with length 20.02 Or 19.98 mm.

And diameter should be not more than 9.98 mm and less than 9.96 mm.

2. Geometrical tolerances.

Less related to dimensions and more related to characteristics of one or

more features.

Example: Step with Diameter 8.00mm should be concentric within circular spread of 0.04 mm  with step diameter represented by datum A i.e. Dia 10.00 mm.ie concentricity at each section over the length in between two cylindrical geometries.