Tolerancing Chapter 11

Objectives Describe nominal size, tolerance, limits, and allowance of two mating parts Identify a clearance fit, interference fit, and transition fit Describe the basic hole and basic shaft systems

Objectives (cont.) Dimension mating parts using limit dimensions, unilateral tolerances, and bilateral tolerances Describe the classes of fit and give examples of each Draw geometric tolerancing symbols Specify geometric tolerances

Understanding Tolerance Tolerancing is an extension of dimensioning It allows you to specify a range of accuracy for every feature of a product so the parts will fit together and function properly when assembled

Understanding Tolerance To provide tolerances in CAD, you must: Understand the fit required between mating parts Have a clear picture of how inspection measurements are performed Be able to apply tolerance symbols to a drawing

Tolerance Tolerance is the total amount a specific dimension is permitted to vary Use generous tolerances when possible because increased precision makes parts more expensive to manufacture

Quality Control Large batches of parts may use statistical methods to control quality where a sample of parts are inspected Specific tolerances are based on the part’s function and fit

Variations in Form Acceptable parts must not extend beyond boundary limits

Implied 90 Degree Angles When lines intersect on a drawing at angles of 90 degrees, it is customary not to dimension the angle Implied 90 degree angles have the same general tolerances applied to them as any other angles covered by a general note

Implied 90 Degree Angles

Fits Between Mating Parts Fit is the term for the range of tightness or looseness resulting from the allowances and tolerances in mating parts

Clearance Fit Where the internal part fits into an external part with space between the parts.

Interference Fit Where the internal part will have to be forced into an external part. Produces a negative clearance or allowance.

Transition Fit Where there the smallest shaft will fit into the largest hole, but the largest shaft will have to be forced into the smallest hole. Refers to either a tight clearance or interference. May or may not produce a negative clearance or allowance.

Definitions for Size Designation Nominal size – used for general identification and usually expressed in decimals Basic size (basic dimension) – the theoretically exact size from which limits of size are determined

Definitions for Size Designation Actual size – the measured size of a finished part Allowance – the minimum clearance or maximum interference specified to achieve a fit between two mating parts

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

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

Specifying Tolerances The primary ways to indicate tolerances in a drawing are: A general tolerance note A note providing a tolerance for a specific dimension A reference on the drawing to another document that specifies the required tolerances

Specifying Tolerances (cont.) Adding limit tolerances to dimensions Adding direct plus/minus tolerances to dimensions Geometric tolerances

GENERAL TOLERANCE NOTES “ALL TOLERANCES ±1 mm UNLESS OTHERWISE NOTED. ANGLES 1 DEGREE.” General notes are usually located in the lower right corner of the drawing sheet near the title block. Often, general tolerance notes are included in the title block itself.

LIMIT TOLERANCES Limit tolerances state the upper and lower limits for the dimension range in place of the dimension values. … is the preferred method of stating tolerances. Method of Stating Limits Note: The upper value is always placed above the lower value.

Plus-or-Minus Tolerances Allows variations in only one direction from the nominal size. Allows variations in both directions from the nominal.

Tolerance Stacking The variations produced by tolerances are stacked in chain dimensioning

Fits between Mating Parts Fit is the range of tightness or looseness resulting from the allowances and tolerances in mating parts. The loosest fit, or maximum clearance, occurs when the smallest internal part (shaft) is in the largest external part (hole),

Fits between Mating Parts continued… Clearance Fit A clearance fit occurs when an internal part fits into an external part with space (or clearance) between the parts. Interference Fit An interference fit occurs when the internal part is larger than the external part, so the parts must be forced together.

USING AMERICAN NATIONAL STANDARD LIMITS AND FIT TABLES The American National Standards Institute has issued ANSI B4.1, Preferred Limits and Fits for Cylindrical Parts, defining terms and recommending preferred standard sizes, allowances, tolerances, and fits in terms of the decimal inch. This standard gives a series of standard classes of fits on a unilateral-hole basis so that the fit produced by mating parts of a class of fit will produce approximately similar performance throughout the range of sizes. Portion of RC8 Fit Table. The International Organization for Standardization (ISO) publishes a similar series of fit tables for metric values.

TOLERANCES AND MACHINING PROCESSES Tolerances Related to Machining Processes

Metric System of Tolerances and Fits The preceding material on limits and fits applies to both systems of measurement The ISO has a system of preferred metric limits and fits The system is specified for holes, cylinders, and shafts and has similar definitions of terms Hole Tolerance Shaft Tolerance 50H8/f7 Basic Size Fit

Geometric Tolerancing Geometric tolerances state the maximum allowable variations of a form or its position from the perfect geometry implied in the drawing The term “geometric” refers to forms such as planes, cylinders, squares, etc.

Geometric Characteristics and Modifying Symbols Tolerances of form and position (or location) control such characteristics as:

Symbols for Tolerances of Position and Form