Types of Tolerances used in a technical drawing mainly indicate the permissible amount of deviations from the original dimension.
Tolerance can be classified as follows
- Unilateral Tolerance
- Bilateral Tolerance
- Compound Tolerance
- Geometric Tolerance
Tolerance or permissible variation of dimension is always allowed during machining of a mating component or any engineering parts to avoid high cost and long time consumption.
The permissible deviation of true dimension of a mating part without hindering any functional accuracy and useful for actual working is known as tolerance.
Unilateral Tolerance: In Unilateral tolerance the variation from the original profile is only allowed in one direction i.e tolerance limits lie only above or below the basic size. Here the variation in dimension is permitted only in one direction, not in both the directions.
Bilateral Tolerance: In Bilateral Tolerance the size of the part varies above and below the basic size, i.e. the variation in dimension is in both the directions.
Compound Tolerance: Compound Tolerance can be calculated by adding or subtracting multiple Tolerances associated with different dimensions. Compound Tolerance is derived by using different established Tolerances (for e.g. lateral, angular etc).
From the above figure it is clear that Compound tolerance on I is dependent on tolerances of L, h and θ.Here I is the combination of three tolerances .The minimum tolerance on ‘l’ will be corresponding to L-b, θ+∝ and h+c.
Geometric Tolerance: Geometrical tolerances are associated with certain geometrical features like flatness of a plane surface, straightness of a cylinder, squareness of a square shape, roundness etc.
While defining the tolerances of a complex structure requiring various tolerances in multiple dimensions, it is important to note the individual tolerance provided are kept on check so that, the cumulative effect of tolerance build-up doesn’t make the overall tolerance to be un undesirable range.
This is particularly true in case of specifying dimensional geometric tolerances which are frequently used to indicate the relationship of one part to another. This is also called positional tolerance which is a 3 dimensional geometric tolerance comprising of position, symmetry and concentricity of various parts making up the object.
What is Compound Tolerance?
Compound Tolerance is a combination of established tolerances on more than one dimensions.
Compound Tolerance is determined by combining more than one tolerances associated with different dimensions, these tolerances may be angular or lateral.
Compound Tolerance Examples
Example of a Compound Tolerance is given below:
Here to calculate tolerance for the dimension L we have to consider tolerances associated with D,H and ϴ.
The maximum limit of L can be obtained when base is D+a, angle is ϴ+a and vertical is H-d.
The minimum limit of L can be obtained when base dimension is D-b , angle is ϴ-b and vertical dimension is H+c.
What is Bilateral Tolerance?
It is the type of tolerance where only the nominal size is defined and error margin is provided on both upper and lower end, both of which are having same absolute value.
In case of Bilateral Tolerance variation from the true profile is permitted in both directions.
Here Basic size:20 mm
Upper limit:20+0.002=20.002 mm
Lower limit:20-0.002=19.998 mm
What is Unilateral Tolerance?
During machining of mating parts Unilateral Tolerance assist the operators to calculate the deviation in a easy and simple way.
In Unilateral tolerance the variation from the original profile is only allowed in one direction i.e tolerance limits lie only above or below the basic size. Here the variation in dimension is permitted only in one direction, not in both the directions.
Example of Unilateral Tolerance for machining a shaft:
Here we try to achieve a diameter of 25 cm, known as basic or nominal size. The deviation from the basic size is permitted as +0.02 cm as upper limit and +0.01 cm as lower limit. Here the variation in dimension is allowed only in positive direction or above the basic size.
Basic size: 25 cm
Upper limit: 25+.02=25.02cm
Lower limit: 25+.01=25.01cm
Bilateral Tolerance Examples
In case of Bilateral Tolerance Upper and Lower limit values lie on both sides (above and below) of basic value.
Examples of Bilateral Tolerance are:
Here
Basic size:25 mm
Upper limit:25+0.02=25.02 mm
Lower limit:25-0.02=24.98 mm
Here
Basic size:25 mm
Upper limit:25+0.02=25.02 mm
Lower limit:25-0.01=24.99 mm
Why Unilateral Tolerance is preferred over Bilateral Tolerance?
In Unilateral Tolerance variation is made only in one direction from the basic size as against bilateral tolerance, where the upper and lower limit of values lies on the both sides of basic value.
There are a good number of reasons for preference of unilateral tolerance as it provides significantly higher advantages over bilateral tolerance.
- The deviation from basic size is only in one direction due to which it is easy and simple to calculate the tolerances.
- suitable for interchangeable manufacturing processes specially where precision fits are required.
- In Unilateral Tolerance Go Gauge ends can be standardized as the holes of different tolerance grades possess same lower limit and all the shafts have same upper limit.
- In case of Unilateral Tolerance the chance of rejection in machining the mating parts is very less. The operator can machine the upper limit of the shaft or lower limit for the hole, because he get extra margin to modify if changes are required later during final fitup.
Why use Unilateral Tolerance?
Reasons to use Unilateral Tolerance are :
- Here deviation is allowed only in one direction from the basic dimension so it is easy and simple to calculate.
- Unilateral Tolerance system is preferred for interchangeable manufacturing processes mainly where precision fits are required.
- Helps to standardise Go gauge end.
- In this system tolerance can be revised without changing the type of fit.
- This system minimizes the chance of rejection during machining of a component. Operators can machine the maximum dimension of the shaft and minimum dimension of the hole without any tension and they have enough margin before anything goes wrong.
Why use Bilateral Tolerance?
In industrial practice, bilateral tolerance is more of a norm than an exception. It provides flexibility to the machinist and in a sense more convenient for the designer to specify.
- Providing a bilateral tolerance allows a machinist the margin of error on both side of the target value
- While specifying a dimension drawing of say, a shaft and a hole, if bilateral tolerance is specified, only changing the nominal size of a shaft or hole is enough to rescale the drawing.
- These are suitable for large scale manufacturing where machine is set for a basic size of a part
I am Sangeeta Das. I have completed my Masters in Mechanical Engineering with specialization in I.C Engine and Automobiles. I have around ten years of experience encompassing industry and academia. My area of interest includes I.C. Engines, Aerodynamics and Fluid Mechanics. You can reach me at