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Engineering Metrology

Engineering Metrology. Tolerances & Allowances. Metrology standards. Sensitivity or resolution is the smallest dimension that can be read on an instrument Standard measurement should do at the temperature 20  0.3 o C

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Engineering Metrology

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  1. Engineering Metrology Tolerances & Allowances

  2. Metrology standards • Sensitivity or resolution is the smallest dimension that can be read on an instrument • Standard measurement should do at the temperature20  0.3oC • Measuring instruments must have the tolerance more than the desired tolerance for a product 10 times. • For example, if we want to measure a product with tolerance equal to 0.01 , the instrument that we use must have the tolerance equal to 0.001

  3. allowance • For assembly parts • The allowance that desire for the different of two or more mating parts • If it is positive,  minimum clearance • If it is negative  maximum interference

  4. Basic size, deviation, and tolerance on a shaft, according to the ISO system

  5. Fit system (ISO) • Hole-basis system • Tolerance is at the zero line of hole • MMC: The basic size is at the smallest hole • Shaft-basis system • Tolerance is at the zero line of shaft • MMC: The basic size is at the biggest shaft.

  6. Fit between mating parts • Clearance • One part is always loose relative to the other one. • Two mating parts have clearance  0 • Transition • Both clearance and interferance • Interference • One part is always forced tight with respect to another. • Two mating parts have clearance  0

  7. Hole basis system

  8. Shaft-basis system

  9. How to calculate allowance • Maximum clearance = max hole – min shaft • Minimum clearance = min hole – max shaft • Maximum interference = max hole – min shaft • Minimum interference = min hole – max shaft • Look up from the allowance table

  10. Clearance fits Transition fits Interference fits

  11. Example We want to assemble shaft and gear together. The nominal size of both part is equal to 5 mm. Using medium drive fit. Chose hole-basis system: H7/u6 andbasic size = 5 mm. From the table, The diameter of the biggest hole = 5.012 mm the smallest hole = 5.000 mm the biggest shaft = 5.027 mm the smallest shaft = 5.019 mm Minimum allowance = -0.007 Maximum allowance = -0.027

  12. Chose shaft-basis system: U7/h6 andbasic size = 5 mm .From the table, The diameter of The biggest hole =4.985mm The smallest hole =4.973 mm The biggest shaft = 5.000 mm The smallest shaft =4.992 mm Minimum allowance = -0.007 Maximum allowance = -0.027

  13. Example Want to assemble gear and bushing with the same shaft. The nominal dimension is equal to 5 mm. Using interference fit with medium drive class for gear and shaft, and clearance with fits-free running class for shaft and busing.

  14. Useshaft-basis system Basic size = 5 mm Gear and shaft:interference fit – medium drive classS7/h6 Shaft and bushing: clearance fits – free running class D9/h9

  15. Usingshaft and bushing max min So, the dimension of the gear has to be changed according to the desired allowance by using the following table, Shaft Gear Fit 4.973 max 5.000 -0.007 0.003 4.963 -0.027 -0.037 min 4.973 4.970 4.963

  16. Usingshaft and gear max min So, the dimension of the bushing has to be changed according to the desired allowance by using the following table, To get the desired allowance, shaft and gear allowance and bushing and shaft allowance is as shown in the tables. Bushing Shaft Fit 5.082 max 5.000 0.090 0.030 min 4.992 5.030

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