Dimensioning: Engineering Graphics

• Dimensioning is used in engineering graphics
• The purpose of dimensioning is to fully communicate engineering drawings
• At the end of this section, the topics are:
  • Introduction to dimensioning, terminology and dimensioning principles
  • Dimensioning techniques (part one and two)
  • Dimensioning for functions

Introduction to Dimensioning

• Dimensions are numerical values that define the attributes of a feature in technical drawings
• Engineering drawings should provide all the information needed to build a part, assembly, or system
• Metric units use millimeters in whole numbers
• English/Imperial units use inches in decimals
• Unless otherwise stated, all dimensions are in millimeters or inches
• Main goals of Dimensioning
  • Use essential dimensions, nothing more
  • Arrange dimensions to support the function and mating relationship of the part
  • Do not specify the manufacturing methods
  • Arrange dimensions for optimum readability
  • Assume angles to be 90 degrees unless otherwise stated

Terminology

• Drawings must include all the necessary dimensions for accurate information
• Elements of dimensioning
  • Angular dimension specifies the angle between lines or edges
  • Dimension value indicates size or distance
  • Reference dimensions are for information or guidance and are in brackets
  • Arrows mark dimension line ends and indicate measurement limits
  • Radius symbol used to denote measurement from the center of a circle, symbol R
  • Diameter symbol represents the full distance across a circle, symbol Ø
  • Dimension lines are between extension lines and include a numerical dimension
  • Extension lines extend away from an object, also known as projection lines
  • Center marks are 2 short intersecting lines that form a cross or plus sign
  • Leader lines are drawn at an angle and used to dimension shapes

Key Principles of Dimensioning

• Dimension line styles and precision
  • Use thin lines with two projection lines marking the start and endpoints
  • Projection lines are drawn perpendicularly but do not touch
  • A leading zero is used with values less than 1, like 0.35 instead of .35
• A simple block needs:
  • Sizes of the block and hole (size dimensioning)
  • The location of the hole (location dimensioning)
• Fully constrained means that designs can be met and achieved at all stages of manufacturing
• Only show information once
• Manufacturing specifications must include:
  • Tolerances
  • Surface finishes
  • Materials
  • Number of parts

Dimensioning Techniques Overview

• The standards of line work, spacing, sizes and placement of dimensions are important
• Chain dimensioning is useful when the accumulation of tolerances does not endanger the part
• Baseline dimensioning is based on a common datum
  • Datum is the left-hand side of the stand
• Running dimensioning is a simplified method of baseline dimensioning and requires little space
  • Should take reference from a single datum
• Combined dimensioning is chain plus baseline dimensioning
  • Dimensions are arranged in a straight line only
• A reference/auxiliary dimension is a simplified method of baseline dimensioning with less space
• Features not drawn to scale use a wide line to underline a particular dimension
• Theoretically exact dimensions (TED) are always used with geometric control
• Dimensioning circles use a 'Ø ' symbol to specify the diameter and it is followed by the value
  • Ø40 denotes diameter of 40mm
• The way of dimensioning depends on the space available
• With dimensioning radii, a center of the arc does not have to be exactly located
• May place as a general note: FILLET R6 AND ROUNDS R3 UNLESS OTHERWISE SPECIFIED
• Various dimensioning of Angular Methods may use degrees, minutes and seconds like 30° 40′ 20″
• Square or flat surfaces can be machined on the end of a shaft so they can be turned by using a spanner
  • Narrow diagonal lines indicate the flat surface

Dimensioning Holes and Screw Threads

• Depth only refers to the depth of the circular portion
• Angles of the spacings are omitted when the intent is evident
• For screw threads
  • M10 = Nominal diameter or size of screw hole
  • 1.5 = Pitch or distance between adjacent threads
  • 6H = Tolerance
• Note: The drafting of major and minor diameters of both male and female threads
• Screw threads require a Continuous narrow line (type 01.1) and a Continuous wide line (type 01.2)

Counterbores and Chamfers

• Dimensioning Counterbores
  • Create a recess for a screw head as a flat surface obtained by spot-facing
  • Flat surface on casting obtained by counterboring or spot-facing
• Dimensioning Countersunk Holes
  • Commonly has angles of 60 Degrees or 90 Degrees
• Dimensioning Chamfers, use a dimension and degrees

Profiles and Dimensions

• Dimensioning Profiles should be metrically aligned and noted
• Unidirectional Dimensions are parallel with the bottom of the drawing sheet
  • 'Notes' use this method
• Aligned Dimensions have writings parallel with the related dimension line
• It Must be readable from the bottom or right-hand side of the drawing

Superfluous Dimensions and Checking

• Some superfluous dimensions are useful for convenience, but must be indicated as a 'Reference Dimension' in parentheses
• Remember to always check the drawing, view it and ask if:
  • Is the information complete?
  • Whether or not the machinist can use or work to the dimension specified to make the item

Tips for Guidelines and Rules with Dimensions

• Dimension lines should be narrow continuous lines 0.35 mm thick
• Keep extension lines away from the drawing feature's outline, leaving a small 2-3 mm gap
• Arrowheads should be approximately triangular
• Limits of sizes are quoted and adequate space must be left between rows of dimensions
• Center lines must never be used as dimension lines
• Use 0.4, rather than .4
• Position figures to be readable from the bottom or right-hand side
• Leader lines point where a specific indication applies
• The following General Rules must be followed:
  • Place dimension on the clearest view and specifically in orthographic projection
  • Circular features should be dimensioned from the front
  • Place dimensions outside the outline of the part as much as possible
  • Put the shortest closest to the part and ensure similar spacing between dimensions

Function Considerations

• The choice of dimensioning is important for the purpose of function
• In choosing between alternatives of dimensioning a given part, it is important to:
  • Consider the function of the part in the entire assembly
  • Consider how to inspect the part to determine acceptability

Functions of Dimensioning Techniques

• Chain, baseline, mating and fabrication are key elements
• Chain dimensioning helps to ensure distance between holes when mating with other parts
  • High risk though of tolerance accumulation than unlike baseline dimensioning
• Baseline dimensioning is also called a datum dimensioning
  • Avoids tolerance accumulation
  • Is for the purposes of function and multiple mating surfaces of a part use common datum
  • Is very Useful in CNC programming
• Dimensioning for Mating are for sliding parts and can be identified using a METRIC indicator
  • One is more controlled than the other
• Consider regular and Irregular spaced holes, and common datums