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Published byIreland Samford Modified about 1 year ago

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FITS and TOLERANCES

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Control Measurements of Finished Parts

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Indicating the Desired Dimension

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Indication of the Real Dimension of Parts Due to the inevitable inaccuracy of manufacturing methods, a part cannot be made precisely to a given dimension.

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Indication of the Real Dimension of Parts

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Selection of Waste Parts

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Waste results when the manufacturing process cannot maintain size within prescribed limits.

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Selection of Acceptable Parts

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There is no such thing as an "exact size".

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Important Terms – Single Part Nominal Size – general size, usually expressed in mms Actual Size – measured size of the finished part Limits – maximum and minimum sizes shown by tolerances (larger value is the upper limit and the smaller value is the lower limit) Tolerance – total allowable variance in dimensions (upper limit – lower limit) – object dimension could be as big as the upper limit or as small as the lower limit or anywhere in between.

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Characteristics of Tolerances

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Connection btw the characteristics: US = N + UD LS = N – LD T = US – LS =

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The Possible Positions of the Tolerance Zone

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The tolerance zone is above the zero (base) line

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The tolerance zone is on the zero (base) line

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The tolerance zone is under the zero (base) line

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Possible positions of the tolerance zone in case of shafts Fundamental Deviation: is the deviation closest to the basic size. ISO standard uses tolerance position letters with lowercase letters for the shafts.

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FYI: The ISO System of Limits and Fits (referred to as the ISO system) is covered in national standards throughout the world, as shown by the following list: Global ISO 286 USA ANSI B4.2 Japan JIS B0401 Germany DIN 7160//61 France NF E UK BSI 4500 Italy UNI 6388 Australia AS 1654

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Possible positions of the tolerance zone in the case of holes Fundamental Deviation: is the deviation closest to the basic size. ISO standard uses tolerance position letters with capital letters for the holes.

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Schematic representation of the placement of the tolerance interval

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A tűrésnagyság szabványos értékei

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IT: International Tolerance

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Example: A shaft of nominal diameter 25 mm is going to be manufactured. IT grade is required to be IT7. Determine the tolerance on the shaft.

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Fits Between Mating Parts

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ILLUSTRATION OF DEFINITIONS

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Fits Between Mating Parts Fit: degree of tightness between two parts. Fit types: –Clearance Fit – tolerance of mating parts always leaves a space –Interference Fit – tolerance of mating parts always results in interference –Transition Fit – sometimes interferes, sometimes clears

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Clearance Fit The mating parts have such upper and lower limits that a clearance always results when the mating parts are assembled.

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Clearance Fit (e.g.: H7/f6)

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Clearance Fit (pl. H7/f6)

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Transition Fit Either a clearance or an interference may result depending on the exact value of the dimensions of the machined shaft and hole within the specified tolerance zones.

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Transition Fit (e.g.: H7/j6)

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Interference Fit The mating parts have such limits that the lowest shaft diameter is larger than the largest hole diameter..

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Interference Fit (e.g. H7/p6)

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Basic Systems for Fit Specification In order to standardize dimensioning of fits, two basic systems are used: 1)Basic Hole System: Minimum hole diameter is taken as the basis. Lower deviation for the hole is equal to zero. D max is prescribed according to the specified tolerance. 2)Basic Shaft System: Maximum shaft diameter is taken as the basis. Upper deviation for the Shaft is equal to zero. d min is prescribed according to the specified tolerance.

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Basic Hole System Toleranced dimensions are commonly determined using the basic hole system in which the minimum hole size is taken as the basic size.

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Fits in Basic Hole System

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Basic Shaft System In this system, the maximum shaft is taken as the basic size and is used only in specific circumstances.

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Fits in Basic Shaft System

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Indicating Tolerances

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Two ways of indicating tolerances on technical drawings Questions? Limits of a dimension or the tolerance values are specified directly with the dimension.

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Indicating tolerances The dimension is given by: a shape symbol, nominal size, a letter indicating the position of the tolerance zone in relation to zero line, a number indicating the width of the tolerance zone. (quality of production?)

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Specifying Fits in Technical Drawings

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DIMENSIONING OF TOLERANCES - RULES The upper deviation should be written above the lower deviation value irrespective of whether it is a shaft or a hole. Both deviations are expressed to the same number of decimal places, except in the cases where the deviation in one direction is nil.

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Example For a nominal diameter of 25 mm and for a fit specification of H7/j5 determine the following: Type of the tolerancing system Tolerance on the hole Tolerance on the shaft Upper and lower limits of the hole (D max, D min ) Upper and lower limits of the shaft (d max, d min ) Type of the fit

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Key a) H7/j5 Basic Hole System b) D = 25 mm, from the given table: nominal size H7 j5 + - c)

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