DESIGN AND MANUFACTURING

DESIGN AND MANUFACTURING

Design and Manufacturing

A machine element, after design, requires to be manufactured to give it a shape of a product. Therefore, in addition to standard design practices like, selection of proper material, ensuring proper strength and dimension to guard against failure, a designer should have knowledge of basic manufacturing aspects. We will discuss briefly about some of the basic manufacturing requirements and processes. First and foremost is assigning proper size to a machine element from manufacturing view point. 

For example, a shaft may be designed to diameter of, say, 40 mm. This means, the nominal diameter of the shaft is 40 mm, but the actual size will be slightly different, because it is impossible to manufacture a shaft of exactly 40 mm diameter, no matter what machine is used.

In case the machine element is a mating part with another one, then dimensions of both the parts become important, because they dictate the nature of assembly. The allowable variation in size for the mating parts is called limits and the nature of assembly due to such variation in size is known as fits.

INTERCHANGEABILITY:

The term interchangeability is normally employed for the mass production of identical items within the prescribed limits of sizes. A little consideration will show that in order to maintain the sizes of the part within a close degree of accuracy, a lot of time is required. But even then there will be small variations. If the variations are within certain limits, all parts of equivalent size will be equally fit for operating in machines and mechanisms. Therefore, certain variations are recognized and allowed in the sizes of the mating parts to give the required fitting. This facilitates to select at random from a large number of parts for an assembly and results in a considerable saving in the cost of production.

In order to control the size of finished part, with due allowance for error, for interchangeable parts is called limit system.

It may be noted that when an assembly is made of two parts, the part which enters into the other, is known as enveloped surface (or shaft for cylindrical part) and the other in which one enters is called enveloping surface (or hole for cylindrical part).

Important Terms used in Limit System

The following terms used in limit system (or interchangeable system) is important from the subject point of view:

1.      Nominal size. It is the size of a part specified in the drawing as a matter of convenience.

2.      Basic size. It is the size of a part to which all limits of variation (i.e. tolerances) are applied to arrive at final dimensioning of the mating parts. The nominal or basic size of a part is often the same.

3.      Actual size. It is the actual measured dimension of the part. The difference between the basic size and the actual size should not exceed a certain limit; otherwise it will interfere with the interchangeability of the mating parts.

4.      Limits of sizes. There are two extreme permissible sizes for a dimension of the part as shown in Fig. 3.1. The largest permissible size for a dimension of the part is called upper or high or maximum limit, whereas the smallest size of the part is known as lower or minimum limit.

5.      Allowance. It is the difference between the basic dimensions of the mating parts. The allowance may be positive or negative. When the shaft size is less than the hole size, then the allowanceis positive and when the shaft size is greater than the hole size, then the allowance is negative.

6.      Tolerance. It is the difference between the upper limit and lower limit of a dimension. Inother words, it is the maximum permissible variation in a dimension. The tolerance may be unilateral or bilateral. When all the tolerance is allowed on one side of the nominal size, e.g.  then it is said to be unilateral system of tolerance. The unilateral system is mostly used in industries as it permits changing the tolerance value while still retaining the same allowance or type of fit

When the tolerance is allowed on both sides of the nominal size, e.g.  , then it is said to be bilateral system of tolerance. In this case + 0.002 is the upper limit and -0.002 is the lower limit.

7.      Tolerance zone. It is the zone between the maximum and minimum limit size

8.      Zero line. It is a straight line corresponding to the basic size. The deviations are measured from this line. The positive and negative deviations are shown above and below the zero line respectively.

9.       Upper deviation. It is the algebraic difference between the maximum size and the basic size. The upper deviation of a hole is represented by a symbol ES (Ecart Superior) and of a shaft, it is represented by es.

10.   Lower deviation. It is the algebraic difference between the minimum size and the basic size. The lower deviation of a hole is represented by a symbol EI (Ecart Inferior) and of a shaft, it is represented by ei.

11.   Actual deviation. It is the algebraic difference between an actual size and the corresponding basic size.

12.  Mean deviation. It is the arithmetical mean between the upper and lower deviations.

13.  Fundamental deviation. It is one of the two deviations which is conventionally chosen to define the position of the tolerance zone in relation to zero line

Fundamental deviation

Fit System

We have learnt above that a machine part when manufactured has a specified tolerance. Therefore, when two mating parts fit with each other, the nature of fit is dependent on the limits of tolerances and fundamental deviations of the mating parts. The nature of assembly of two mating parts is defined by three types of fit system,

ü  Clearance Fit,

ü  Transition Fit

ü  Interference Fit.

The fit system is shown schematically in Fig.3.

Clearance Fit In this type of fit, the shaft of largest possible diameter can also be fitted easily even in the hole of smallest possible diameter.

Transition Fit In this case, there will be a clearance between the minimum dimension of the shaft and the minimum dimension of the hole. If we look at the figure carefully, then it is observed that if the shaft dimension is maximum and the hole dimension is minimum then an overlap will result and this creates a certain amount of tightness in the fitting of the shaft inside the hole. Hence, transition fit may have either clearance or overlap in the fit.

Interference Fit In this case, no matter whatever may be the tolerance level in shaft and the hole, there is always a overlapping of the matting parts. This is known as interference fit. Interference fit is a form of a tight fit.

There are two ways of representing a system.

ü  Hole basis

ü  Shaft basis.

Hole basis system. When the hole is kept as a constant member (i.e. when the lower deviation of the hole is zero) and different fits are obtained by varying the shaft size, then the limit system is said to be on a hole basis.

Shaft basis system. When the shaft is kept as a constant member (i.e. when the upper deviation of the shaft is zero) and different fits are obtained by varying the hole size, then the limit system is said to be on a shaft basis.

The hole basis and shaft basis system may also be shown as in Fig., with respect to the zero line.

                                                                                           

Common manufacturing processes

The types of common manufacturing processes are given below in the Fig.

The types of shaping processes are given below in the Fig

Various joining processes are shown in Fig

The surface finishing processes are given below

The non-conventional machining processes are as follows