It is not possible to manufacture 100% accurate parts. Limits, fit and tolerance on a part ensures product assembly on assembly line. Therefore manufacturers always try to approach for true value that are within tolerance limits.
Limits, tolerance and fit in a part provides flexibility to manufacturer. It helps in reducing rejection rate and overall part cost.
Following terms and definitions are important to understand Limits, Fit and Tolerances.
Basic Size is theoretical exact size from which limits of size are defined. In Example-1 shaft basic size is 4 mm.
Actual Size is manufactured part actual measured size. It’s value lies in between upper and lower size limits.
Maximum Material Condition (MMC)
MMC is a condition where a feature of a finished part contains the maximum amount of material. In other words, the largest shaft or smallest hole size is MMC condition for shaft and hole respectively.
MMC (Hole) = 9.5 mm and 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 other words, the smallest shaft and the largest hole size is LMC conditions for shaft and hole respectively.
LMC (hole) = 10.5 mm and LMC (shaft) = 3.8 mm
It is the maximum space available between the hole and the shaft or between two parts.
Max. Clearance = LMC (hole) – LMC (shaft).
Allowance is the minimum space available between the hole and the shaft. In other words, allowance is minimum clearance between two parts.
Allowance (Min. Clearance) = MMC (hole) – MMC (shaft)
Limits Of Size
Limit of size is extreme permissible sizes for a feature or dimension of a part. Actual Dimension of measured part must lie within these limits.
In Example-1, shaft basic size is 4 mm and its upper and lower tolerance are +0.2 mm and -0.1 mm respectively.
Therefore limits of size for shaft are :
Upper limit = 4.2 mm and Lower Limit = 3.9 mm.
As per ASME, Tolerance is the total amount a specific dimension is permitted to vary. For example, If a dimension represented as 25± 0.5 . It’s upper and lower tolerance are 0.5 mm.
Tolerance stack-up analysis is used to ensure parts assembly is acceptable even in the worst conditions.
Why tolerance is Required?
Production of closely mating parts, without tolerances is economically unfeasible. It results in increased rejection rate and manufacturing cost. cost will increase. Therefore, tolerance ensure:
- Reduced Rejection Rate
- Smooth Product Assembly.
- interchangeability of manufactured parts.
Type of Tolerances
Engineering tolerances are further classified in two categories:
- Unilateral Tolerance
- Bilateral Tolerance
In unilateral tolerance, Feature Dimensions are allowed to vary only on one side of the basic size. In other words tolerance lies only on one side of the basic dimension.
For example, if a hole basic size is 20 mm and its value can vary from 19.85 mm to 20 mm. This type of tolerance is known as unilateral tolerance.
- They are used to define precision fits.
- Unilateral tolerance are easy to control in machining parts. Operator machines part to upper limit of shaft or lower limit for hole. As a result operator still has some margin left for further correction.
In bilateral tolerance, Feature Dimensions are allowed to vary on both sides of the basic size. in other words, tolerance limit lies on both sides of the basic dimension.
For example, if a hole basic size is 20 mm and it’s value can vary from 19.8 mm to 20.1 mm. This type of tolerance is known as Bilateral tolerance.
- Bilateral Tolerance system is used in mass production.
- In this system, operator always target to basic size.
Engineering Fits are used as part tolerances in an assembly. They define the clearance between two mating parts.
Types of Fits
This clearance between two parts determines the type of Fit. Engineering Fit can be divided into three types.
- Clearance Fit
- Interference Fit
- Transition Fit
In clearance fit, size of hole is always larger than size of shaft. Therefore clearance fit ensures always a clearance between assembled mating parts. Clearance fit has positive allowance.
Examples of clearance fit are door hinges, shaft sliding inside a bearing, bearing inside ceiling fan, electrical switch and board e.t.c
Interference (Force/ Shrink) Fit
In Interference fit, size of shaft is always larger than size of hole. Therefore Interference fit ensures interference between assembled mating parts. Interference fit has negative allowance.
Examples of interference fit are : push fit inserts in plastic part, Axle and bearing assembly e.t.c
Transition fit is either a clearance fit or interference fit. Therefore In transition fit, shaft may be either larger or smaller than the hole in a mating part. Transition fits is a compromise between clearance and interference fits.
Transition fit has overlapping tolerance zones of the hole and shaft. Therefore they are used where accurate location is important. But small amount of clearance or interference is permissible.
Systems of Fit
To obtain a required type of fit. Basic hole system and basic shaft system are 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 fit.
e.g 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 fit.
e.g 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.
To sum up, Engineering tolerance is very important and critical part of product design. Products can not be manufactured without tolerances. You can also use tolerance stackup calculator to define tolerance.
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