Reading welding symbols on blueprints can feel overwhelming at first. I’ve spent 15 years in fabrication shops watching new welders struggle to interpret these essential markings.
Resistance spot and seam weld symbols are among the most common symbols you’ll encounter. These compact symbols convey complete welding specifications in a standardized format.
What Are Resistance Spot and Seam Weld Symbols?
Resistance spot weld symbols use a circle placed above, below, or centered on the reference line. Resistance seam weld symbols use a circle with two parallel lines running through it. Both symbols indicate resistance welding processes defined by the AWS A2.4 standard.
- Spot Weld: Single circle symbol (diameter specified)
- Seam Weld: Circle with two parallel lines (width specified)
- Standard: AWS A2.4: Standard Symbols for Welding
These symbols tell welders exactly where and how to make resistance welds without written instructions. The American Welding Society (AWS) A2.4 standard establishes these symbols for consistent communication across the fabrication industry.
In automotive manufacturing alone, resistance spot welds account for over 4,000 welds per vehicle. Understanding these symbols is essential for anyone working with sheet metal fabrication or assembly drawings.
Resistance Spot Weld Symbol Explained
Quick Summary: The spot weld symbol consists of a circle that can appear above, below, or centered on the reference line. Dimensions including weld diameter and shear strength are placed to the left of the symbol.
The resistance spot weld (RSW) symbol is one of the simplest welding symbols to identify. It appears as a single circle on the reference line of the welding symbol.
Symbol Components
| Component | Description |
|---|---|
| Circle | Indicates spot weld type |
| Reference Line | Horizontal line showing weld location |
| Arrow | Points to weld joint |
| Dimensions | Size, strength, pitch, and spacing |
Dimension Placement for Spot Welds
Spot weld dimensions follow a specific placement rule according to AWS A2.4. Understanding where each number goes prevents costly interpretation errors.
Spot Weld Dimension Placement
- Left of symbol: Weld diameter (in inches or millimeters)
- Right of symbol: Pitch (center-to-center spacing)
- Above symbol: Shear strength in pounds or newtons
- Below symbol: Number of spots (if grouped)
Example Spot Weld Symbol
A spot weld symbol showing “5” to the left and “50” to the right indicates:
- Weld diameter: 5mm
- Pitch: 50mm center-to-center spacing
I’ve seen fabricators misinterpret pitch as weld count. This mistake leads to incorrect spacing and rejected parts. Always remember: left equals size, right equals spacing.
Spot Weld Strength Specifications
When shear strength appears above the spot weld symbol, it indicates the minimum load the weld must withstand. For example, “500” above the symbol means 500 pounds minimum shear strength.
Automotive body specifications typically require shear strengths between 500-2,000 pounds per spot weld depending on the application. Structural components require higher strength specifications than cosmetic panels.
Resistance Seam Weld Symbol Explained
Quick Summary: The seam weld symbol features a circle with two parallel horizontal lines. It indicates a continuous resistance weld along a seam, with dimensions specifying weld width and length.
Resistance seam welding (RSEW) creates a continuous watertight joint. The symbol distinguishes itself from spot welding by adding two parallel lines through the circle.
Symbol Identification
The seam weld symbol looks like a spot weld symbol with a horizontal line through the center. This visual distinction makes it easy to identify on drawings.
Visual Memory Aid: Think of the two parallel lines as the continuous seam being welded. Unlike a spot (single point), a seam extends across the joint.
Dimension Specifications for Seam Welds
Seam weld dimensions follow the same left/right placement rules as spot welds, but specify different measurements.
| Dimension | Placement | Meaning |
|---|---|---|
| Weld Width | Left of symbol | Width of the seam weld |
| Weld Length | Right of symbol | Length of weld seam (if not continuous) |
| Pitch | Right of symbol | Spacing between intermittent seams |
| Strength | Above symbol | Shear strength per inch of weld |
Seam Weld Example
A seam weld symbol with “8” on the left and “100” on the right indicates:
- Weld width: 8mm
- Weld length: 100mm (or 100mm spacing for intermittent welds)
Seam welds commonly appear in fuel tanks, fluid containers, and anywhere requiring a liquid-tight joint. The continuous nature of the weld makes it ideal for sealing applications.
Intermittent Seam Welds
Not all seam welds run continuously. When specifications call for intermittent seams, the right-side dimension indicates spacing rather than total length.
For example, a length dimension of “25-100” means 25mm of weld followed by 75mm of unwelded space, repeating this pattern along the seam.
Reference Line Positioning Guide
The position of the spot or seam weld symbol relative to the reference line indicates which side of the joint receives the weld.
| Symbol Position | Weld Location |
|---|---|
| Below reference line | Arrow side (side arrow points to) |
| Above reference line | Other side (opposite arrow) |
| Centered on line | No significance (weld can be on either side) |
| Both above and below | Both sides (double weld) |
Arrow Side vs. Other Side: The arrow side is the joint surface the arrow points toward. The other side is the opposite surface of the joint. This distinction matters when materials differ or when access limitations exist.
Resistance welding symbols centered on the reference line have no arrow side significance. This means the welder can perform the weld from whichever side provides better access or equipment positioning.
Spot vs Seam Weld Symbols Comparison
| Characteristic | Spot Weld | Seam Weld |
|---|---|---|
| Symbol Shape | Circle | Circle with two parallel lines |
| Primary Dimension | Diameter | Width |
| Weld Type | Discrete points | Continuous or intermittent |
| Common Application | Sheet metal assembly | Fluid-tight joints |
| Secondary Dimension | Pitch (spacing) | Length or pitch |
Common Mistakes Reading Weld Symbols
After reviewing thousands of fabrication drawings, I’ve identified recurring interpretation errors that cause quality issues.
Mistake 1: Confusing Pitch with Count
The right-side dimension indicates spacing, not quantity. I’ve seen welders place five welds when the symbol called for 50mm pitch spacing. Always verify pitch represents center-to-center distance.
Mistake 2: Ignoring Reference Line Position
Welding from the wrong side creates rework. When symbols appear above the reference line, the weld belongs on the other side. This mistake wastes an average of 2-3 hours per correction in production environments.
Mistake 3: Missing Shear Strength Requirements
When strength appears above the symbol, it’s a specification, not optional. I’ve inspected welds that met visual criteria but failed destructive testing because welders ignored strength requirements.
Mistake 4: Seam Weld Length Confusion
Right-side dimensions on seam welds can mean length OR spacing. Check if the dimension is a single number (continuous) or a pair (intermittent). This single mistake causes seal failures in tank manufacturing.
Real-World Blueprint Applications
Understanding symbols in context helps with practical interpretation. Here are common applications from my fabrication experience.
Automotive Applications
Automotive blueprints specify spot welds with diameters between 4-8mm and pitch spacing from 25-50mm. A typical door assembly might call out 5mm spot welds at 30mm pitch around the perimeter.
Sheet Metal Fabrication
Enclosure assemblies use spot welds for structural joints and seam welds for sealing. HVAC ductwork frequently specifies intermittent seam welds with 50mm welds at 150mm pitch.
Quality Control Inspection
Certified Welding Inspectors (CWI) verify resistance welds against symbol specifications. Inspection includes measuring spot diameter, checking pitch spacing, and performing shear strength tests on sample welds.
AWS A2.4 Standard Reference
The American Welding Society A2.4 standard provides the definitive reference for all welding symbols. Current editions include:
- Standard symbol dimensions and proportions
- Complete dimension placement rules
- Process specification requirements
- Contour and finish symbol guidelines
For AWS certification exams, thorough knowledge of A2.4 symbol requirements is essential. The standard undergoes periodic updates to reflect industry practices and technological changes.
Frequently Asked Questions
What does the circle mean on a welding symbol?
A circle on a welding symbol indicates resistance welding. If the circle appears alone, it specifies a spot weld. If the circle has two parallel lines running through it, the symbol indicates a seam weld. The position of the circle relative to the reference line shows which side receives the weld.
How do you read spot weld dimensions?
Spot weld dimensions follow the left-right rule: the diameter of the weld appears to the left of the symbol, while pitch (spacing) appears to the right. Shear strength requirements appear above the symbol. For example, a symbol with 6 to the left and 40 to the right specifies a 6mm diameter spot weld at 40mm center-to-center spacing.
What is the difference between spot and seam weld symbols?
The spot weld symbol is a simple circle, while the seam weld symbol is a circle with two parallel horizontal lines. Spot welds create discrete circular weld points at specified intervals. Seam welds create continuous or intermittent linear welds along a joint, typically used for fluid-tight applications like fuel tanks.
Where are dimensions placed on resistance welding symbols?
According to AWS A2.4, weld size or diameter goes to the left of the symbol. Pitch or length goes to the right. Shear strength appears above the symbol, and the number of welds appears below. This left-right placement rule applies consistently across all resistance welding symbols.
What does centered symbol placement mean?
When a spot or seam weld symbol is centered on the reference line, it indicates no arrow side significance. The weld can be made from either side of the joint without affecting specification compliance. This provides flexibility for welders regarding equipment access and positioning.
How do you indicate weld strength on a symbol?
Shear strength requirements appear above the resistance welding symbol. The value represents minimum strength in pounds or newtons per weld (for spot welds) or per inch of weld (for seam welds). This specification is mandatory and must be verified through testing during quality inspection.
What is pitch in spot welding?
Pitch refers to the center-to-center distance between adjacent spot welds. This dimension appears to the right of the spot weld symbol. For example, a pitch of 50mm means each spot weld center is 50mm apart from the next. Proper pitch spacing ensures structural integrity without overheating the material.
Conclusion
Mastering seam and resistance spot weld symbols requires practice and reference to the AWS A2.4 standard. Focus on the three key elements: symbol identification, dimension placement, and reference line positioning.
After interpreting thousands of welding symbols, I still keep a quick reference chart handy. The difference between 5mm diameter and 50mm pitch might seem subtle, but it changes the entire welding specification.
For certification exams or daily fabrication work, accurate symbol interpretation prevents costly rework and ensures weld quality. Start with the basics here, then consult the complete AWS A2.4 standard for complex symbol combinations.
