basics of mechanical drawing and dimensioning

Basics of Mechanical Drawing and Dimensioning

ChE 126

Borrowed mostly from the Fundamentals of Engineering Honors program at Ohio State

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How Would You Describe This?

• Describe this using only words• How effective is it?

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• Freehand sketches

• Instrument drawings

• Computer drawings and models

Three Basic Types of Technical Drawings

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Introduction to Projections

• Present 3-D objects with 2-D media• Two Basic Categories

Orthographic Pictorial

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Orthographic Projection

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Defining the SixPrincipal Views

orOrthographic

Views

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• Hidden Lines – represent features that cannot be seen in the current view

• Centerlines – represent symmetry and mark the center of circles, the axes of cylinders, and the axes of symmetrical parts, such as bolts

Hidden and Center Lines in Orthographic Projections

• Object Lines – represent visible features for an object

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For Example:

1. Visible

2. Hidden3. Center

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Step 1 – Lightly Block Three Views

Use very light lines for drawing in

the construction

lines

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Step 2 – Lightly Block Major Features

Use very light lines for

drawing in the construction

lines

• Holes• Arcs• Cutouts

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Step 3 – Add Final Lines

Use very light lines for drawing in

the construction

lines

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Completed Sketch

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Dimensioning

• Orthographic and isometric views define the shape and general features of the object

• Dimensioning adds information that specifies

– Size of the object

– Location of features (e.g. holes)

– Characteristics of features (e.g. depth and diameter of hole)

• Dimensions also communicate the tolerance (or accuracy) required

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Units of Measure

• Length– English: Inches, unless

otherwise stated• Up to 72"• Feet and inches over 72"

– SI: millimeter, mm

• Angle– degrees, minutes, seconds

Angle Dimensions

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Dimensioning Basic Shapes – Assumptions

• Perpendicularity

• Symmetry

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Dimensioning Basic Shapes

• Rectangular Prism

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Dimensioning Shows:

ALWAYS give DIAMETER " " for full circles (360 degrees) and RADIUS "R" for arcs (less than 360

degrees)

A) Size B) Location and Orientation

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General Guidelines: Clarity is the Goal

• Dimension Outside of View

Avoid Good Practice

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General Dimensioning Guidelines

• Start with basic outside dimensions of the object– Height– Width– Depth

• Add dimension for location and size of removed features

• Add general and specific notes – such as tolerances

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Practice Problem

How many Dimensionsare needed?

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Practice Problem

How many Dimensionsare needed?

Answer: 8