Tolerancing

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# Tolerancing - PowerPoint PPT Presentation

Tolerancing. Deviation: . The difference between the actual size and the corresponding basic size. Tolerance:. The total amount by which a specific dimension is permitted to vary. The tolerance is the difference between the maximum and minimum limits.

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## Tolerancing

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Presentation Transcript

### Tolerancing

Deviation:
• The difference between the actual size and the corresponding basic size.
Tolerance:
• The total amount by which a specific dimension is permitted to vary. The tolerance is the difference between the maximum and minimum limits.
• For example: a dimension given as 1.625 +/- .002 means
• that the manufactured part may be 1.627” or 1.623”
• or anywhere between these limit dimensions
• The tolerance, or total amount of variation “tolerated,” is .004
Example: Two mating parts
• To control the dimensions of the quantities of the two parts so that any two mating parts will be interchangeable, it is necessary to assign tolerance to the dimensions of the parts.
Allowance
• Is the minimum clearance space (or maximum interference) intended between the maximum material condition (MMC) or mating parts.
• Smallest Hole 1.250”
• Largest shaft 1.248”
• Allowance = .002”
General Tolerancing Rules
• All Tolerancing limits are considered to be absolute.
• i.e.,

1.22 = 1.22000…0

1.20 = 1.20000…0

• All Dimensions and Tolerances are understood to apply at 68 degrees Fahrenheit.
• The system of tolerances does not necessarily require the use of any particular method of production.
Setting Tolerances:
• Unilateral: The tolerance is applied in one direction, the other value is zero.
• Bilateral: The tolerance is applied in both directions from the nominal.
Tolerance Accumulation
• Chain
• Baseline
• Direct
Functional Dimensions
• Tolerances can have a cumulative effect.
• Most important function needs to be considered.
• Used where there is a uniformity in tolerances.
• Nominal (Basic) dimension alone is given on drawing face.
• Based on the number of decimal places specified.
Limits and Fits
• Production and inspection benefit from the use of standard limits.
• ISO 286 has more than 500 possible tolerance zones for holes and shafts.
• ANSI B4.2 has about 150 possible tolerance zones for holes and shafts.
• Goals is to maximize standardization.
International Organization of Standards (ISO)
• Rapid growth to world\wide commerce has fostered an international system of units - SI units -
• Seven basic units: 1) meter (length), 2) kilogram (mass), 3) second (time), 4) ampere (electric current), 5) kelvin (thermodynamic temp..), 6) mole (amount of substance), and candela (luminous).
• SI system gradually coming into use in the United States.
• Effort to convert all standards of the American National Standards Institute (ANSI) to the SI units in conformity with the ISO standards.
Fit:
• Fit is the general terms used to signify the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in mating parts.
• Clearance Fit
• Transition Fit
• Interference Fit
Clearance fit:
• The relationship between assembled parts when clearance occurs under all tolerance conditions.
• There is always clearance.
Interference fit:
• The relationship between assembled parts when interference occurs under all tolerance conditions.
• The internal member is larger than the external member such that there is always an actual interference of material.
• Where there is always interference.
Transition fit:
• The relationship between assembled parts when either a clearance or interference fit results, depending on the tolerance conditions of the mating parts.
• Where there may be clearance or interference based on where the parts fall in the tolerance range.
Preferred Metric System of Tolerances and Fits
• A system of preferred metric limits and fits by the ISO in in the ANSI B4.2 standard.
• The system is specified for holes, cylinders, and shafts.
• Terms for metric fits are similar to those for decimal-inch fits.
Examples: Close Running -Metric Hole Basis Clearance Fits
• 50H8/f7
• 50 = 50 millimeters in diameter
• H8 = hole deviation of 8
• f7 = shaft deviation of 7
• Refer to page A153 - Appendix C.13 (Lamit) for max. and min. deviations
• hole = 50.039 - 50.000
• shaft = 49.975 - 49.950
Preferred Inch Fits
• ANSI has issued the ANSI B 4.1 - 1967 (R1994), “Preferred Limits and Fits for Cylindrical Parts”
• Recommends preferred sizes, allowances, tolerances, and fits in terms of the decimal inch.
• Used to produce similar performance throughout the range of sizes (hole sizes)
• Three groups: running and sliding fits, locational fits, and force fits.
Running and Sliding Fits
• Designed to provide a similar running performance throughout the range of sizes.
• Examples preferences:
• RC 1 = Close sliding fits are intended for the accurate location of parts that must assemble without perceptible play.
• RC 3 = Precision running fits are about the closest fits that can be expected to run freely, and they ar intended for precision work at low speeds.
• RC1 - RC9 (close sliding - loose running)
Locational Fits
• Intended to determine only the location of the mating parts.
• Divided into three groups
• LC = Clearance fits
• LT = Transition Fits
• LN = Interference Fits
Force or Shrink fit
• Special type of Interference fit
• Maintains constant bore pressure throughout the range of sizes.
Example: 2.0000” basic diameter with a Class R1 fit
• Standard Limits (HOLE):
• upper limit = +0.5 = .0005”
• Lower limit = 0 = 0
• Dimension the hole as . . . 2.0000 +.0005/-.0000
Example (continued)
• 2.000” - .0004” = 1.9996 (upper limit)
• 2.000” - .0007” = 1.9993 (lower limit)
• Shaft dimensioned as 1.9996 +.0000/-.0003