Understanding Engineering Drawings

technical drawings are everywhere in engineering they can be used to define how a part should be manufactured and inspected or to explain how the different parts of a system fit together ultimately they're tools that Engineers use to communicate so being able to understand them is an important skill but it can sometimes feel like they're quite difficult to decipher in this video we'll cover everything you need to know to make sense of them so let's get started engineering drawings come in all shapes and sizes assembly drawings show how all of the different components of an assembly

fit together and the functional relationship between them detail drawings on the other hand fully Define the geometry of a single component although they don't normally Define the manufacturing methods that should be used they provide all the information needed to fabricate and inspect a specific part other types of drawings include layout drawings that are used to illustrate a design approach and so don't include much detail and interface control drawings that identify any interfaces with other components drawings often follow conventions that are defined in technical standards with the most common being as me y14 and the various

ISO standards standards go into a lot of detail and companies usually have their own requirements anyway so in this video we'll just cover the fundamentals and some best practices without focusing on any one particular standard regardless of the standard being used drawings all have the same general structure the title block is usually located in the bottom right corner it contains important information like the company logo drawing title drawing number the scale of the drawing and information about the author Checker and approver on detail drawings the title block might also include information about the part material

finish and surface roughness next there's the revision history table usually in the top right corner that lists changes to the drawing and finally there's the drawing space which is where views of the component or assembly are shown if there are a lot of different views on the same page it can be confusing but there's always an underlying structure to how they're laid out let's start by looking at primary views which are the front side and top and bottom views that are a key part of any detailed drawing we'll illustrate how they're drawn using this bracket

a primary view is created by projecting the visible edges of the part onto an imaginary plane that's located between the part and the Observer and is aligned with the face of the object this is called an orthographic projected View the orthographic part means that the projection lines are at right angles to the plane one view is chosen to be the front view this is usually the view that provides the most information about the object additional projected views showing its other sides are then needed to fully Define the object in three dimensions this is the view

of the left side of the object this is the rear view [Music] and we can generate the top bottom and right views in the same way [Music] the projection planes are then unfolded and this is how the views are placed on a drawing because the projection plane was between the camera and the object the left view ends up being placed to the left of the front view the top view is placed above it and so on only enough views need to be used to ensure that the part is fully defined here the right bottom and

rear views aren't needed and can be removed this way of arranging the views based on the projection plane being placed between the camera and the object is called third angle projection but there's another common method called first angle projection where the projection plane is located behind the object instead of in front of it thank you when this method is used the left view is placed to the right of the front view and the top view is placed below it first angle projection is more commonly used in Europe and third angle projection is more common in

North America to understand why these methods are called first and third angle projections consider two perpendicular planes that create four quadrants if an object is located in quadrant one and is being viewed either from above or from the right the views are projected onto planes that are behind the object which is first angle production but in the third quadrant the views are projected onto planes that are in front of the object which is third angle projection foreign showing a tapered cone is used to indicate which projection method has been used on a drawing when drawn

using first angle projection the view from the left where both circular edges are visible is placed to the right of the front view and for third angle projection it's placed to the left one of these two symbols is usually added to the drawing title block on this drawing of the bracket there's a third angle projection symbol in the title block so we immediately know that the view above the front view shows the top of the part in the view to the left of it shows the left side of the part primary views always line up

with each other perfectly which makes it easy to locate the same feature in different views but other views can be added to the drawing to complement the primary views and they can be placed anywhere on the page they don't have to be in alignment a very common additional view is the isometric view it shows the object in three dimensions which can do a lot to improve the clarity of a drawing [Music] assembly drawings sometimes contain just a single isometric View a variant on this is the exploded view which illustrates how the different parts of an

assembly fit together if a drawing includes small features that can't be properly shown in the primary views detailed views showing them at a larger scale are used foreign has internal geometry hidden lines can be shown on any of the views usually as dashed lines but another option is by turning a view into a sectional view which shows the object as if it's been sliced solid areas that have been cut through or drawn as hatched surfaces and The Cutting plane and viewing direction are defined on one of the other views foreign of course there's always more

than one way to lay out a drawing it's up to the author to use the different views in a way that best presents the important information drawings will often contain tables and notes that provide additional information assembly drawings for example usually include a bill of materials a table that lists the parts that make up the assembly and the required quantities foreign balloons are added to the drawing to identify the different parts in the table [Music] notes are used to specify any important additional information on assembly drawings this might be recommended torque ranges for bolts or

instructions to be followed during assembly if a note is shown inside a flag it means it refers to a specific part of the drawing on detailed drawings the notes could include material and coding information if these haven't been specified in the title block or information about how Parts should be marked for example foreign pieces of information on a detailed drawing is dimensions the drawing should provide all of the dimensions needed to manufacture the part which includes lengths the diameter of holes or the radii of fillets for example some features like this fillet are defined by

an arrow and a short string of text which is called a call out the same feature appears more than once in a view it's common to only Dimension at once the word typical is sometimes added to the callout to indicate that the feature appears several times although the best approach is usually to explicitly State how many times it appears each Dimension is normally only defined once although redundant Dimensions can sometimes be shown if it makes the drawing clear to indicate the total length of a part for example these are called auxiliary dimensions and are enclosed

in Brackets to make it clear that they're provided for information only foreign if you're preparing a drawing there are quite a few best practices to bear in mind when it comes to dimensioning here are a few examples Dimensions shouldn't really be placed inside a part they should always be on the outside hidden lines shouldn't normally be used for dimensioning if a specific detail can only be dimensioned from a hidden line you should probably be using a sectional view instead it's not usually necessary to Dimension 90 degree angles lines that look like they're at 90 degrees

can be assumed to be right angles it's a good idea to add center lines to Circular features Central lines reinforce the fact that the features are circular which may not be obvious from a single View and can be useful for dimensioning holes appear in so many drawings that it's worth discussing them in a bit more detail the call out for a plain hole is simple it needs to include two things the diameter of the hole and the depth of the hole if no depth is specified it's assumed that the hole goes all the way through

although often this will be stated explicitly if a hole is counter bored or countersunk this will be indicated in the callout by using the correct symbol and specifying the diameter and depth or the diameter and angle as appropriate things get a bit more complicated when it comes to threaded holes which are drawn as two concentric circles that represent the crest and root of the thread the call out has to Define all the information needed to create the thread and this is usually done by referring to one of several standardized thread types the two most common

are the iso standard for metric units and the unified thread standard for U.S customary units the call out for an iso thread starts with the letter M for metric followed by the thread nominal diameter in millimeters and then the letter X and the thread pitch the pitch is the distance between threads the ISO standards Define preferred combinations of diameter and Pitch which includes options for coarse and fine thread pitches the call out might also include information about the thread class of fit which essentially describes how tight the fit will be between the two mating parts

6h and 6G are standard fits with uppercase or lowercase letters being used to indicate whether the fit refers to the internal or external thread and that's all the threat information that needs to be specified on the drawing because everything else is defined in the ISO standards the call out for a unified thread follows a similar approach it starts with a nominal size which is the diameter in inches for sizes larger than a quarter inch or a number from 0 to 12 for smaller sizes the size is followed by a dash in the thread pitch which

is specified in Threads per inch instead of as a distance next is some text specifying the thread grade which is usually UNC for coarse pitch UNF or a fine pitch or unef for an extra fine pitch and finally there's the class of fifth which is a number from one to five that defines the closeness of the fit followed by the letter A for an external thread or B or an internal thread it's obvious that no part can be manufactured exactly to a set of specified dimensions if we take this drawing and make 10 of these

brackets they'll all have slightly different sizes there needs to be a way of defining what's an acceptable deviation from the nominal size and that's what tolerances are used for there are a few different ways of applying tolerances to a specific dimension there's the limit approach that states the upper and lower acceptable limits for the dimension or the plus and minus approach that states the acceptable deviations then there are general tolerances which are specified in the title block they apply to any Dimension that doesn't have an explicitly defined tolerance a drawing that contains Dimensions but no

tolerances is incomplete manufactured parts will be inspected and their Dimensions measured using tools like calipers or a coordinate measuring machine depending on the required accuracy and the measurements will be compared against the tolerances on the drawing to determine whether the part is acceptable the Golden Rule when it comes to tolerancing is to avoid specifying tolerances that are tighter than is absolutely necessary this chart shows typical tolerances that can be achieved for different manufacturing processes specifying very tight tolerances will limit the methods that can be used to fabricate the part and will result in it being

far more expensive to manufacture and to inspect the way a part has been dimensioned can also have a big impact on tolerancing and this should be taken into account when adding Dimensions to a drawing [Music] this part has been dimension in two different ways the method on the left is called chain dimensioning where dimensions are applied from one feature to the next and the method on the right is called datum dimensioning where the dimensions are applied from a chosen feature or Surface called a datum which in this case is the left surface of the part

because each Dimension has a tolerance chain dimensioning can result in an unwanted accumulation of tolerances datum tolerancing is often preferred because it avoids stacking up tolerances it can also make it easier to inspect the part since all measurements are made from the same surface of course there are scenarios where chain dimensioning might be the best approach for example when you want to control the relative distances between several holes the important thing is to think carefully about how your dimensions and their tolerances will affect the shape of the part the traditional approach to applying tolerances does

have limitations for one thing it only considers the size of an object it doesn't consider function or shape it doesn't allow you to properly control how flat a particular surface should be or how round the cross section of a cylinder should be for example geometric dimensioning and tolerancing is a different method for applying tolerances that allows you to do exactly that it complements traditional dimensional tolerancing by allowing you to control a range of different characteristics [Music] you'll know if a drawing has used gdnt because it will contain blocks called feature control frames that Define these

additional requirements this feature control frame controls the perpendicularity of this surface relative to datum a for example and this one controls the position of the center of the hole if a drawing has Dimensions enclosed in a box it means that normal tolerances don't apply to that feature because the position is controlled using gdnt instead g d and T is quite a complex topic that deserves its own video so I'll cover it separately on this channel engineering drawings as we know them have been around since the time of the Industrial Revolution and GD T came along

in the 1940s but the established drawing conventions and the way parts are designed are slowly changing two topics that may give us some idea of where the future of engineering drawing and design is headed are model based definition and statistical tolerance analysis these are particularly interesting topics I wanted to cover but they didn't quite fit into this video so I've created a companion video that you can watch right now over on nebula where I explore how model-based design works and where I cover the basics of Tolerance Stack Up analysis nebula was built by a group

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that's it for this look at engineering drawings thanks for watching