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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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**Indication of the Real Dimension of Parts**

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**Selection of Waste Parts**

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**Selection of Waste Parts**

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

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**Selection of Acceptable Parts**

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**Selection of Acceptable Parts**

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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**Characteristics of Tolerances**

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**

ISO standard uses tolerance position letters with lowercase letters for the shafts. Fundamental Deviation: is the deviation closest to the basic size.

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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**

ISO standard uses tolerance position letters with capital letters for the holes. Fundamental Deviation: is the deviation closest to the basic size.

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**Schematic representation of the placement of the tolerance interval**

EI, ES fundamental deviations of holes (examples) ei, es fundamental deviations of shafts (examples) a Nominal size.

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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**

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 Note these terms are used with multiple parts.

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

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

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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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**Interference Fit (e.g. H7/p6)**

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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: Basic Hole System: Minimum hole diameter is taken as the basis. Lower deviation for the hole is equal to zero. Dmax is prescribed according to the specified tolerance. Basic Shaft System: Maximum shaft diameter is taken as the basis. Upper deviation for the Shaft is equal to zero. dmin 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**

Limits of a dimension or the tolerance values are specified directly with the dimension. Questions ?

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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 (Dmax, Dmin) Upper and lower limits of the shaft (dmax, dmin) 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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Application of Limits and Fits BS 4500

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