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Fits and Tolerances

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INTRODUCTION 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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**HISTORY OF THE ISO SYSTEM**

The present ISO system is based on the ISA System of Limits and Fits published in ISA Bulletin 25 (1940), and on comments included in the Draft Final Report of ISA Committee 3, December The unification of the various national systems of limits and fits was one of the essential tasks discussed at the initial conference of the ISA in New York, in April, The same year the Secretariat of ISA Committee 3, Limits and Fits, was entrusted to the Germany Standardizing Association, and needless to say, the system was all metric from the start.

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**Tolerancing ∙ Tolerances are used to control the variation that exists**

on all manufactured parts. ∙ Toleranced dimensions control the amount of variation on each part of an assembly. ∙ The amount each part is allowed to vary depends on the function of the part and of the assembly. For example: the tolerances placed on a swing set is not as stringent as those placed on jet engine parts. ∙ The more accuracy needed in the machined part – the higher the manufacturing cost.

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**Representing Tolerance Values**

∙ Tolerance is the total amount a dimension may vary and is the difference between the maximum and minimum limits. (A) Tolerance = .04 (B) Tolerance = .006 ∙ Tolerances are represented as Direct Limits (A) or as Tolerance Values (B). Which part costs more to manufacture?

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**Tolerances can also be expressed as:**

1. Geometric Tolerances. “GDT” 2. Notes Referring to Specific Conditions. 3. A General Tolerance Note in the Title Block. Example: ALL DECIMAL DIMENSIONS TO BE HELD TO ± .002”

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**Plus and Minus Dimensions**

With this approach, the basic size is given, followed by a plus/minus sign and the tolerance value. Notice that a Unilateral Tolerance varies in only one direction, while Bilateral Tolerances varies in both directions from the basic size.

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**Important Terms of Toleranced Parts**

A System is two or more mating parts. Nominal Size is used to describe the general size (usually in fractions). The parts above have a nominal size of 1/2” Basic Size – theoretical size used as a starting point for the application of Tolerances. The parts above have a basic size of .500”

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**Important Terms of Toleranced Parts**

Limits – the maximum and minimum sizes shown by the tolerance dimension. The slot has limits of .502 & .498, and the mating part has limits of .495 & .497. The large value on each part is the Upper Limit, the small value = Lower Limit. Actual Size is the measured size of the finished part after machining. The Actual Size of the machined part above is .501”

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**Important Terms of Toleranced Parts**

Allowance – the tightest fit between two mating parts. (The minimum clearance or maximum interference). For these two parts, the allowance is .001, meaning that the tightest fit occurs when the slot is machined to it’s smallest allowable size of .498 and the mating part is machined to its largest allowable size of The difference between .498 and .497, or .001, is the allowance.

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**Important Terms of Toleranced Parts**

Tolerance – the total allowable variance in a dimension; the difference between the upper and lower limits. The tolerance of the mating part is .002” ( = .002) The tolerance of the slot is .004” ( = .004)

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**Important Terms of Toleranced Parts**

Maximum Material Condition (MMC) The condition of a part when it contains the greatest amount of material. The MMC of an external feature, such as a shaft, is the upper limit. The MMC of an internal feature, such as a hole, is the lower limit. Least Material Condition (LMC) The condition of a part when it contains the least amount of material possible. The LMC of an external feature is the lower limit. The LMC of an internal feature is the upper limit.

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**Important Terms of Toleranced Parts**

Piece tolerance The difference between the upper and lower limits of a single part (.002 on the insert in this example, .004 on the slot.). System tolerance The sum of all the piece tolerances. For this example (.006)

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**Fit Types: Clearance & Interference fits between two shafts and a hole**

Shaft A is a Clearance fit, shaft B is an Interference fit

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**Fit Types: Transition Fit**

A Clearance Fit occurs when two toleranced mating parts will always leave a space or clearance when assembled. An Interference Fit occurs when two toleranced mating parts will always interfere when assembled. A Transition Fit occurs when two toleranced mating parts are sometimes an interference fit and sometimes a clearance fit when assembled.

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**Functional Dimensioning**

Functional Dimensioning begins with tolerancing the most important features. Then, the material around the holes is dimensioned (at a much looser tolerance). Functional features are those that come in contact with other parts, especially moving parts. Holes are usually functional features.

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**Tolerance Stack-up AVOID THIS!!! Occurs when dimensions are taken**

from opposite directions of separate parts to the same point of an assembly. Tolerance Stack-up Dimensioned from the left. Dimensioned from the right. AVOID THIS!!!

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**Avoiding Tolerance Stack-up Tolerance stack-up can**

Better still, relate the two holes directly to each other, not to either side of the part. The result will be the best tolerance possible of ±0.005. Tolerance stack-up can be eliminated by careful consideration and placement of dimensions. (Dimension from same side).

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**Basic Hole System The basic hole system is used to**

apply tolerances to a hole and shaft assembly. The smallest hole is assigned the basic diameter from which the tolerance and allowance is applied.

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**Creating a Clearance Fit Using The Basic Hole System**

Check the work by determining the piece tolerances for the shaft and the hole. To do so, first find the difference between the upper and lower limits for the hole. Subtract .500” from .503” to get .003” as a piece tolerance. This value matches the tolerance applied in Step 4. For the shaft, subtract .493” from .496 to get .003” as the piece tolerance. The value matches the tolerance applied in Step 3. The difference between the largest hole (.503” upper limit) and the smallest shaft (.493” lower limit) equals a positive .010”. Because both the tightest and loosest fits are positive, there will always be clearance between the shaft and the hole, no matter which manufactured parts are assembled. Using the basic hole system, assign a value of .500” to the smallest diameter of the hole, which is the lower limit. The allowance of .004” is subtracted from the diameter of the smallest hole to determine the diameter of the largest shaft, .496”, which is the upper limit. The lower limit for the shaft is determined by subtracting the part tolerance from .496”. If the tolerance of the part is .003”, the lower limit of the shaft is .493” Using the assigned values results in a clearance fit between the shaft and the hole. This is determined by finding the difference between the smallest hole (.500” lower limit) and the largest shaft (.496” upper limit), which is a positive .004”. As a check, this value should equal the allowance used in step 2 The system tolerance is the sum of all the piece tolerances. T o determine the system tolerances for the shaft and the hole, add the piece tolerances of .003” and .003” to get .006” The upper limit of the hole is determined by adding the tolerance of the part to .500”. If the tolerance of the part is .003”, the upper limit of the hole is .503” The parts are dimensioned on the drawing.

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**Creating an Interference Fit Using The Basic Hole System**

ADD here Follow the same sequence of steps as you did for a Clearance Fit, except that you ADD the allowance in Step 2, instead of subtract.

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**Cylindrical Fits – Metric Units**

ANSI B4.2 standard basic size – the diameter from which limits are calculated upper and lower deviation – the difference between the hole or shaft size and the basic size tolerance - the difference between the maximum and minimum sizes

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**Cylindrical Fits – Metric Units**

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**Cylindrical Fits – Metric Units**

fundamental deviation – a letter grade that describes the deviation closest to the basic size International Tolerance (IT) grade – a series of tolerances that vary with the basic size to provide a uniform level of accuracy within a given grade there are 18 IT grades: IT01, IT0, IT1, …, IT16

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**Cylindrical Fits – Metric Units**

Hole basis a system of fits based on the minimum hole size as the basic diameter the fundamental deviation for a hole-basis system is “H” Appendices 35 and 36 give hole-basis data for tolerances Shaft basis a system of fits based on the maximum shaft size as the basic diameter the fundamental deviation for a hole-basis system is “h” Appendices 37 and 38 give shaft-basis data for tolerances

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**Cylindrical Fits – Metric Units**

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**Cylindrical Fits – Metric Units**

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**Example 1 determine the shaft and hole limits for: hole-basis system**

a close running fit a basic diameter of 49 mm use a preferred size

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Example 1 use a preferred basic diameter of 50 mm use a fit of H8/f7

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**Example 2 determine the shaft and hole limits for: hole-basis system**

a location transition fit a basic diameter of 57 mm use a preferred size

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Example 2 use a preferred basic diameter of 60 mm use a fit of H7/k6

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**Example 3 determine the shaft and hole limits for: hole-basis system**

a medium drive fit a basic diameter of 96 mm use a preferred size

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Example 3 use a preferred basic diameter of 100 mm use a fit of H7/s6

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**Nonstandard Fits, Nonpreferred Sizes**

determine the shaft and hole limits for: hole-basis system a close running fit a basic diameter of 45 mm (do not change to a preferred size)

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**Nonstandard Fits, Nonpreferred Sizes**

use a fit of H8/f7

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Dimensioning

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**Measurement is used to tell us:**

* How tall? * How heavy? * How many? * How much?

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**Standards of Measurement:**

1. Inch System 2. Metric (ISO) 3. Military (diverse) 4. Associations (also diverse)

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**Dimensions are applied to objects in a variety of styles:**

1. Chain i. Aligned ii. Unidirectional 2. Tabular 3. Coordinate/Baseline 4. Ordinate

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Dimension elements….

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**Dimensions are used to show an object’s:**

1. Overall: Width Depth Height 2. The actual size of features (rounds, fillets, holes, arcs, etc.)

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**3. And where features are located such as centers, angles, etc.**

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The proper placement of dimensions is critical to ensure that the part can be read and manufactured to specifications. Here are some practices that need to be followed when applying dimensions to an object:

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1. Dimensions should be stacked in a “broken chain” format to aid in the readability of the plate. “Breaking the Chain” refers to leaving out one dimension as shown above so that manufacturing tolerances are maintained.

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**Stacked dimensions that show position and size.**

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2. Do not place dimensions directly on the object unless it is unavoidable.

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**As a general rule…Stay off the object as much as possible.**

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3. Extension lines can be shared and even broken to clarify crowded dimensions.

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**4. Some features are dimensioned from their center lines.**

The center line may also be used as an extension line.

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5. Dimensions may be laid out in different configurations. Unidirectional dimensioning is the current standard in most industrial applications today.

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6. Leaders with dimensions are used to show negative cylinders (holes). The leader should always be placed to penetrate the center of all round features.

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Features such as counterbores, countersinks and spot faces are all dimensioned using a leader. Note: Each of these features has a special dimensioning symbol that can be used to show: a. Diameter b. Shape c. Depth

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**Here we see several examples where notes are used instead of symbols to dimension features.**

Notes that describe a specific feature are known as “local notes”. “General notes” are used to describe a characteristic that effects the entire part, i. e. materials, production instructions, etc.

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7. Arcs are always dimensioned as a radius. Full circles are dimensioned showing their diameter value.

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8. When dimensioning a part, always start with the inner-most dimensions and work to the outer-most values. Remember: Dimensions are used to show both the size and location of features.

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9. Always dimension features and not lines…..and remember…. NEVER, NEVER, NEVER dimension to hidden lines!

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**Here are some rules (See Table. 15.1) on how to dimension properly:**

1. The overall Width, Height, and Depth must be shown on the object. 2. Each feature should be dimensioned in the view where it appears true shape and size. Never dimension a feature where it appears only as a line. 3. NEVER dimension to hidden lines!!!

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**4. The measurement standards must be**

4. The measurement standards must be maintained and clearly noted on the print. 5. Dimension lines should never cross. 6. Leaders must point at an angle that allows them to penetrate the centre of the object which they are describing. 7. Chains MUST be broken to allow for tolerance variation in the part.

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**8. Dimensions should be placed to allow**

8. Dimensions should be placed to allow order to the print and promote ease of reading. 9. Negative cylinders (holes) must always be measured by their diameter. 10. Arcs are always shown by specifying a radius value. 11. When laying out centres, specify one common reference point for the X and Y axes.

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**12. Dual dimensions may be used, but they must**

12. Dual dimensions may be used, but they must be consistent and clearly noted. 13. Angles may be dimensioned by showing their degree(s), or their 3 point location. 14. Only dimension each feature once!!!

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