Session 10 Dimensions, Fits and Tolerances for Assembly

Transcription

1 Session 10 Dimensions, Fits and Tolerances for Assembly Lecture delivered by Prof. M. N. Sudhindra Kumar Professor MSRSAS-Bangalore 1

2 Variations in Production It is necessary that the dimensions, shape and mutual position of surfaces of individual parts of mechanical engineering products are kept within a certain accuracy to achieve their correct and reliable functioning. Production processes do not maintain dimensions, geometrical properties and surface roughness with absolute accuracy. Actual surfaces of the produced parts therefore differ from ideal surfaces prescribed in the part models 2

3 Variations in Production Limits for dimensions, geometric properties and surface roughness are therefore to be specified by designers to ensure correct functioning of engineering products. The part accuracy is decided based on the functionality of the product and economy of production, and ensured by the manufacturing processes and inspection methods used 3

4 Variations in Production Variations of actual surfaces are divided into three groups to enable specification and measurement of inaccuracies during production: Dimensional variations Variations in Geometric Properties Surface roughness variations 4

5 Variations in Production Variations in dimensions, geometric properties and surface roughness are a result of the inaccuracies of the manufacturing process, tooling and machine - - sheet metal process, tool and machine; - machining process, tool and machine; - casting or moulding process, tool and machine Dimensional and geometric property tolerances, and surface roughness tolerances are to be provided by the designer keeping in mind the functional requirement of the part and also on the limitations of the manufacturing process, tooling and machine used, and the planned cost of the part. 5

6 Variations in Production Parts modelled on 3D software contain features with basic dimensions. Critical dimensional tolerances, geometric properties and surface roughness tolerances have to be specified in 2D drawings These are functional requirements of the design and are required for tool development, process control and part inspection The Designer should understand the ISO system for specifying these tolerances, and should be able to specify appropriate tolerances in the drawings to meet the intended functional requirements 6

7 Assembly of parts and Fits Designs require Assembly of two parts to provide a Fit which performs an intended function location or movement with respect to each other. Fits can be classified as - clearances fit - transition fit - interference fit ISO has specified a system of Tolerance Grades and Tolerance Zones for Holes and Shafts which are used for specifying component tolerances to obtain the desired fit on assembly 7

8 Basic Hole System 8

9 Types of Fits A clearance fit B transition fit C interference fit 9

10 Dimensional Tolerances The tolerance of a dimension is defined as the difference between the upper and lower limit dimensions of the part. In order to meet the requirements of various production branches for accuracy of the product, the ISO system specifies 20 Tolerance Grades Each tolerance Grade on this system is marked "IT" with attached grade of accuracy (IT01, IT0, IT1... IT16). 10

11 ISO Tolerance Grades IT01 to IT6 For production of gauges and measuring instruments IT5 to IT12 For fits in precision and general engineering IT11 to IT16 For specification of limit deviations of non-tolerated dimensions 11

12 Tolerance Grades obtainable from Manufacturing Processes PEMP Tolerence grades and Tolerance values in microns for hole manufacturing processes diameter fine boring int grinding boring boring drilling drilling steps reaming reaming reaming reaming slot milling punching honing broaching H6 H7 H8 H9 H10 H11 upto to to to to to

13 ISO Tolerance Zones The tolerance zone is defined as a spherical zone limited by the upper and lower limit dimensions of the part. It is determined by the amount of the tolerance and its position related to the basic size. The position of the tolerance zone, related to the basic size is called the basic deviation. ISO defines 28 classes of basic deviations for holes. These classes are marked by capital letters (A, B, C,... ZC). The tolerance zone for the specified dimensions is prescribed in the drawing by a tolerance mark, which consists of a letter marking of the basic deviation and a numerical marking of the tolerance grade (e.g. H7, H8, D5, etc.). 13

14 ISO Tolerance Zones ISO defines 28 classes of basic deviations for shafts. These classes are marked by lower case letters (a, b, c,... zc). The tolerance zone for the specified dimensions is prescribed in the drawing by a tolerance mark, which consists of a letter marking of the basic deviation and a numerical marking of the tolerance grade (e.g. h7, h6, g5, etc.). 14

15 ISO Tolerance Grades and Zones 3.3 of PSG Design Data Book 15

16 16

17 Running and Sliding Fits 3.4 of PSG Design Data Book 17

18 Location and Assembly Fits 18

19 Transition and Interference Fits 3.5 of PSG Design Data Book 19

20 Transition and Interference Fits 3.6 of PSG Design Data Book 20

21 Mean Fit and Variation Values 21

22 Geometric Tolerances and Symbols 22

23 Geometric Tolerances 23

24 Geometric Tolerances 24

25 Geometric Tolerances obtainable from manufacturing Processes PEMP Table on 3.11 of PSG Design Data Book 25

26 Surface Roughness Obtainable from Manufacturing processes PEMP 26

27 Surface Roughness Obtainable from Manufacturing processes PEMP 27

28 Gauging Gauges are limit checking devices used for Go-No Go inspection of components during mass production The Go Gauge checks the maximum material condition and is designed to check dimension as well as geometry over its useful length No Go Gauge checks the minimum material condition and is designed to check only dimension at any point over its length 28

29 Plug Gauge For a Hole, the Go Gauge checks the minimum diameter and the No Go Gauge checks the maximum diameter. Holes which are within the specified tolerance will permit entry of the Go Gauge but will not permit entry of the No Go Gauge The type of Gauge used is Plug Gauge 29

30 Ring Gauge/Snap Gauge For a Shaft, the Go Gauge checks the maximum diameter and the No Go Gauge checks the minimum diameter. Shafts which are within the specified tolerance will enter the Go Gauge but not enter the No Go Gauge The type of gauges used is Ring Gauge and Snap Gauge 30

31 Thread Plug and Ring Gauges 31

32 Pin Gauges and Thickness Gauge 32

33 Case Study for Fits and Tolerances for assembly PEMP 33

34 Summary Parts made in production have an inherent variation in dimensions, geometric properties and surface finish due to variations in the materials and manufacturing equipment used Dimensions, fits and tolerances are therefore to be provided in all manufacturing drawings in addition to the 3D part models Parts and assemblies need to be made and checked with limit gauging to confirm that they meet design requirements with respect to assembly product performance, reliability, servicability and life 34