2D Detailing

AIM :

To study about the various design aspects regarding BIW fixtures.

OBJECTIVE :

• To explain about the Drawing templates.
• To brief on the Angle of Projection.
• To explain about the First angle & Third angle projection.
• To explain in detail about GD & T and its benefits.
• To list out the symbols used in GD & T.

1. Drawing Templates

engineering drawing is the graphical representation of structures, machines, and their component parts that communicates the engineering intent of a technical design to the craftsman or worker who makes the product.

2. Angle of Projection

The angle at which an observer draw an object to convert it from 3D to 2D plane such as real object image to a plane paper is known as angle of projection.

Projection is the process or technique of representing a three-dimensional object by projecting all its points by straight lines, either parallel or converging to a picture plane.

There are two types of Angle of Projection,

• First Angle Projection
• Third Angle Projection

Orthographic projections are 2-D representations of 3-D objects. Features of 3-D parts are presented on a flat plane, as though the outlines of the part had been projected onto a screen.

The viewing angles are split into quadrants of first angle, second angle, third angle, and fourth angle.

In practice only first angle quadrant and the third angle quadrant are used since these are pure views. The second angle quadrant and fourth angle quadrant are combination of first and third angle projections and cause confusion to the reader.

3. First and Third Angle projection

First angle projection and Third angle projection are two different methods of creating a 2D drawing of a 3D part.

First Angle Projection is commonly used in all countries other than United States.

It is mainly used in Europe and Asia countries.

The Indian Standard Institution (ISI) recommend the use of First Angle Projection method now in all the institutions.

Third Angle Projection is commonly used in United States of America.

First Angle Projection

• The object is imagined to be in first quadrant.
• The object is lies between the observer and plane of projection.
• The plane of projection is assumed to be non transparent.
• When view are drawn in their relative position Top view comes below Front view, Right side view drawn to the left side of elevation.
• A First Angle Projection drawing is identified by the first angle projection symbol. The angle of projection symbol typically appears in angle of projection block near to Title block.

Third Angle Projection

• The object is imagined to be in third quadrant.
• The plane of projection lies between the observer and object.
• The plane of projection is assumed to be transparent.
• When view are drawn in their relative position Top view comes above Front view, Right side view drawn to the light side of elevation.
• A Third Angle Projection drawing is identified by the third angle projection symbol. The angle of projection symbol typically appears in angle of projection block near to Title block.

Second and Fourth angle Projection

In Mechanical Engineering,drawings are made either in first angle projection or in third angle projection.We do not make drawings in second or fourth angle projection.
When there are Vertical Plane (VP) and Horizontal Plane (HP) perpendicular to each other , we have 4 quadrants.

For 1st angle projection system, the object is placed in 1st quadrant, for 2nd angle projection system, the object is placed in 2nd quadrant and so on.

In Second quadrant, Vertical plane (VP) lies in between object and observer. Therefore front view of the object will lie on vertical plane whereas top view will lie on horizontal plane.

As per rule of projection when horizontal plane is rotated 90 degree in clockwise direction, top and front view will overlap.

Overlapping projection views create confusion in the drawing. Therefore 2nd angle projection system is not used.

Similarly when object is placed in 4th quadrant both top and front view will overlap. Therefore fourth angle projection is also not used.

To sum up, because of overlapping of front and top views 2nd and 4th angle projections are not used. First and third angle projection systems are recommended for orthographic projections.

4. Geometric Dimensioning & Tolerancing

Manufactured items differ in size and dimensions from the original CAD model due to variations in the manufacturing processes. To optimally control and communicate these variations, engineers and manufacturers use a symbolic language called GD&T, short for Geometric Dimensioning and Tolerancing.

GD&T tells manufacturing partners and inspectors the allowable variation within the product assembly and standardizes how that variation is measured.

GD&T, short for Geometric Dimensioning and Tolerancing, is a system for defining and communicating design intent and engineering tolerances that helps engineers and manufacturers optimally control variations in manufacturing processes.

With functional assemblies, multi-part products, or parts with complex functionality, it is crucial that all components work well together. All relevant fits and features need to be specified in a way that impacts the manufacturing process and its related investments the least, while still guaranteeing functionality. Tightening tolerances by a factor two can raise the costs twofold or even more, due to higher reject rates and tooling changes. GD&T is the system that allows developers and inspectors to optimize functionality without increasing cost.

The most important benefit of GD&T is that the system describes the design intent rather than the resulting geometry itself. Like a vector or formula, it is not the actual object but a representation of it.

For example, a feature standing at 90 degrees to a base surface can be toleranced on its perpendicularity to that surface. This will define two planes spaced apart, that the center plane of the feature must fall within. Or, when drilling a hole, it makes the most sense to tolerance it in terms of alignment to other features.

Describing product geometry related to its intended functionality and manufacturing approach is ultimately simpler than having to describe everything in linear dimensions. It also provides a communication tool with manufacturing vendors, customers, as well as quality inspectors.

When performed well, GD&T even allows statistical process control (SPC), reducing product reject rates, assembly failures, and the effort needed for quality control, saving organizations substantial resources. As a result, multiple departments are able to work more in parallel because they have a shared vision and language for what they want to achieve.

Benefits of GD & T

The language of geometric dimensioning and tolerancing can be intimidating at first, but when used properly, it can yield many advantages such as,

• Standardized, international system.
• More flexibility, particularly for complex shapes.
• Allows cylindrical tolerance zones.
• Datums are clearly defined.
• Based on the fit and function of a part or assembly.
• Eliminates the need for many notes.
• Allows for more tolerance.

GD & T Guidelines

• Clarity of a drawing is the most important, even more so than its accuracy and completeness. To improve clarity, draw dimensions and tolerances outside of the part's boundaries and applied to visible lines in true profiles, employ a unidirectional reading direction, convey the function of the part, group and/or stagger dimensions, and make use of white space.

• Always design for the loosest feasible tolerance to keep costs down.

• Use a general tolerance defined at the bottom of the drawing for all dimensions of the part. Specific tighter or looser tolerances indicated in the drawing will then supersede the general tolerance.

• Tolerance functional features and their interrelations first, then move on to the rest of the part.

• Whenever possible, leave GD&T work to the manufacturing experts and do not describe manufacturing processes in the engineering drawing.

• Do not specify a 90-degree angle since it is assumed.

• Dimensions and tolerances are valid at 20 °C / 101.3 kPa unless stated otherwise.

5. Symbols used in GD & T

GD&T is feature-based, with each feature specified by different controls. GD&T symbols fall into five groups:

Form controls specify the shape of features, including:

• Straightness is divided into line element straightness and axis straightness.

• Flatness means straightness in multiple dimensions, measured between the highest and lowest points on a surface.

• Circularity or roundness can be described as straightness bent into a circle.

• Cylindricity is basically flatness bent into a barrel. It includes straightness, roundness, and taper, which makes it expensive to inspect.

Profile controls describe the three-dimensional tolerance zone around a surface:

• Line Profile compares a two-dimensional cross-section to an ideal shape. The tolerance zone is defined by two offset curves unless otherwise specified.

• Surface Profile creates through two offset surfaces between which the feature surface must fall. This is a complex control typically measured with a CMM.

Orientation controls concern dimensions that vary at angles, including:

• Angularity is flatness at an angle to a datum and is also determined through two reference planes spaced the tolerance value apart.

• Perpendicularity means flatness at 90 degrees to a datum. It specifies two perfect planes the feature plane must lie in between.

• Parallelism means straightness at a distance. Parallelism for axes can be defined by defining a cylindrical tolerance zone by placing a diameter symbol in front of the tolerance value.

Location controls define feature locations using linear dimensions:

• Position is the location of features relative to one another or to datums and is the most used control.

• Concentricity compares the location of a feature axis to the datum axis.

• Symmetry ensures that non-cylindrical parts are similar across a datum plane. This is a complex control typically measured with a CMM.

Runout controls define the amount by which a particular feature can vary with respect to the datums:

• Circular Runout is used when there is a need to account for many different errors, such as ball-bearing mounted parts. During inspection, the part is rotated on a spindle to measure the variation or ‘wobble’ around the rotational axis.

• Total Runout is measured on multiple points of a surface, not just describing the runout of a circular feature but of an entire surface. This controls straightness, profile, angularity, and other variations.

CONCLUSION :

All the given objectives were satisfied with atmost perfection.

The images were attached wherever necessary