what is difference between accuracy and precision?

Accuracy and precision:-

Accuracy is defined as closeness to the exact value and precision is repeatability. accuracy is something related to the mean value of the parts produced and precision is related to the process dispersion. in fig. 1.8 (1) mean value of the observations is very close to the targeted value but there are large fluctuations in the observations due to higher process capability, such systems are accurate but not precise.

In fig. 1.8 (2) standard deviations of observations due to low and also the mean is close to the targeted value, so such systems are precise and accurate. In fig. 1.8 (3) although the process dispersion of the system is low due to the improper setting of the machine mean value is far from the targeted value. such systems are called precise but are not accurate.

in fig. 1.8 (4) the process dispersion is large and the mean is far from the targeted value. such systems are said to be neither precise nor accurate.

what is difference between accuracy and precision?

INTERCHANGEABILITY:-

Any assembly is made up of a combination of holes and shafts. The hole is any internal feature of the part and may not be circular and similarly, any external feature of the part is called a shaft, and may not be circular. In most cases, it is the hole that is made first because the standard size of drills and reamers are available. if a shaft is made first holes are called make-to-suit. As discussed earlier that it is impossible to duplicate the part, so limits are provided on the dimensions which are acceptable. This target value of the hole and shaft is called basic size and the nearest round number as per the standard is called nominal size.

suppose there are two different machines producing holes and shafts. if it so happens that process capability is equal to the desired tolerance individually (as shown in Fig. 1.9(1) and (2) if a hole is selected randomly from the hole lot and a shaft is selected randomly from the shaft lot, the assembly can be made such a system is called a fully interchangeable system which does not require any inspection after machining. Interchangeability is having following advantages:-

  1. The cost of the assembly decreased because holes and shafts can be made at a different place where the material and the labor is cheap.
  2. standardization of hole and shaft is possible.
  3. Quality of product increases.
  4. maintenance if the assembly becomes cheap and easy because when either hole or shaft fails one does not have to follow away the complete assembly and the failed parts only need to be replaced.

SELECTIVE ASSEMBLY

To increases the quality of assembly the desired tolerances for both hole and shaft are decreases but machines will have certain process capabilities. By converting conventional machines to NC process papability decreases but desired tolerance are decreasing more than that. Selective assembly is a mathematicla procedure to achive full interchangeability althrough machines are not capable i.e. process capability is uch larger than the desired tolerance. let us understand it by discussing some cases.

case: 1 process capability of hole and shaft are equal but more than the desired tolerance.

Let us assume that process capability is 4 times the desired tolerance of both hoel and shaft and shaft. the process capability range of both hole and shaft is divided into 4 equal parts as shown in figure . this means in case of selective assembly each component has to be inspected after manufacturing and it is put into different subgroups according to its dimensions. The variation of dimensions with in one word the subgroup is equal to the desired tolerance. So when randomly a hole is selected from subgroup ‘A’ and a shaft is selected randomly from subgroup ‘a’ assembly can be made.

case: 2 process capability of hoel is equal to the desired tolerance and the process capability of the shaft is worse than the desired tolerance.

In this case process capability, range of the shaft is divided into 4 equal divisions so that variation within the subgroup is equal to the desired tolerance. For producing holes of a subgroup of shafts ‘a’, the machine targeted the mean of a subgroup of shafts ‘a’ and the holes will be directly categorized as subgroup ‘A’ without any inspection because process capability of machine producing hole is equal to the desired tolerance. When variations are equal to the desired tolerance assembly can be give a quality of assembly.

case: 3 process capability of both holes and shafts are different and worse than the desired tolerance.

Let us consider that the process capability of the hole is 4 times the desired tolerance and the process capability of the shaft is not an integer multiple of tolerance. similar to the previous cases’ process  capability of the hole is divided into 4 subgroups by measuring each and every part. for shaft e.g. in subgroup ‘b’ shaft will be produced in two different settings. so in this case also each and every component must be inspected after manufacturing.

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