Engineering Drawing Dimensions
Su Mcdowall
The alphabet of lines is a set of standard line types established by the American National Standards Institute (ANSI) for technical drawing. The alphabet of lines and the approximate dimensions used to create different line types, are referred to as linestyles when used with CAD.
Each line should have one of the two thicknesses, thick or thin, and the ratio of the thicknesses must be not less than 2:1. In many textbooks (especially, of the US authors) you will come across the thicknesses of lines: 0.3 mm and 0.6 mm. In general cases it is good enough in practical work.
In cases where other than shown types of lines are used for special drawings (for example, electrical drawings, pipeline diagrams, or building drawings) the conventions adopted must be clearly indicated by reference to specific standards or by notes on the drawings.
Note that all these break lines were designed for preparing drawings in the old-fashioned manner by using a draft board. Sometimes engineers and designers still work on drawings with pencils and rulers. However, modern 3D CAD software, which prevails in our days, can offer other line types for the break lines in the computer-based drawings.
Extension lines should not cross dimension lines, and they should avoid crossing other extension lines whenever possible. When extension lines cross object lines or other extension lines, they should not be broken. When extension lines cross or are close to arrowheads, they should be broken for the arrowhead.
(a) by the angle; (b) as a ratio combined with the slope symbol; (c) by the dimensions showing the difference in the heights of two points from the base line and the distance between them; (d) by the slope symbol, the base line length and the slope height.
Normally, gear drawings include a table of information, called cutting data, for manufacturing. A detail drawing of a gear would also include other dimensions not found in the table (root diameter, bore diameter, keyway dimensions).
Projections and dimension lines should be drawn as thin continuous lines. Projection lines should extend slightly beyond the respective dimension line. Projection lines should be drawn perpendicular to the feature being dimensioned. If the space for dimensioning is insufficient, the arrowheads may be reversed and the adjacent arrowheads may be replaced by a dot. However, they may be drawn obliquely, but parallel to each other in special cases, such as on tapered features.
A leader line is a line referring to a feature (object, outline, and dimension). Termination and Origin Indication Dimension lines should show distinct termination in the form of arrow heads or oblique strokes or where applicable an origin indication Leader lines should be inclined to the horizontal at an angle greater than 30. The leader line should terminate
When a radius is dimensioned only one arrowhead, with its point on the arc end of the dimension line should be used. The arrowhead termination may be either on the inside or outside of the feature outline, depending on the size of the feature.
The indications that are used with dimensions to show applicable shape identification and to improve drawing interpretation: The diameter and square symbols may be omitted where the shape is indicated. The applicable indication (symbol) shall precede the value for dimension.
The arrangement of dimensions on a drawing must indicate the purpose of the design of the object. They are arranged in three ways. 1. Chain dimensioning 2. Parallel dimensioning 3. Combined dimensioning.
As with many questions, the real answer is "it depends." The biggest variables are what kind of engineer you are, who your audience is, and what industry you're working in. In a class setting, it is probably a good idea to consult your textbook or your teacher for their opinions in the context of the class, but here are some general factors in making the decision when you don't have someone to ask. In industry, if I know the department or company that is going to do the actual work, I do usually ask them what information they need and try to tailor the drawing to them. More formally many companies will have internal policies or follow consensus standards like ASME Y145 or the ISO ICS 01.100 series. But often, you won't have that luxury. Maybe you are drawing something to go out to competitive bid, or for a project that has no manufacturing pipeline set up yet. In this case you have to make decisions based on the context. A number of other answers here provide good guidelines for relatively conventional machined parts, but don't necessarily address other types of drawings you may need to produce.
Dimension critical features: These are the items that must be right for your object to serve its purpose. For example if you are drawing a sprocket the center bore must be correct, or it will not fit on the shaft. For mechanical drawings, you should pay particular attention to the tolerances on these dimensions and make sure they are very clear. Make sure you are dimensioning what really matters to you, even if this isn't the most convenient way to dimension it for your audience. In architecture and some other disciplines, you can also add "HOLD TO" to the dimension, to indicate that other dimensions may have to be adjusted slightly to make this dimension correct.
Dimension all functional requirements: These are all things that matter to your design, but are not as essential as your critical features. You should make sure that they are dimensioned and clear, but you don't have to draw extra attention to them like you do with a critical dimension. For example on the sprocket, the thickness of the sprocket needs to be machined correctly for it to fit, but it won't be a very tight tolerance.
Dimension things your audience needs to know: In many designs there are some aspects that are arbitrary. Maybe in your situation the thickness of the sprocket hub doesn't matter at all - it's under very little load and going in the middle of a big empty shaft for example. All the same, you should dimension it so that the person making it doesn't have to make the choice for you. If you want to allow them to make some economical decisions (eg start with a material size they have in stock) you could chose to put a very wide tolerance on the dimension. If it impacts what the next person has to do, you should provide some information.
Note the this section can be a little different for different industries. Machinists for example are used to having a part fully defined on a drawing so there's no guesswork for them to do. On the other hand if you are drafting a stud wall for a carpenter to frame out in a building, you would typically show them the minimum stud spacing, any special conditions (doorways, windows, etc.) but not every other detail. For example, it typically wouldn't matter to the engineer which side they started the studs from and where they ended as long as there is no gap greater than you specified. It might be easier for them to start from the north side because that's where the lumber is, or easier to start from the south side because the adjacent wall has already been framed. They might get clever and start the pattern centered on the wall to save one stud's worth of material. As long as the drawing contains all of the dimensions that matter to you, it's OK to leave it under-defined in this situation. For this step, it is really helpful to understand the process that someone else will be using to make your part whether it's machined, welded, carved, molded, etc.
Dimension things that other audiences would like to know (optional:) Often there is useful information you could provide, even though it doesn't matter to the person you're sending the drawing to. In the sprocket example, the machinist won't care what the pitch diameter is if you've already given them the profile to cut. All the same, you might decide to add it. This information will be useful if another mechanical designer wants to use your sprocket and comes across your drawing first. Generally since these dimensions are just for information, not an instruction to the next person, they will be indicated as reference dimensions. That should be done by placing them in parenthesis or adding the abbreviation 'REF' after the word.
Once all of these dimensions are listed, and the appropriate tolerances, materials, processes, etc are specified, your drawing should be complete. Now it's possible that if your object is complicated, the drawing will be cluttered with lots of dimensions that may seem redundant. There are a few options to reduce the total number of dimensions but retain the important information. This list is certainly not exhaustive, but it should get you started in most applications. Note that some of these make more work for your audience, so they may not always be received warmly.
Lastly, there are a number of good textbooks on drafting that will go into much more detail with better examples and style guidance. One that I would recommend is Technical Drawing with Engineering Graphics (Giesecke et al.)
I am not a professional engineer yet, but in my engineering design courses we were taught to show certain dimensions. For example, you choose a datum line and show certain dimensions. You don't need to label every single dimension, but you should show your dimensions in a way such that the dimensions which are NOT shown can be calculated arithmetically.
For example, if you have a single line, with 4 points starting at A and ending at D with B and C somewhere along the line, and A is your datum, then you could just show the dimensions for A-D, A-B and A-C.Simple arithmetic will then allow you to calculate the distance from B-C, for example (by taking A-C minus A-B). I believe this is done so that the person looking at your drawings doesn't* need to get a ruler out to check the dimension!
The way I do it is to show bounding dimensions for the plate, dotted lines to indicate vertical and horizontal centerlines (no dimensions there as it's understood they're half the bounding dimensions), then just show the distance from each centerline to one hole. For the hole, have a callout that gives the hole diameter and tolerance and/or threading requirements. Again, if all four holes are the same, then just use, "4x 6mm Thru All" or similar.
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