Drawing Basic

Q.1 What are the Drawing templates?

Drawing templates are the standard drawing files that are used to detail the 3D parts like mylars, pins, weldments, blades, pin retainers and L blocks that are designed and 3D finished.

Drawing Templates provides a platform where a designer can convey the details of the design of a part or assembly with the help of a drawing.

Drawing Templates contains a lot of information about the drawing as given below:

• Scale
• GD&T
• Roughness symbol and standards
• A person who has designed, checked and approved the drawing
• Part size
• Quantity
• Material
• Tolerances
• Sheet Size and sheet number
• Type of Projection(1st angle or 3rd angle)
• Notes about surface hardening and stress relieving 
• Project Code
• Project Name
• Title

Every company has its own standard design template which contains all the symbols and standards that the company follows to detail the parts or assemblies.

Usually, Mylars and Pins are detailed on the A4 sheets while A3 sheets are used to detail the key sheets.

Depending on the size of part or assembly, we can change the scale so that the drawing fits the sheet. We need to make sure that all the details about the drawing like linear dimensions, hole dimensions, GD&T, roughness etc. are clearly visible.




[Note: It is very important that while detailing we should check the angle of projection in which customer requires us to give the drawing. According to the customer standards, we should change the drawing angle of projection setting in our software and only then we should start detailing. not doing this can lead to errors in manufacturing and increase the overall cost.]





Q.2 What do you understand by Angle of projection?

To define an object, technical drawings use 2-dimensional views. This is called orthogonal projection. The angle of projection is of the quadrant in which we have kept the object. The projection of the object is projected on to the planes as it is viewed by the observer. Images projected on the horizontal planes are rotated clockwise so that the projections come on the same plane.

Orthogonal projection uses different methods to project the 3D object namely:

1   First Angle Projection 


The first angle of projection is a type of orthographic projection in which the object lies between the observer and the plane of projection.  In this projection, the top view comes on the bottom and the bottom view comes on the top of the front view. Similarly, the left view of the object is projected on the right and right view projected on the left. It is most commonly used in Europe and Asia region. Drawing templates contain the symbol of the type of projection to be used to project the images. Knowledge about the angle of projection symbols can help us identify the angle of projection in which the drawing is to be made.


2   Third Angle Projection

Third angle of projection is a type of orthographic projection in which plane of projection lies between object and observer. In this, the top view comes on the top of the front view and the bottom view on the bottom. Similarly, the left view is projected on the left and right on the right side of the front view. 
It is used mostly in the USA and Australia.


Differences between the First angle projection and third angle of projection:

1 In first angle projection object lies in the first quadrant while in the third angle of projection, object lies in the third quadrant.

2  In first angle projection, the top view comes on the bottom and the bottom view comes on the top of the front view while in third angle projection the top view comes on the top of a front view and the bottom view on the bottom.

3 The difference in symbols of first and third angle are given
       

First Angle Projection                  Third Angle Projection


4 Plane of projection is assumed to be non-transparent in first angle projection while it is assumed to be transparent in third angle projection.

These two projections types vary from customer to customer depending on their standards. Knowing about the angles of projection helps us to get an image of the object for which drawings are drawn.




Q.3 What is the First angle & Third angle projection? and why can\'t we use the 2nd and 4th angle of projection?

 

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

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

Q.4 What is GD&T? What are the benefits of GD&T?

Geometric Dimensioning and Tolerancing (GD&T) is a design approach and manufacturing mechanism that helps engineers and designers communicate how to bring a part design to life. When documented correctly using GD&T, it is possible to build a part that exactly matches its on-paper plans.

GD&T uses a symbolic language to indicate how significantly part features can deviate from the geometries listed in the design model. This language contains all relevant details involved in fabrication, including dimensions, tolerances, definitions, rules, and symbols that communicate a component’s functional requirements.

These symbols are placed in the first compartment which shows the type of tolerance to be applied. The characteristics are grouped into different features like form, orientation, location, runout. Their primary use and description are shown in the table. GD&T is applied to features using a feature control frame. Most used tolerance categories are form, location and orientation. Among the 14 symbols that are there, the most used symbols are only 10.

The figure above shows an object being controlled by three datums and a control frame. By datums, we mean the reference points that we take to measure the basic dimensions, A being the 1st datum, B being the second and C the third datum.

Form tolerances control the shape of the part

Orientation tolerances control the tilt of the part and have to do with the angle at which particular part is tilted.

Location tolerance deal with the location of a particular feature and has to do with linear dimensions of a part. Any plane or side is taken as a reference and a corresponding linear dimension is drawn mentioning the tolerance and feature applied. Profile locates feature surfaces. A profile is one of the strongest features and controls form and orientation.


Benefits of GD&T:

• Saving Money — GD&T enhances design accuracy by allowing for appropriate tolerances that maximize production. For many projects, the process provides extra or bonus tolerances, further increasing cost-effectiveness.

• Ensuring Dimensional and Tolerance Requirements — By explicitly stating all design requirements, a thorough GD&T process guarantees accurate fulfilment of all dimensional and tolerance specifications.

• Assisting Digital Design Methods — Clear, concise GD&T data is readily adaptable to digital design programs, including nearly universally used 2D and 3D CAD files.

• Offering Uniformity and Convenience — As a single, consistent language, GD&T reduces guesswork and interpretation while ensuring consistent geometries across design and manufacturing.

• Providing Accurate Communication — Today’s intricate designs demand the most accurate and reliable communication. GD&T enables designers, manufacturers, and inspectors to communicate clearly with one other, saving time and making the process more efficient.

Q.5 Explain all the symbols used in GD&T?

GD&T has 14 symbols out of which 10 are most commonly used features. They are categorised base on a form, orientation, location, profile and runout. They are described below:


Form:

Form controls the shape of surfaces. It does allow datum reference.

1) Flatness- In this feature, all the elements are supposed to be in the same plane. Flatness error is calculated by the difference in the highest and lowest points on the plane. You are trying making sure that any point along the surface does not go above or below the tolerance zone. This feature doesn\'t require any reference.



2) Straightness- In this feature, all the elements are supposed to be in a straight line. It is used for a line to communicate the variation allowed in a line, hence maintaining it straight. It doesn\'t require a datum. If it is to be used for a cylinder, it is given along with the diameter dimension. It controls each dimension separately.



3) Cylindricity- This feature ensures that the surface of the cylinder is smooth and within the tolerance zone. Each surface is at a fixed distance from the axis of the cylinder in this feature. It does not require a datum.


4) Circularity- Circularity requires no datum. Circularity ensures all the points to be equidistant from the centre. Circularity error is the radial distance between coaxial diameters. it limits the circularity error.



Orientation:

5) Perpendicularity- When this feature is applied to a surface, the tolerance zone is between two parallel surfaces perpendicular to the datum surface. It requires a datum surface.



6) Parallelism-When this feature is applied to a surface, the tolerance zone is between two parallel surfaces parallel to the datum surface. It basically ensures that all the points on the referenced surface lies in the tolerance zone at a certain distance away from the datum surface. It also ensures that the surface to be referenced is parallel to the datum surface. Parallelism is quite simple to measure. 



7) Angularity- Angularity is a feature that is specified between two lines. It requires a datum reference from which the angular tolerance is maintained. In case of a plane, angularity feature maintains the angular tolerance between two planes.



8) Position- Position tolerance shows the exact location of a FOS(a feature of size) with respect to a datum. It tells us how far feature location can vary from its true position. It requires datum for reference.



Profile:

9) Profile of a surface- Profile of a line is usually used at a place where we require a surface to be in the tolerance range. In this case, the profile directly mimics the design of the surface. Every point of the surface should fall within the tolerance zone. For example, if a callout is given on fillet of a weldment, each point on the surface should fall in the tolerance zone. It is used only for advanced curve surfaces. A CMM is used to track the entire surface.


10) Profile of Line- This feature is used at places where a curve is to be maintained within the tolerance zone. The tolerance zone is two parallel curves from the main curve. It is used for advanced curves. This tolerance zone may or may not be referenced by a datum.


Runout:

11) Total Runout- Total Runout is how much a surface or feature varies with respect to a datum when the part is rotated 360 degrees about datum axis. It controls both amounts of variation in the surface when the part is rotated and also the variation in the axial dimension.



12) Circular Runout- Runout feature is used to check how much one feature of a part varies with respect to another when the part is rotated about a datum axis. It is basically used to check how much a part wobbles. Runout can be called out on any feature that is rotated about an axis. Runout is measured using a simple height gauge on the reference surface.



Location(Derived Median points): 

13) Concentricity- Concentricity feature ensures the smoothness of a cylindrical surface with respect to a datum surface. A surface is chosen as a datum. The axis about which this surface acts as a datum axis for the feature we wish to control. The feature must fall in the tolerance zone. It is very difficult to maintain concentricity as it is not to be checked directly w.r.t a surface but w.r.t an axis derived from another surface.



14) Symmetry- Symmetry feature ensures symmetry of a 3d object about a virtual datum plane. Points from either surface are calculated using the CMM and checked if they fall in the positive and negative side of the tolerance zone. Symmetry is very difficult to calculate as it does not have a fixed surface about which we can check the deviation. 

In short, refer below Image;