Slide 1. Slide 2 CHAPTER -3 LIMITS, FITS & TOLERANCE IMPORTANT TERMS

Transcription

1 Slide 1 CHAPTER -3 LIMITS, FITS & TOLERANCE 1 Slide 2 IMPORTANT TERMS ACCURACY: Capability to get accurate measure of dimension (Micrometer & Scale) accurate upto one millionth of metre(micron) PRECISION : How close together of readings TOLERANCE: Permissible deviation from the desired value, Basic size SURFACE FINISH: Amount of geometric irregularity on the surface RELIABILITY: Probability of product functioning in a intended manner 2

2 Slide 3 TOLERANCE Largest acceptable dimension Basic size Smallest acceptable dimension Tolerance 3 Slide 4 MEASURING AND INSPECTION INSTRUMENTS DIRECT MEASURING INSTRUMENTS Scale, Vernier Caliper, Micrometer INDIRECT MEASURING INSTRUMENTS Simple Caliper, comparator GAUGES: To check the acceptability of a specified dimension 4

3 Slide 5 THE GUAGE Workpiece tolerance GO NOGO GO NO GO Upper limit (Maximum acceptable size) Lower limit (Minimum acceptable size) Dimension of GO opening Dimension of NOGO opening Gage 5 Slide 6 GO AND NOGO GAUGE GO NOGO GO NOGO GO NOGO Part 6

4 Slide 7 LIMITS AND FITS A system of limits and fits helps in: Interchangeability and ease of assembly. Eliminating the need for minor rectifications (fitting) during assembly. Eliminating unnecessary effort being spent in producing dimensions to a higher degree of accuracy or surface finish than what is needed for satisfactory operation. For example, is uneconomical to limit the accuracy of a dimension to mm when 1 mm would prove satisfactory 7 Slide 8 TOLERANCE It is the acceptance of an error in mfg. Difference between the maximum and minimum limit of size Upper deviation Tolerance Upper deviation Lower deviation Dmax Dn (a) Dmin Dn Dmax (b) Dmin Lower deviation (Not to scale) 8

5 Slide 9 TOLERANCE SELECTION OF TOLERANCE DEPENDS ON The nominal size. Manufacturing method used. Accuracy and quality desired. Type of fit required 9 Slide 10 TOLERANCE UNILATERAL AND BILATERAL TOLERANCE Nominal size = Tolerance = Specified as: or Specified as: or (Not to scale) 10

6 Slide 11 Specifications of Tolerances Unilateral Tolerance Bilateral Tolerance 11 Slide 12 Allowance & Clearance 12

7 Slide 13 FITS It is the degree of tightness or looseness between two mating parts It is the characteristic of the assembly TYPES OF FITS Clearance fits Interference fits Transition fits 13 Slide 14 14

8 Slide 15 CLEARANCE FIT an internal member fits in an external member (as a shaft in a hole) and always leaves a space or clearance between the parts. Minimum air space is This is the allowance and is always positive in a clearance fit 15 Slide 16 16

9 Slide 17 INTERFERENCE FIT Ex: dowel pins, bearings in castings Allowance- Negative Clearance Maximum interference Minimum interference Shaft Hole (Not to scale) 17 Slide 18 INTERFERENCE FIT- CONTD The internal member is larger than the external member such that there is always an actual interference of material. The smallest shaft is and the largest hole is , so that there is an actual interference of metal amounting to at least Under maximum material conditions the interference would be This interference is the allowance, and in an interference fit it is always negative. 18

10 Slide Slide 20 TRANSITION FIT Maximum clearance Maximum interference Shaft Hole (Not to scale) 20

11 Slide 21 TRANSITION FIT CONTD--- may result in either a clearance or interference condition. In the figure below, the smallest shaft will fit in the largest hole , with to spare. But the largest shaft, will have to be forced into the smallest hole, with an interference of metal of Slide 22 22

12 Slide 23 GRADES OF TOLERANCE 23 Slide 24 24

13 Slide 25 EXAMPLES Find the type of fit obtained in the following case :- Size of the hole : Size of the shaft : Maximum clearance = Maximum size of hole Minimum size of shaft = = mm Minimum clearance = Minimum size of hole Maximum size of shaft = = 0 mm Since, both clearances are greater than or equal to zero, the resulting fit is clearance fit. 25 Slide 26 EXAMPLES Solution: Maximum size of shaft = 50 0 = 50 mm Minimum size of shaft = = mm Maximum size of hole = = mm Minimum size of hole = = mm 26