Brief Overview of Design & Manufacturing - 2 - Mechanical Engineering PDF Download

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Standard limit and fit system

Fig. 1.3.4 shows the schematic view of a standard limit and fit system. In this figure tolerance is denoted as IT and it has 18 grades; greater the number, more is the tolerance limit. The fundamental deviations for the hole are denoted by capital letters from A and ZC, having altogether 25 divisions. Similarly, the fundamental deviations for the shaft is denoted by small letters from a to zc.

Here H or h is a typical case, where the fundamental deviation is zero having an unilateral tolerance of a specified IT grade.

Therefore in standard limits and fit system we find that,

Standard tolerances 

18 grades: IT01 ,IT0 and IT1-1T16

Fundamental deviations
25 types: A- ZC (For holes)
               a- zc (For shafts)

The values of standard tolerances and fundamental deviations can be obtained by consulting design hand book. It is to be noted that the choice of tolerance grade is related to the type of manufacturing process; for example, attainable tolerance grade for lapping process is lower compared to plain milling. Similarly, choice of fundamental deviation largely depends on the nature of fit, running fit or tight fit etc. The approximate zones for fit are shown in Fig. 1.3.5. Manufacturing processes involving lower tolerance grade are generally costly. Hence the designer has to keep in view the manufacturing processes to make the design effective and inexpensive.

Sample designation of limit and fit, 50H6/g5. 

The designation means that the nominal size of the hole and the shaft is 50 mm. H is the nature of fit for the hole basis system and its fundamental deviation is zero. The tolerance grade for making the hole is IT6. Similarly, the shaft has the fit type g, for which the fundamental deviation is negative, that is, its dimension is lower than the nominal size, and tolerance grade is IT5.

Preferred numbers 

A designed product needs standardization. It means that some of its important specified parameter should be common in nature. For example, the sizes of the ingots available in the market have standard sizes. A manufacturer does not produce ingots of sizes of his wish, he follows a definite pattern and for that matter designer can choose the dimensions from those standard available sizes. Motor speed, engine power of a tractor, machine tool speed and feed, all follow a definite pattern or series. This also helps in interchangeability of products. It has been observed that if the sizes are put in the form of geometric progression, then wide ranges are covered with a definite sequence. These numbers are called preferred numbers having common ratios as, 5 10 20 40 10 1.58, 10 1.26, 10 and 10 ≈ ≈ ≈ 1.12 ≈ 1.06

Depending on the common ratio, four basic series are formed; these are R₅ , R₁₀ , R₂₀ and R₄₀ . These are named as Renard series. Many other derived series are formed by multiplying or dividing the basic series by 10, 100 etc.

Typical values of the common ratio for four basic G.P. series are given below.

Few examples

R₁₀ , R₂₀ and R₄₀ :     Thickness of sheet metals, wire diameter

R₅ , R₁₀ , R₂₀ :           Speed layout in a machine tool (R₁₀ : 1000, 1250,1600, 2000)

R₂₀ or R₄₀ :                Machine tool feed

R₅ :                             Capacities of hydraulic cylinder

Common manufacturing processes 

The types of common manufacturing processes are given below in the Fig.1.3.6.

The types of shaping processes are given below in the Fig.1.3.7.

Following are the type of machining processes, shown in Fig.1.3.8.

Various joining processes are shown in Fig.1.3.9.

The surface finishing processes are given below (Fig.1.3.10),

The non-conventional machining processes are as follows (Fig.1.3.11),