Frank Wilson
Senior Editor
Blueprints and production drawings are an essential part of any production process, whether milling, turning, or welding. Traditionally, this has been the work of a draftsman using specialized tools and a drawing board or drafting machine. These days, most drawings or blueprints are produced using CAD/CAM (computer-aided design/computer-aided manufacturing) software.
A professional or prospective welder must understand the welding symbols and terms used in production drawings to efficiently perform their job.
What is a Blueprint Drawing?
Blueprints and whiteprints are production or working drawings that skilled tradespeople use in a variety of manufacturing disciplines. The purpose of these drawings is to convey a set of instructions as diagrams, written instructions, symbols, and other visual information to tell a worker how to manufacture a part, assembly, or product.
These drawings may take the form of isometric, 3D, and other depictions to help the welder understand the workpiece’s position and orientation. These drawings may also include bills of materials and assembly instructions for components.
The term “blueprint” is more commonly associated with architectural drawings and the building trades. However, as welding is often an integral part of construction projects, there is some overlap.
You'll learn most of this in welding schools and colleges where you can get certification for this job.
Tolerances
One of the first things to know about how to read production drawings is tolerance. A tolerance is an acceptable degree of deviation from a specified dimension. You’ll often see + or - (plus or minus) used following a dimension.
Sometimes the tolerance is strictly plus or strictly minus, showing that it’s only acceptable for the deviation to be in one direction.
Tolerances are used to reflect that neither humans nor machines are 100% uniform, and there must be a margin for error. Engineers account for this when designing parts or assemblies to be made.
When welding, an industry-standard is called the 1/16” rule. This a tolerance, indicating that a weld should be 1/16” smaller or larger than the dimension specified in the drawing. This allows a certain level of precision to be maintained.
Welding Procedure Specification (WPS)
When beginning a new welding operation, it’s always a good idea to consult the Welding Procedure Specification (WPS). The WPS is a document that lays out the information necessary for the creation of a weld. This allows welders to achieve repeatable welds in compliance with best practices and code requirements.
Difference Between Weld and Welding Symbols
As with machining and other manufacturing processes, welding has its own set of special symbols to convey different instructions regarding welding processes, angles, material thicknesses, types of welds, and more. When learning how to read welding blueprints, it’s important to understand the difference between a weld symbol and a welding symbol.
A weld symbol denotes the type of weld that the welder should apply to the workpiece. Welding symbols are composed of a reference line, arrow, and a weld symbol (if needed).
Reference Line
The horizontal line, called the reference line, acts as an anchor point or datum. This is the point to which other welding symbols are connected. An arrow connects the reference to the joint that you need to create on the workpiece.
Don’t confuse the reference line with leader lines that you may have seen on other production drawings designed to connect written text or a dimension to a picture or diagram.
Welding Symbol Identification
Connected to the reference line is an arrow that points upward or downward, depending on where you need to place the weld. The tail of the reference line may point to additional information regarding the welding process.
Attached to the reference line may be a geometric shape or parallel lines, showing what kind of weld you need to perform. If the shape is below the reference line, it indicates that you should apply the weld on the join corresponding to the arrow side. If it’s below the reference line, it shows that you need to apply the weld to the other side.
Some of the most common symbols include:
Fillet
A fillet weld, where you join two metal pieces that are perpendicular to a tee joint, is indicated by a fin-like shape on the reference line.
Plug or Slot
Plug and slot welds are indicated by a square or rectangular shape on the reference line. These welds involve joining overlapping parts, at least one of which has a round or elongated hole.
Spot or Projection
A circle on the reference line indicates spot and projection welds. Both types of welding involve using electrodes to join sheet metal pieces, but you can use projection electrodes to weld thicker pieces.
Stud
Stud welds, which you apply to join a fastener to a metal part, are represented by a circle on the reference line with an “X” inside it.
Seam
Seam welds are indicated by a circle on the reference line with two parallel lines running through it on the horizontal plane. A seam weld is a continuous weld that you apply to join two metal pieces.
Back or Backing
Back and backing welds are indicated by a half-circle on the reference line. You apply a back weld at the back of a single groove weld, whereas you apply a backing weld before a groove weld to act as a support.
Surfacing
Surfacing welds are indicated by two half circles on the reference line. The purpose of a surfacing weld is not to join metal parts but to add a hard metal layer to a part. You may apply a surfacing weld for several reasons, such as to increase a dimension or reinforce the material.
Edge
Two vertical rectangular shapes on the reference line indicate edge welds. An edge weld involves joining two adjacent pieces at their respective edges.
When learning how to read welding blueprints, it’s worth taking the time to familiarize yourself with the wide array of symbols and the processes that they correspond to.
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Final Thoughts
Knowing how to read blueprints and other manufacturing drawings can help you avoid errors and effectively accomplish your work. As welding can require complex processes and methods, you must interpret the symbols and terms correctly.
