Engineering Tolerance & Fits

As per ASME, Tolerance is the total amount a specific dimension is permitted to vary. For example, dimension represented as 25± 0.5 means, this dimension value can be in between 24.5 mm to 25.5 mm.

Tolerance stack-up analysis is performed to make sure parts assembly is acceptable even in worst conditions.

Why it’s important to use Tolerance?

Consider if you are working on a product with 100 parts. For assembly of 100 parts, we need to make sure all parts are within designed limit.This makes Interchangeability of manufactured parts very critical. 

The production of closely mating parts, without tolerances is economically unfeasible. This results in increase in rejection rate. This will also increase overall product cost. This is the reason, we use tolerance in part dimensions.

Type of Tolerances

Unilateral Tolerance

In unilateral tolerance system, dimension of a part is allowed to vary only on one side of the basic size.

In another words, tolerance lies only in one side of the basic dimension.

As shown in image, basic size of hole is 20 mm and its value can vary from 20 mm to 19.85 mm.

Unilateral system is used when precision fits are required. One side tolerance is easy to control while manufacturing machining parts.This is used for small quantities. Operator machines to the upper limit of shaft (lower limit for hole). As a result operator still has some margin left for further correction.

Bilateral Tolerance

With Bilateral Tolerance, dimension of the part is allowed to vary on both the sides of the basic size. 

In another words, limits of tolerance lie on both sides of the basic dimension.

As shown in image, basic size for hole is 100 mm and it’s value can vary from 19.8 mm to 20.1 mm. 

 

Bilateral Tolerance system is used in mass production.In this system machine setting is done for the basic size. 

The following terms and definitions are important to understand Limits, Fits and Tolerances:

Maximum Material Condition (MMC) :

MMC is a condition where a feature of a finished part contains the maximum amount of material. In another words, largest shaft or smallest hole.

For Example (refer above picture) :  MMC (Hole) = 9.5 mm, MMC (Shaft) =4.2 mm

Least Material Condition (LMC) : 

LMC is a Condition where a feature of a finished part contains the least amount of material. In another words, the smallest shaft or the largest hole.

For Example (refer above picture) :  LMC (hole)=  10.5 mm, LMC (shaft) = 3.8 mm

Basic/Nominal Size : 

This is theoretical exact size from which limits of size are defined.

For Example (refer above picture) : Shaft basic size is 4mm

Max Clearance : 

Maximum amount of space available between the hole and the shaft.

For Example (refer above picture): Max. Clearance = LMC (hole) – LMC (shaft). In above example maximum clearance is (10.2 – 3.9) mm 

Allowance :

Allowance is the minimum amount of space available between the hole and the shaft. In another words we can say, allowance is minimum clearance.

Min. Clearance = MMC (hole) – MMC (shaft)

Types of Fits in Engineering

Engineering fits are generally used as part tolerances in an assembly. In another words, “Engineering Fit” is the clearance between two mating parts. This clearance determines the type of Fit. Engineering fit can be divided in three types.

Clearance Fit

In clearance fit in all conditions, size of hole is larger than size of shaft. In another words clearance fit assures a clearance between assembled mating parts. Clearance fit has positive allowance.

Interference (Force/ Shrink) Fit

In Interference fit in all conditions, size of shaft is larger than size of hole. In another words Interference fit assures interference between assembled mating parts. Clearance fit has negative allowance.

Transition fit

Transition fit is either a clearance or interference fit. In this case shaft may be either larger or smaller than the hole in a mating part. Transition fits are a compromise between clearance and interference fits.

This Fit is used where accurate location is important but small amount of clearance or interference is permissible. Transition fit has overlapping tolerance zones of the hole and shaft.

Systems of Fit

To obtain a required type of fit, Basic hole system and basic shaft system is used. Selection of type of system is done based on availability of standard parts and process used. 

Basic Hole System

In basic hole system, size of the hole is kept constant and shaft size is varied to obtain required type of fits.

In this system, minimum hole diameter is considered as the basic diameter (basic size) from which the tolerance and allowance are calculated.

Basic hole system is widely used because standard Drills, reamers are used to produce holes. On the other hand, size of the shaft produced by turning, grinding can be modified.

Basic Shaft System

In basic shaft system diameter of the shaft is kept constant and hole size is varied to obtain required type of fit.

In this system, largest shaft diameter is considered as basic diameter from which tolerances and allowance are calculated.

The basic shaft system is used for standard size shafts. 

Conclusion :

To sum up, Engineering tolerance is very important and critical part of product design. Products can not be manufactured without tolerances.

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