1.1.6: Dimensioning and Tolerancing
- Page ID
- 125138
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The important thing to remember is that dimensioning and tolerancing must clearly define design intent. The ASME Y14.5 standard governs geometric dimensioning and tolerancing practices and covers a whole class’s worth of material, as you may know. We will review the more nuanced topics in the standard that aid in drafting a technical drawing that is clear and easy to read. Let us start by comparing a poorly dimensioned drawing, and a properly dimensioned drawing. (ASME, 2019)
Here are common mistakes to look out for on your drawings and the drawings you peer review.
Dimension Rules
Careful placement of dimensions on a drawing will greatly enhance the readability. By grouping dimensions of a common feature, staggering dimensions that are too close together, and including a visible gap between the object and dimension’s extension line, you can improve drawing clarity.
Always dimension a feature where you can see the outline the best. Additionally, by grouping dimensions of a feature together in one drawing view it reduces the time a print reader must search for the dimension they are looking for.
Staggering the dimension text will enhance the readability of a technical drawing.
A visible gap between object lines and extension lines enhances readability. To ensure a visible gap in your Inventor drawings, do the following: select the dimension by single clicking it, click and drag the end grip of the extension line without a visible gap, drag and release it on the outermost object vertex or line. Repeat on the other side, as necessary. You may have to do this after arranging the dimensions for better dimension grouping.
TIP: For simple dimensions without tolerance, it can sometimes be quicker to delete the dimension and replace it with careful selection of object lines or vertices to achieve the desired look. However, if there are tolerances or feature control frames attached to the dimension, it may be more efficient to drag the end grip of the extension lines to reduce the time spent recreating the dimension tolerance.
Leader Rules
There are rules you could follow when placing leaders to help make your drawing more readable. The first rule is that leaders “float up.” That is, they are easier to find if they are toward the top of the drawing, so inclining the leader above the horizontal is ideal. Sometimes other dimensions prevent leaders from being placed above the horizontal, and they must be placed below the horizontal, and that is okay if it improves the clarity of the drawing. The preference is that they are placed above the horizontal, but the ultimate goal is to produce a readable drawing.
Another rule with leaders is to avoid placing leaders too horizontally or too vertically. The best angle is 45 degrees, followed by 30 and 60 degrees, but certainly between 15 and 75 degrees. This avoids leader lines being mistaken for object lines and improves readability. The figure on the right display leader notes in the first quadrant, however the rule applies to leader notes placed in any quadrant.
Diameter and Radius Dimension Rules
The Leader Rules above also apply to diameter and radius dimensions. Additionally, diameter and radius dimensions should always point to the center point of the arc to which they are attached. If it is not, that dimension is likely an overtyped leader note. The downside there is that the leader note is not linked to the model parameter and will not update automatically.
Full circles such as holes and cylinders should be dimensioned as diameters, and partial circles should be dimensioned as a radius. This is the default behavior of Inventor, but occasionally you will want to change the dimension from one to the other. One example is below. In this example, the shape would be best interpreted as a full circle and therefore would be best dimensioned as a diameter. To change it: after the dimension is placed, press <esc> key to back out of the dimension command, if needed, then right click the dimension you would like to change, hover over Dimension Type, and choose the desired option.
In the same way we dimension a feature where its outline is shown best, holes and cylinders should be dimensioned following the rules in the ASME standard. Cylinders should be dimensioned from the side, and holes should be dimensioned where they appear as a true circle. Whether it is a cylinder or a hole, they should always be located in the circle view with extension lines extending from the center point.
Threaded Holes
The example below shows a 1/4-20 UNC – 2B ↧ .500 hole callout. Let us examine how to read a thread note like this.
| Thread Note Measurement | Explanation |
|---|---|
| 1/4 | .250-inch major diameter. A 1/4” diameter bolt would thread into this hole. |
| 20 UNC | 20 threads-per-inch, in the Unified National Course thread form. |
| 2B | Thread tolerance. This is optional and if omitted 2B is assumed. |
| ↧ .500 | Thread depth. This is the depth of the fully formed threads. |
Threaded Hole Manufacturing Process
It is important to understand how holes are manufactured. Below we see a threaded hole with a ¼-20 UNC thread profile cut to a depth of .500 inches.
- Blind Hole: A blind hole is a hole that does not go through the other side of the material. When a threaded hole must be made, it starts with a plain hole 1X diameter deeper than the threaded hole finished depth.
- Threaded Hole: A cutting tool called a “tap” is used to cut the threads for a threaded hole. The end of the tap is tapered to make it easier to start the cutting tool, resulting in the first four threads to not be fully formed threads. This is why the blind hole was cut deeper.
- Chamfer/Countersink: If desired, a chamfer or countersink is cut to facilitate easy assembly. The diameter shown for the countersink is the diameter measured at the surface of the part.
Dashed lines are used to represent threads in both section view and hidden line views.
Clearance Holes
Selecting dimensions for clearance holes has never been easier with the convenience of moder CAD systems like Autodesk Inventor. Clearance hole dimensions have long been standardized but they have not always been built into Inventor’s Hole tool. Before then, designers were required to dig out their copy of Machinery’s Handbook and look up clearance hole sizes out of the book. (Oberg, 2016)
- Countersink: This is a clearance hole for a machine screw, which has a conical head profile to match the angle of the countersink. Note that the countersink diameter is the diameter measured at the surface of the part.
- Counterbore: This is a clearance hole for a cap head screw. Holes (E), (F), and (G) are all counterbored clearance holes for the same 1/4-inch cap head screw, but different fit. This is a normal fit.
- Same as (E), but a close fit.
- Same as (E), but a loose fit.
Assigning Fit in the CAD Model
In the modeling environment you can choose the type of fit of your clearance hole by changing the Fit in the Hole tool.
