This page answers some common questions about drafting and CAD, as well as some administrative matters.
Some parts of an assignment must be done in pencil; other parts must be done in pen.
The rule is simple: all engineering drawings/sketches must be entirely in pencil; everything else must be in indelible ink.
Read the rules for cover pages.
It can be difficult to decide which view is the best front view for a given drawing. There are some guidelines, but sometimes - depending on the complexity of the part - they can conflict. Sometimes, there is no one “best” front view; in these cases, graders will take the ambiguity into account. The guidelines are:
Natural bottom. Many parts have a “natural bottom” face. Imagine holding the part, and placing it on a tabletop. You will likely choose the flattest and largest face as the bottom, to ensure the piece won't tip over. That's the “natural bottom” of the part. This also identifies which view will be the top view.
Most interesting geometry. The front view is the view that should have as much “interesting” geometry as possible facing forward. The more holes, edges, faces, chamfers, slots, lugs, flanges, etc are visible, the better.
Fewest hidden lines. While this may seem to be the same as most interesting geometry, it often isn't. For instance, because an automobile body is generally symmetrical left-to-right, but asymmetrical front-to-back, one may argue that the best front view is that of the side of the automobile (because it will have the fewest hidden lines.
Fewest hidden lines in other views. If the first three guidelines aren't enough, one can choose a view as the front view such that it will lead to the fewest hidden lines in the other (side & top) views.
Besides CAD, MEC222 also focuses on technical sketching.
A technical sketch is a carefully drawn freehand sketch. Sketching is covered in Chapter 2 of the textbook. It is expected that by the end of the semester, students will have learned to make freehand sketches of parts and assemblies.
You can use your drafting instruments to help you execute a sketch, but in only certain limited ways. For instance, if you need to draw a line between two points that are 5cm apart, you can use a ruler, straight-edge, and/or compasses to determine where those two points should be. However, you must learn to draw the actual line freehand. Likewise for circles and other curves: by the end of the semester, you should be drawing all these shapes freehand.
To encourage students to learn to draw freehand, marks will be deducted for lines and curves not drawn freehand. Deductions will follow a sliding scale. At the beginning of the semester, no marks will be deducted. However, as the semester progresses, grading will become more and more stringent.
It is easy to use quad-ruled paper to help you sketch. Use two paperclips to fix a sheet of blank paper onto your pad of quad paper. You will be able to see the quad paper's grid through the blank paper. You can use the grid to help you draw freehand. The grid size of typical quad paper is 1/4in or 1/2cm, so it's quite useful for estimating dimensions too.
Drawings on the final exam will be expected to be done freehand; grid paper will be provided.
There is a whole chapter (ch 2) on sketching in the textbook. Read it.
Also, Table 5.1 (page 232), the Alphabet of Lines, enumerates all the different kinds of lines we use in sketching, with examples. You should put a post-it on this page till you know it by heart.
A technical sketch or just sketch is done free-hand, but follows all the rules of engineering drawings as described in lecture and in the textbook. A drawing that violates the rules below is always unacceptable and will get a failing grade.
You may leave construction lines on your drawings so long as they are faint enough that they cannot be mistaken for anything else. This is rather like “showing your work” and can help the TA understand what you were trying to do.
See also other drafting notes for further information.
A CAD drawing is created via CAD software, and is a proper engineering drawing; that is, it is composed of views of object(s), has a proper border and title block, etc. as described in lecture and in the textbook.
There are some rules that must be followed for both technical sketches and CAD drawings. A drawing that violates these rules is always unacceptable and will get a failing grade.
A solid model or rendering is not a drawing; there will also be conventional homework questions. These can be treated as conventional homework questions in any other course.
Use indelible ink for all non-drawing homework.
In mechanical/industrial drafting there are really only two units used: millimetres if the drawing is in SI and decimal inches (e.g. 1.375) if the drawing is in American units. Never mix the two.
Here are three rules to help you determine what units to use. Check each one in the order given here.
A scale is a ratio that is used to shrink or expand the size of the image you draw so that it fits on the page.
Scales are always written as ratios of integers, where the left number represents a unit on the drawing and the right number represents a unit in real life. So a scale of
1:2 means one unit on the drawing is two units in real life - so-called “half-scale.”
Scaling a drawing by 1:2 does not mean you also divide dimensions by 1/2. Dimensions are always indicated at 1:1 scale. For instance, if you have to draw a 10cm line at a scale of 1:2, you draw a line 5cm long, but you dimension it as 10.
There are no units of measurement in scales. One must never write something like
1 in : 1 cm. Units of measurement are completely different from scales.
If the scale is 1:1, it can be omitted. Conversely, if no scale is given on one of your drawings, we will assume you intended 1:1.
Historically, only a few scales have been used: 1:1, 1:2, 1:5, 1:10, and their inverses. However, because we typically draw on letter-sized paper in this course, scales of 1:3 and 1:4 are also acceptable.
To select a good scale for your drawing:
All parts on one sheet must be to same scale.
Different scales can be used on different sheets.
Refer to Chapter 10 of the textbook for detailed information.
A working set of drawings applies to assemblies of parts. They consist of:
Here are some further guidelines about each of these elements.
Assembly Drawing. An assembly drawing is used by someone who has to assemble the product.
Bill of Materials. The Bill of Materials (BOM) lists in order of part number (see Assembly Drawing, above) every part needed to make the product.
Part Drawings. These are drawings of the individual parts needed to make the product described in the Assembly Drawing.
In some cases, the assignment will clearly state how many - and what kind of - views you must draw.
If this is not given, then draw the fewest principal views needed to show the entire geometry. If you are drawing a sphere, you only need one view. If you are drawing a flat plate, you only need one view, and specify “constant thickness” with a dimension in a note on the drawing. If two views are enough to show all sides of the geometry, then only draw two views.
If the “interesting” geometry of a part lies on an oblique plane in all principal views, then you need to provide an auxiliary view of the oblique features.
If the part has “interesting” geometry that is hidden or “inside” the part, then you need to add a section view.
Remember, a drawing may consist of multiple sheets. If you need five views in a drawing, you can use more than one sheet to show them.
It is up to each student to learn and decide how many, and what type of, views to draw.
Refer to this Google Doc.
U.O.S. = Unless Otherwise Stated.
This abbreviation is often seen on drawings after default values for common dimensions.
All fillets 0.25 R U.O.S.
Threads (on bolts, nuts, screws, etc.) are almost always omitted from sketches in practice. You do not need to draw threads in your sketches.
The same is true of CAD drawings. Unless you are actually drawing a bolt or other threaded item for the sake of its manufacture, you need not draw threads on bolts in CAD drawings.
If you are sketching, it is useful to place section views and auxiliary views in locations that facilitate drawing them. A section view usually goes near the view upon which it is based, and an auxiliary view is usually aligned with the projection lines from which the auxiliary view is based. Refer to the textbook for examples.
If you are using CAD, you can place the views in any reasonable location.
See section 5.4.2, page 266 of the sixth edition of the textbook. We use third angle projections in Canada.