Linear Dimensions

Prints are used to convey all necessary information in the manufacture or fabrication of a product. Views provide an illustration of the object, and dimensions provide the units of measure to which the finished product must conform.

Linear dimensions are by far the most common dimensions on prints. They provide a dimension from one reference point to another. The dimension can be provided in whatever unit or convention of measurement being used. Metric dimensions are most often given in millimeters, in the case of U.S. measurements it may be in fractional inches (½”, 1 ½” and so on) or decimal inches (0.5”, 1.5” and so on). Depending on the industry a measurement may simply be given in inches (50”) or a feet-inch measurement (4’ 2”).

When linear dimensions are expressed on a drawing they will make use of extension and dimension lines. The extension lines extend from the reference points on the assembly where the measurement is pulled, a dimension line terminated in arrows will carry the numbers of the measurement. The extension lines may reference the edge of an object or feature, but it is not uncommon for dimensions to be pulled from the center of an object so the dimension line may run from a centerline to an extension line.

Linear dimensions, and all dimensions, will serve one of two purposes, both of which are necessary for fabrication of the product. Size dimensions are linear dimensions which give the required size of the material, while location dimensions call out the location of specific features in relation to some other reference point.

There are two methods for giving linear dimensions. Conventional dimensioning is what we have shown in the images of this section thus far. In this method two extension and dimension lines are used. Each dimension pulls from one reference point and goes to another. In a method called baseline dimensioning many linear dimensions will pull from a single baseline reference or zero point. If this method of dimensioning is used it can be recognized by the lack of dimension lines, baseline dimensions will be carried on an extension line. Baseline dimensions are sometimes called running dimensions, the zero point for these dimensions is indicated by a 0 or the letters RD for running dimension, in some cases the symbol in the Figure 16.12 will be used to indicate the zero point.

Angular Dimensions

Angular measurements are only necessary when two surfaces lie at an odd angle to one another. If an angular measurement is not present it should be assumed that the edges lie at 90o (square) or 180o (parallel) to one another. It is most common for angles to be given in degrees, even down to tenths (0.1) or hundredths (0.01) depending on the accuracy required.

Arcs, Diameters, and Radii

Radius and diameter are two sides of the same coin. Or, more accurately, a radius is half a coin, while a diameter is the whole thing. A radius measures from the center of the coin, or any circular feature, to the outside. A diameter is a measurement all the way across a circular feature, so is twice the radius.

A diameter is usually given only when the circular feature is a full 360o. If the feature is less than a full circle a radius is used instead. These measurements will be called out on curved edges or surfaces with a leader line followed by a measurement. Radius measurements are denoted with the letter R, while diameter may sometimes be indicated by the letter D but more commonly by the symbol pictured in Figure 16.5.

Depending on the complexity of the part being detailed, it may become necessary to provide more than just a radius. Below are three examples, each more complex than the last, which show different ways a radius might be detailed.

Let’s explore why each example is dimensioned just so:

• The left example is the simplest one, only a radius of 1 is dimensioned. In this case it is safe to assume that the radius is a full quarter (90o). If a radius is all that is provided the part is a full 90o or 180o.
• In the middle example we see that not only a radius but an angular dimension are needed when the radius is not in an increment of 90o
• In the right example, it is shown that linear dimensions may also be needed to locate the center of the radius. This is only necessary when the end or beginning of the radiused part or feature does not lie in line with the center of the circle.

The center of a radius is also denoted with a special mark called a center mark. The center mark looks like a plus sign (+) and can be seen in the right most example above. A center mark may have centerlines extending out from it to aid in dimensioning.

Hole Dimensions and Specifications

Holes are put into material in order to accept bolts, screws or other fasteners. Holes are also used for plug welds (see Chapter 15 for more information). The simplest type of hole is a through hole, often abbreviated “thru”. Through holes go all the way through the thickness of the material, the dimensions are called out with a leader line and diameter symbol shown in Figure 16.15. If the hole is viewed in a cross section a linear dimension can be used.

When a hole does not extend all the way through the material it is called a blind hole. With a blind hole it is necessary to call out not only the diameter but also the depth of the hole. Depth can be called out on a leader line and may be indicated by the word “DEEP”, the abbreviation “DP” or a depth symbol, shown below, can be used.

When a hole is made to accept a bolt or other fastener it may be necessary to prepare the opening of the hole in one of several ways. Below are cross sectional views of these preparations and explanations of their use:

• Countersunk holes are used to accept fasteners with countersunk heads. The angle of the countersink in the hole must match the angle on the countersunk head of the fastener.
• Special countersink cutting tools are used for this and are manufactured at angles of 60o, 82o, 90o, 100o, 110o, and 120o.
• When a counter sink is called for the diameter of the hole, angle of countersink, and diameter of the countersink at the opening must be given. In the Figure 16.20 the diameter of the hole is called out with a leader line, then the major diameter and angle of the countersink, followed by the symbol shown below. Rather than the symbol countersink may also be called out with the abbreviation “CSK”.

Counterboring is the practice of cutting a larger diameter, flat-bottomed, hole in line with a smaller diameter hole. This is done to accommodate flat headed fasteners, like socket cap screws. The counterbore is drilled deep enough so when the fastener is inserted in the hole the head sits flush with, or below, the surface of the material.

Special cutting tools are used for counterboring, they are made in a variety of diameters and have a pilot pin that ensures the counterbore is centered on the smaller hole.

When a counterbore is required the diameter and depth of the counterbore must be called out. In Figure 16.23 Counterbored Hole we see the diameter of the through hole called out, followed on the next line by the diameter and depth of the counterbore. The symbol shown below indicates counterbore, it may alternatively be abbreviated “CBORE”.

The shape of a spotface is essentially the same as a counterbore. The difference is that only the diameter, not the depth, of the spotface is shown. This is because a spotface is only used to give a flat, clean mating surface for the head of a fastener, so the depth is not important. Holes may be spotfaced in materials with rough or uneven surfaces, or on rounded surfaces.

The symbol indicating spot face is the same one used for counterbores, it may also be abbreviated “SF”

Attributions

1. Figure 16.10: Dimensions & Extra Lines by Cameron Kjeldgaard, for WA Open ProfTech, © SBCTC, CC BY 4.0
2. Figure 16.11: Size & Location Dimensions by Cameron Kjeldgaard, for WA Open ProfTech, © SBCTC, CC BY 4.0
3. Figure 16.12: Common Running Dimension or Baseline Dimension Symbol by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
4. Figure 16.13: Dimensional Methods by Cameron Kjeldgaard, for WA Open ProfTech, © SBCTC, CC BY 4.0
5. Figure 16.14: Angular Dimension by Cameron Kjeldgaard, for WA Open ProfTech, © SBCTC, CC BY 4.0
6. Figure 16.15: Diameter Symbol by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
7. Figure 16.16: Radius Dimensions by Cameron Kjeldgaard, for WA Open ProfTech, © SBCTC, CC BY 4.0
8. Figure 16.17: Through Hole by Cameron Kjeldgaard, for WA Open ProfTech, © SBCTC, CC BY 4.0
9. Figure 16.18: Symbol Denoting Depth Of Drilling In Blind Holes by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
10. Figure 16.19: Blind Hole Dimensions by Cameron Kjeldgaard, for WA Open ProfTech, © SBCTC, CC BY 4.0
11. Figure 16.20: Symbol For Countersunk Holes by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
12. Figure 16.21: Countersunk Hole by Cameron Kjeldgaard, for WA Open ProfTech, © SBCTC, CC BY 4.0
13. Figure 16.22: Symbol For Counterbored & Spotfaced Holes by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
14. Figure 16.23: Counterbored Hole by Cameron Kjeldgaard, for WA Open ProfTech, © SBCTC, CC BY 4.0
15. Figure 16.24: Spotfaced Hole by Cameron Kjeldgaard, for WA Open ProfTech, © SBCTC, CC BY 4.0