The horizontal stick figure you see below is featured in three variations on the theme:
First, the symbol has an arrow, which points to the location of the weld. Then there's a horizontal line known as the reference line, and a tail at one end that forks off in two directions. The arrow includes what's called a leader line, which connects it to the reference line. The forked end of the horizontal line is where instructions are written and is optional.
The geometric shape you see at the center of a reference lines is known as the weld symbol, indicating the type of weld to be performed. The weld symbols you see in the drawings above represent a square, fillet and V-groove weld, respectively. The weld symbol may be placed above the reference line, or below it (as shown).
This location is important. When the weld symbol sits below the reference line, it means the weld is performed on the "arrow side" of the joint. In the next drawing, a fillet weld is specified on the side of the joint nearest to the arrow.
If the weld symbol appears on top of the reference line, then the weld should be made on the opposite side of the joint from where the arrow points. Here's an example:
And if the shape appears on both sides of the reference line, as shown below, it specifies a weld for both sides of the joint.
Numerous weld symbols have been devised to represent all the different joint or weld types used in the trade. Here are the most common ones:
If you're not familiar with welds and joints, be sure to check out those topics in the green menu box located on the top right side of this page. And since it takes time to memorize and interpret all these symbols, it's a good idea to print out or photocopy a chart with both the symbols and drawings of the completed welds. For a sample chart, click here.
Numbered dimensions for a weld are indicated on one or both sides of the weld symbol (which is why the reference line is long). For instance:
In most cases, the weld width is located left of the weld symbol. (As explained in Anatomy of a Weld, the width is measured using one or both legs of a weld.) The length of the weld is written on the right side. Often, no length is indicated, which means the weld should be laid down from the beginning to the end of the joint, or where there's an abrupt change in the joint.
Naturally, any dimensions provided below the reference line apply to the joint on the arrow side, while dimensions above the reference line apply to the joint on the other side.
Sometimes, a series of separate welds are indicated, rather than a single long weld. This is common when thin or heat-sensitive metals are welded on, or where the joint is a really long one. In the following symbol and drawing, 3-inch intermittent fillet welds are specified. The "pitch" indicates that these welds should be centered at 5-inch intervals.
Notice that the fillet shapes on either side of the reference line are offset, rather than mirroring each other. This means the welds should be located at staggered spots on either side of the joint, as depicted in the drawing.
A weld symbol may sometimes specify an angle, root opening or root face dimension. This is common for metal components thicker than 1/4 inch. The following example is a symbol and drawing of a V-groove joint:
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Here, the groove weld has dimensions written inside the symbol. The first is 1/8 inch, which pertains to the root opening. The larger number below it signifies 45 degrees, which represents the included angle between the plates. "Included" means the sum of the angles beveled on each side. In other words, the bevel made on each plate is 22 1/2 inches.
Moving to another part of the overall welding symbol, at the intersection of the reference line and the leader line (which is attached to the arrow), two common instructions may be provided:
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A flagpole indicates a field weld, which simply tells the welder to perform the work on site, rather than in the shop. The weld-all-around circle, located at the same juncture, means just that. While this symbol is often used in pipe and tubing, a non-circular structural component (as shown above right) may likewise need welding on all sides.
Here are a few other types of instructions you might see on a drawing:
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A curve located above the weld symbol's face specifies that the finished weld should be either flat, convex or concave. (If you see a straight line, then it's a flat - i.e. flush face.) As shown on the top right, a V-groove weld symbol with a box above it indicates a backing strip or bar is required for this joint. The strip or bar must be welded onto the back side of the joint before the groove weld is performed.
A backing strip or bar is sometimes confused with a "back weld " or a "backing weld". They are not the same thing as using a backing strip. A back weld is where a second weld is created on the back side of the joint after the primary groove weld is completed. Conversely, a backing weld is a weld that the welder performs first (so it serves the same function as a backing strip). The symbols for each of these three options are illustrated below using the tail of the welding symbol:
As you can see, the only difference between the back and backing welds is when they're performed. The symbols look the same, so both must be specified by name. In the third symbol, the dimensions and type of steel (A-38) for the backing strip are specified.
When a welding operation involves a lot of steps, you will sometimes see multiple reference lines on the welding symbol, as shown below.
To keep the instructions clear, several reference lines may extend from the leader line at a parallel trajectory. Each line represents a separate operation and is performed in order, beginning with the line closest to the arrow.
The forked tail of the welding symbol is used to convey details that aren't part of the normal parameters specified on the reference line. For instance, the engineer or designer might want the welder to use stick welding (i.e. SMAW), or another welding process. Or there may be other information to convey:
Of course, when no special instructions are needed, the tail is ommitted from the welding symbol, leaving just the reference line, arrow and leader line.
Once you master the basics, you'll be ready to absorb the many other particulars conveyed on shop drawings and blueprints. Among the most common:
Below you'll find the standard chart that's used to communicate information with a welding symbol. You may need to refer to it when there are lots of dimensions listed or uncommon specifications to sort out.
To research welding symbols further, follow the links in the resource box on the upper right of this page. The Lincoln Foundation also publishes a book, "How to Read Shop Drawings", which costs $10 if you purchase it from their website. (See the link above right.)
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Tutorial: Welding Symbols TheRangerStation.com
Diagrams and interpretations of symbols
Welding Symbol Chart (PDF) AWS
Welding Symbol Reference Guide (110 pages) (PDF)
Blueprint Reading for Welders Google book
How to Read Shop Drawings Lincoln Electric Foundation
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Welder Qualification Test Via Advice section