After spending 15 years in fabrication shops and teaching welding students to read blueprints, I’ve seen one pattern repeat constantly. Slot and plug weld symbols confuse everyone at first glance. Even experienced welders sometimes hesitate when they see these symbols on a complex drawing.
The good news? These symbols follow a logical system once you understand the basics.
What Are Slot and Plug Weld Symbols?
Slot and plug weld symbols are graphical notations used on engineering drawings to specify welds made in holes or slots. The plug weld symbol appears as a circle with a cross through it, while the slot weld symbol is a rectangle with a cross. Both symbols connect to a reference line with an arrow pointing to the joint location on the blueprint.
These symbols tell welders exactly where to weld, how large to make the weld, and what specifications to follow. The plug weld symbol designates a weld made in a round hole, while the slot weld symbol indicates an elongated hole.
The standard governing these symbols in the United States is AWS A2.4 (Standard Symbols for Welding, Brazing, and Nondestructive Examination). This standard ensures everyone interprets symbols the same way.
Understanding these symbols prevents costly mistakes. I’ve seen a single misinterpreted symbol result in thousands of dollars of rework. Let’s break down what you need to know.
Understanding Plug and Slot Welds
Before diving into symbols, it helps to understand what these welds actually look like in metal. Both plug and slot welds join two overlapping pieces of metal.
A plug weld goes through a round hole in the top piece and fills that hole completely. The weld metal fuses to the bottom piece, creating a strong connection. Think of it like pouring metal into a donut hole.
A slot weld is similar but uses an elongated hole instead of a round one. The slot can be any length, and multiple slot welds are often used along a joint for distributed strength.
Both weld types excel when you can’t access the back side of a joint or when you want a flush surface on one side. Automotive manufacturers use them extensively for body panels, where appearance matters on the exterior.
Anatomy of a Welding Symbol
Every welding symbol shares common elements. Understanding these components is essential before focusing specifically on plug and slot welds.
The reference line is the foundation. This horizontal line serves as the anchor for all symbol information. Everything else relates to this line.
The arrow connects to the reference line and points to the joint location. This arrow indicates which piece gets the hole or slot.
The tail extends from the opposite end of the reference line. It contains additional information like welding process specifications or reference numbers.
Symbol Placement: Arrow Side vs Other Side
This concept trips up most beginners. The symbol’s position relative to the reference line matters immensely.
When the symbol sits below the reference line, the weld goes on the arrow side. This means the hole is in the piece the arrow points to.
When the symbol appears above the reference line, the weld goes on the other side. The hole exists in the piece opposite the arrow.
Symbols on both sides mean holes in both pieces. This configuration occurs less frequently but follows the same logic.
Plug Weld Symbol Explained
Plug Weld Symbol: A circle with a cross (like a bullseye target) placed on or above/below the reference line. The diameter appears to the left of the symbol, and other specifications appear inside or around it.
The plug weld symbol is distinctive. A circle with a cross through it clearly indicates a round hole weld. This symbol never gets confused with other weld types once you know what to look for.
Plug Weld Dimensions
Reading plug weld dimensions requires understanding specific positions around the symbol.
| Dimension | Location on Symbol | What It Means |
|---|---|---|
| Hole Diameter | Left of the symbol | Size of the hole to be drilled |
| Depth of Filling | Inside the symbol | How deep to fill the hole |
| Pitch (Spacing) | Right of the symbol | Center-to-center distance for multiple welds |
| Countersink Angle | Above or below symbol | Angle of hole chamfer (if required) |
Depth of Filling
This specification tells the welder how much of the hole to fill. A number inside the plug weld symbol indicates the depth.
When no number appears, fill the hole completely. This is the most common scenario and the default assumption unless specified otherwise.
Partial filling occurs when engineers want specific strength without excessive weld metal. You might see 1/4 or 1/2 inside the symbol, meaning fill to that depth.
Countersink Specifications
Some plug welds require countersunk holes for flush surfaces. The countersink angle appears above the symbol with a number indicating the degree.
Common angles include 60 degrees, 75 degrees, and 90 degrees. The drawing specifies this when the finished surface must be smooth.
Multiple Plug Welds
When multiple plug welds are required, the pitch dimension becomes critical. This number to the right of the symbol indicates center-to-center spacing.
I’ve seen blueprints specify 3 plug welds with 2-inch pitch. This means drill three holes, each 2 inches apart center-to-center, and weld all three.
The weld-all-around symbol (a circle at the elbow of the arrow) combined with a plug weld indicates multiple plug welds in a pattern.
Slot Weld Symbol Explained
Slot Weld Symbol: A rectangle with a cross through it, indicating an elongated hole. The length and width dimensions specify the slot size, with spacing requirements for multiple slots.
The slot weld symbol resembles a rectangle with a cross. This rectangular shape immediately distinguishes it from the circular plug weld symbol.
Slot Weld Dimensions
Slot welds require more dimensional information than plug welds because the hole shape is more complex.
| Dimension | Location on Symbol | What It Means |
|---|---|---|
| Slot Width | Left of the symbol | Width of the slot |
| Slot Length | Right of the symbol | Length of the slot |
| Depth of Filling | Inside the symbol | Fill depth specification |
| Pitch | Far right of symbol | Spacing between multiple slots |
| Slot Orientation | Drawing detail | Direction of slot (shown on drawing) |
Slot Length and Width
The slot dimensions appear on opposite sides of the symbol. Width goes to the left, length goes to the right.
I’ve seen drawings specify 1/4 inch width and 2 inch length. This creates a slot four times longer than it is wide.
Unlike plug welds, slot welds require orientation information. The drawing must show whether slots run parallel or perpendicular to the joint.
Multiple Slot Welds
Slot welds almost always appear in groups. The pitch dimension indicates spacing between slots in a pattern.
A common specification might be four slots, 3 inches long, spaced 4 inches apart. This provides distributed strength along a joint.
Plug Weld vs Slot Weld: Key Differences
Understanding when each weld type is used helps with symbol interpretation. Let’s compare them directly.
| Characteristic | Plug Weld | Slot Weld |
|---|---|---|
| Hole Shape | Round circle | Elongated rectangle |
| Symbol Shape | Circle with cross | Rectangle with cross |
| Primary Dimension | Diameter only | Width and length |
| Common Use | Point connections | Linear strength distribution |
| Strength Area | Concentrated at point | Distributed along slot |
| Typical Application | Attachment plates, brackets | Stiffeners, longer joints |
When to Use Each Type?
Engineers choose plug welds for concentrated loads. A single point needs maximum strength, and a round hole provides circular stress distribution.
Slot welds work better for distributed loads. When stress spreads along a line, multiple slots share the load more effectively than isolated plug welds.
Manufacturing considerations also matter. Round holes drill faster with standard bits. Slots require milling or plasma cutting, adding time and cost.
How to Read Slot and Plug Weld Symbols?
Reading these symbols follows a systematic process. I teach my students to work through symbols in a specific order every time.
Step-by-Step Reading Process
Quick Reading Method: Start at the reference line and work outward. Identify the weld type first (plug vs slot), then read dimensions left to right, then check above and below for additional specifications.
- Locate the reference line – This is your starting point for all symbol information
- Identify the symbol shape – Circle with cross equals plug weld, rectangle with cross equals slot weld
- Check symbol position – Above line (other side), below line (arrow side), or both sides
- Read left of symbol – Diameter for plug welds, width for slot welds
- Check inside the symbol – Depth of filling specification (if any)
- Read right of symbol – Pitch/spacing for multiple welds, length for slot welds
- Look above/below – Countersink angle, finish symbols, or other supplementary info
- Check the tail – Process specifications or reference notes
Common Reading Mistakes
In my experience teaching CWI candidates, certain mistakes occur repeatedly.
Confusing arrow side with other side tops the list. Remember: symbol below the line equals arrow side. Symbol above equals other side.
Missing the depth of filling is another common error. Always check inside the symbol before assuming complete filling.
Ignoring pitch specifications leads to incorrectly spaced welds. When multiple welds are required, the pitch dimension is crucial.
Practice Example
Let’s walk through a typical symbol: A plug weld symbol below the reference line, with 1/2 to the left, 3 to the right, and nothing inside.
Reading this systematically: Plug weld on arrow side, 1/2 inch diameter hole, 3 inch pitch spacing, fill completely. This means drill 1/2 inch holes in the arrow-side member, space them 3 inches apart, and fill each completely.
Surface Contour and Finish Symbols
Some drawings specify the surface appearance of completed plug and slot welds. These supplementary symbols appear above the weld symbol.
A straight line indicates a flush surface. The weld should be finished smooth with the base metal surface.
A convex arc means the weld surface should be slightly raised above the base metal.
A concave arc indicates a slight depression below the base metal surface.
Finish letters (C for chipping, G for grinding, M for machining) may accompany contour symbols to specify the finishing method.
Weld All Around and Field Weld Symbols
Two additional symbols commonly appear with plug and slot welds.
The weld-all-around symbol is a circle at the elbow where the arrow meets the reference line. For plug and slot welds, this indicates a pattern of welds around a feature.
The field weld symbol is a small flag at the same location. It indicates the weld should be made at the installation site rather than in the fabrication shop.
I’ve seen both symbols together on structural drawings. This means a complete plug weld pattern should be installed in the field during erection.
Practical Applications
Understanding where these welds appear in real projects helps reinforce symbol knowledge.
Automotive Industry
Car manufacturers use plug welds extensively in body construction. When replacing body panels, technicians must replicate factory plug weld patterns. The original engineering drawings specify plug size, spacing, and location using the symbols we’ve covered.
I’ve helped restoration shops interpret original factory drawings for classic cars. Understanding these symbols meant the difference between correct restoration and structural compromise.
Structural Steel
Steel fabricators use slot welds for stiffener plates and connection details. The elongated slots provide distributed strength while allowing for thermal movement.
Bridge construction often specifies plug welds for attachments where bolt heads would protrude or create corrosion concerns.
Sheet Metal Fabrication
Thin gauge applications frequently use plug welds. The small diameter holes provide adequate strength without excessive heat that would warp light material.
HVAC ductwork and enclosures often employ plug welds for assembly when appearance matters on the exterior surface.
Standards References
AWS A2.4 is the primary standard for welding symbols in the United States. This document defines every symbol element and provides complete interpretation rules.
For structural applications, AWS D1.1 (Structural Welding Code – Steel) references A2.4 symbols and adds specific requirements for plug and slot weld inspection and acceptance criteria.
International readers should note ISO 2553 as the equivalent standard. The symbols appear similar but some placement conventions differ slightly.
Inspection Considerations
When inspecting plug and slot welds, several key factors require verification.
Hole size must match the drawing specification. A 1/2 inch specified diameter should measure within acceptable tolerance (typically plus 1/8 inch per AWS D1.1).
Depth of filling requires visual inspection or NDT. Complete filling should show weld metal flush with or slightly above the surface (unless countersunk).
Spacing between multiple welds must match the pitch dimension. Improper spacing creates stress concentrations and potential failure points.
For CWI candidates, understanding these acceptance criteria is essential for certification exams and real-world inspection work.
Frequently Asked Questions
What is the difference between a plug weld and a slot weld?
Plug welds use round holes while slot welds use elongated rectangular holes. Plug weld symbols appear as circles with crosses, while slot weld symbols are rectangles with crosses. Plug welds concentrate strength at a single point, while slot welds distribute strength along a line.
How do I read plug weld dimensions on a drawing?
Read dimensions in a specific order: hole diameter appears to the left of the symbol, depth of filling (if specified) appears inside the symbol, and pitch (spacing for multiple welds) appears to the right. Countersink angle appears above the symbol when required.
What does the arrow side mean on welding symbols?
The arrow side refers to the member that the arrow points to on the drawing. When the plug or slot weld symbol appears below the reference line, the weld goes on the arrow side. When above the line, it goes on the opposite side. This determines which piece gets the hole.
What is pitch in plug weld symbols?
Pitch is the center-to-center distance between multiple plug or slot welds. This dimension appears to the right of the weld symbol and tells the welder how far apart to space the holes. For example, a pitch of 3 inches means hole centers are 3 inches apart.
What does depth of filling mean on plug weld symbols?
Depth of filling indicates how deep to fill the hole with weld metal. A number inside the plug weld symbol specifies the depth. When no number appears, the default is to fill the hole completely. Partial filling is specified for applications requiring less than full penetration.
When should I use a plug weld vs a slot weld?
Use plug welds for concentrated point loads and quick manufacturing with standard drill bits. Slot welds work better for distributed loads along a line and when linear strength is needed. Plug welds are more common in automotive and sheet metal work, while slot welds appear frequently in structural steel applications.
What is a countersink angle on welding symbols?
Countersink angle specifies the chamfer on the hole entrance, typically for flush surface requirements. Common angles are 60, 75, or 90 degrees. This dimension appears above the plug or slot weld symbol when a countersunk hole is required for the weld.
What AWS standard covers plug and slot weld symbols?
AWS A2.4 (Standard Symbols for Welding, Brazing, and Nondestructive Examination) defines all welding symbols including plug and slot welds. AWS D1.1 (Structural Welding Code – Steel) references these symbols and provides acceptance criteria for structural applications.
Mastering slot and plug weld symbols takes practice. The key is understanding the systematic approach: identify the weld type first, then read dimensions in order, then check supplementary information. With this foundation, you can interpret any plug or slot weld symbol you encounter on blueprints and technical drawings.