The selective assembly can be referred to as a concept where the subcomponents are assembled to form a proper final assembly which will in return provide the highest tolerance specification.
Using selective assembly when the parts are being manufactured, the selective assembly resembles an old concept of inspection, in which the component is being identified as good or bad. If all the parts are correct and the component is good then it will be used for assembly or else it will be used for scrap purposes.
In selective assembly, if the components are divided into different categories, then the groups will be organized according to the sizes and dimensions. We have to make sure that all the parts are grouped together and all are ready for mating so that every component will match with the corresponding sized component to form an assembled part.
If a small shaft is mated with small holes, then large holes will be mated with large shafts. For clearance and interference fits in selective assembly, the minimum value increases and maximum value decreases, where as for transition fit the maximum value of clearance and interference fit decreases.
Interchangeability can be defined as it is a system of producing the mating parts.
A single operator was confined with a number of units to assemble it which will take a long period of time and it won't be economical. To reduce the cost and time, mass production of the system was developed.
In the production systems, the components will be produced in one or more batches by different operations on different machines.
This system will be applied to interchangeability and then any one of the components will be selected at random to mate with other components which will also be selected at random. When this system is used for such purposes then it is called interchangeability. This system ensures that the cost will be reduced and the output will be increased.
In these systems, especially in the interchangeable systems, the system does some portions of the work so that the workload will be decreased for utilization purposes. Then that person can easily specialize in that work. Then that person can use the mating parts to fit the components by trial and error method and the assembly time is reduced completely and considerably.
If it is the case of big assemblies where big parts are being manufactured, the manufacturing of parts may be very tricky and hard because it is very difficult to find parts which are similar, because these parts need to be grouped together so that they can fit in the same batch.