Dimensioning and Tolerancing in Engineering Drawings

• Dimensioning is key part of detailed drawings, but not necessarily using the dimensions that were used to draw the object.
• Dimensions used should follow accepted standards to allow parts to be manufactured.
• Aim when dimensioning a part is to provide dimensions clearly and concisely.
• Provide everything required to produce and inspect the part exactly as the designer intended.

Proper Dimensioning Techniques Require Knowing

• Dimension Appearance/Technique
• Dimensioning and Locating Features
• Dimension Choice

Dimension Appearance/Technique Includes

• Lines used in dimensioning
• Types of dimensions
• Dimension symbols
• Dimension spacing and readability
• Dimension placement

Dimensioning and Locating Features

• Different types of features use unique methods of dimensioning.

Dimension Choice Factors

• Your choice of dimensions influences the manufacturing method for the part.
• Units and decimal places used
• Locating feature using datums
• Dimension accuracy and error build up

Lines for Dimensioning

• Dimension lines
• Extension lines
• Leader lines
• All three line types are drawn thin not to be confused with visible lines.

Dimension Line

• Lines terminated by arrowheads indicate the direction and extent of a dimension.

Extension Line

• Thin solid line extends from a point on the drawing to which the dimension refers.
• Long extension lines should be avoided.

Leader Line

• Straight inclined thin solid line terminated by an arrowhead, dot or nothing.
• Use arrows if it ends on the outline of an object.
• Use a dot if it ends within the outline of an object.
• Use nothing if it ends within the outline of an object.

Avoid Leader Line Errors

• Crossing leaders
• Long leaders.
• Leaders that are parallel to adjacent dimension, extension, or section lines.
• Small angles between the leader and the terminating surface.

Arrowheads

• Drawn between the extension lines if possible, or on the outside if there is not enough space.

Types of Dimensions

• Linear distances: Arranged horizontally or vertically but may be aligned with a feature of the part.
• Angles: Give the angle between two surfaces or features.
• Notes: Dimension diameters, radii, chamfers, threads, and other features that cannot be dimensioned with the other two methods.

Lettering Rules

• Lettering should be legible, easy to read, and uniform.
• Use upper-case letters unless a lower case is required.
• Minimum lettering height is 0.12 in (3 mm).

Dimensioning Symbols

• Dimensioning symbols replace text allowing for no language translation
• Ø = Diameter
• SØ = Spherical Diameter
• R = Radius
• SR = Spherical Radius
• (8) = Reference dimension
• "Box" = Counterbore/spot face
• upside down triangle = Countersink
• 4X = Number of times or places
• inverted T = Depth/Deep
• "10 in box" = Dimension not to scale
• small "Box" = Square shape.
• arc on top of a line = Are Length

• l = Conical Taper

• |= Slope
• "3 horizontal lines' = Symmetry.

Spacing and Readability Rules

• Dimensions should be easy to read, and minimize the possibility for conflicting interpretations.
• The spacing between dimension lines should be uniform.
• Avoid dimensioning inside an object or have the dimension line touch the object unless clearness is gained.
• Dimension text should be horizontal from the bottom of the drawing.
• Dimension text shouldn't cross dimension, extension or visible lines.
• Dimension lines should not cross extension lines or other dimension lines.
• Extension lines can cross other extension lines or visible lines.
• Extension lines and centerlines should not connect between views.
• Leader lines should be straight, and point to the center of the are/circle at an angle between 30 and 60 degrees.
• Dimensions should not be duplicated or the same information given in two different ways.
• Use parentheses for reference dimensions (e.g. (10)).

Dimension Placement Rules

• Place dimensions to enhance the communications of the design.
• Group dimensions whenever possible.
• Place dimensions between views unless clearness is promoted by placing them outside.
• Attach dimensions to the view where the shape is shown best.
• Do not dimension hidden lines.

Dimensioning Features

• Circles are dimensioned by diameter and arcs by radius using a leader line and a note.
• Holes are dimensioned by giving their diameter and location in the circular view.
• Cylinders are dimensioned by giving its diameter and length in the rectangular view and located in the circular view.
• Repetitive features or dimensions are specified using the symbol "X" along with the repetition number.
  • No space between the number and "X" but should have space between the symbol "X" and the dimension.

Advanced DImensioning Feature Techniques

• If a dimension is given to the center of a radius, draw a small cross at the center.
• Unimportant Center Location: Clearly show the arc location is controlled by other dimensioned features such as tangent surfaces.
• Spheres: Complete spheres are dimensioned by diameter and partial spheres by radius w/ "S" prefix.
• Blind Hole Depth: Specify in a note and is the depth of the full diameter from object's surface.
• Use "THRU" to indicate if a whole goes all the way through the shape.
• Symmetric Parts: Only dimension to one side of the center line of symmetry.

Drilled Holes

• Counterbored holes: show the drill DIA, C'Bore DIA and C'Bore depth
• Show thickness rather than the counterbore depth if it is significant.
• Spoftfaced Holes: The difference between a C'BORE and a Spotface is that the machining occurs on a curved surface. The depth cannot be specified in the note.
• Countersunk Holes: Drill DIA, Drill Depth, Space, C'Sink DIA C'Sink angle

Chamfers

• Dimensioned by a linear dimension and an angle, or by two linear dimensions.

Drawing Notes

drawing notes provide additional information to complement the design.

• Manufacturing requirements.
• Treatments and finishes.
• Blanket dimensions, e.g. size of all rounds and fillets on a casting or a blanket tolerance.
• Use "NOTE:" to identify note area.

Dimension Choice Factors

• Dimension placement and dimension text influences how a part is manufactured.
• Manufacturing process should not be specifically stated.
• Base dimensions on function first then manufacturing.

Units and Decimal Places

• Decimal dimensions should be used for all machining dimensions.
• Drawings specify standard drills, broaches, and the like by size.
• For drill sizes given by number or letter, also include a decimal size.
• Metric dimensions given in 'mm' and to 0 or 1 decimal place (e.g. 10, 10.2).
• When given in millimeters and less than 1, use a leading zero (e.g. 0.5).
• English are given in 'inches' and to 2 decimal places (e.g. 1.25).
• Do not use a leading zero for values less than one inch (e.g. .75).
• Metric 3rd angle drawings are designated by the SI symbol.

Locating Features Using Datums

• Use three mutually perpendicular datum planes.
• Planes are imaginary and theoretically exact.
• Surfaces of the part that touch the datum planes are called datum features.
• Features on a part are located with respect to a datum feature.
• Datum dimensioning is preferred over continuous dimensioning.
• Base dimensions between points/surfaces that have a functional relation to each other.
  • e.g., Slots, mating hole patterns,

Dimension Accuracy

• There is no "exact" measurement.
• Every dimension has an implied or stated tolerance.
• Tolerance is the amount a dimension is allowed to vary.

Rounding off fractional dimensions

• More accurate dimensions mean a more expensive manufacturing cost, so to cut costs round off fractional dimensions.
  • If the third decimal place number is less than 5, truncate after the second decimal place. Ex. 1.123 -> 1.12.
  • If the third decimal place number is greater than 5, round up and increase the second decimal place number by 1. Ex. 1.126 -> 1.13.
• If the third decimal place number is exactly 5, whether or not we round up depends on if the second decimal place number is odd or even. If it is odd, we round up and if it is even, it is kept the same. Ex. 1.165 -> 1.16 1.135 -> 1.14

Cumulative Tolerances

• Continuous dimensioning lead to error accumulation.
• Use datum dimensioning to reduce error buildup.

Test your knowledge of dimensioning and tolerancing in engineering drawings. This quiz covers symbols, notations, and best practices for accurately representing dimensions for manufacturing. Understand how dimensioning choices impact manufacturing processes.