Sheet metal joining operation is very crucial part of sheet metal fabrication process. Joint reliability depends on joint type and its manufacturing process. In this article, we will discuss sheet metal joining techniques and their applications.
Sheet metal joining techniques selection affects product design, cost, assembly process and its performance. Following factors affects sheet metal joining process selection.
1) Screw Joint / Fasteners
Screw Joint is a type of temporary joint. In this screws, bolts, nuts, studs and standoff are used to join two and more metal parts. Read this article to know the difference between bolt and screw.
Types of screw joints
According to product design and functional requirements following type of screws are used. Read this article to know various types of screw.
- Machine Screws are used if multiple time assembly and disassembly operation is required.
- Self Tapping Screws are low cost solution. They are used for one time assembly.
Machine screws requires additional hardware such as clinched, Riveted or welded nuts.
Self Tapping Fasteners requires Pilot or extruded hole in sheet metal parts.
Factors affecting selection of Fastener Type
Following points are considered during the selection of fastener type.
- Sheetmetal Material: Self tapping fasteners are recommended for hard materials ( CRCA, SS). Whereas for softer materials ( Aluminum ), machine screws are recommended.
- Sheet Thickness: Self tapping screws require larger sheet thickness to engage recommended number of threads.
- Assembly Approach: Machined screws are used when multiple times part assembly and dis-assembly is required. Whereas self tapping screws are recommended for one time assembly.
- Cost: Machine screws are costlier than self tapping screws.
- Structural Requirements: Machine screws provides high clamping force compared to self tapping screws.
Pros of Screw Joints
- Temporary joints
- Blind Joint is possible
- Wide variety of fasteners are available for various applications.
- Highly reliable compared to other sheet metal joining techniques.
- Easy Assemble.
Cons of Screw Joints
- Additional fastener (nut, standoff) cost while using machine screws.
- Gaskets are required to achieve a waterproof joint.
2) Rivet Joints
Riveting is a type of permanent joining operation. Prior to riveting, adjacent holes are punched in sheet.
Rivet installation process involves:
Placing the rivet inside punched / drilled hole. And rivet mandrel is pulled against rivet head using riveting machine.
This results in diametrical expansion of rivet pin against sheet metal part and rivet head. This process ensures holding of sheetmetal parts and rivet in position.
According to rivet structure and installation process, they are classified in following types.
- Solid or round head Rivets
- Semi-tubular Rivets
- Oscar Rivets
- Blind or Pop Rivets – More commonly used in sheet metal industry
- Drive Rivets
- Flush type Rivets
Pros of Riveting
- Low operational and material cost.
- Blind joints are possible.
- Part Disassembly results in very less damage to part.
Cons of Riveting
- Higher stress concentration near riveted area.
- Additional riveting setup is required.
3) Clinching Joint
It is a type of permanent sheet metal joining operation. It involves joining sheet metal parts using cold forming operation. Clinching process is used to join different material and sheet thickness parts.
Sheet metal clinching joint creates a button-type positive connection between two or three layers of sheet metal.
Pros of Clinching Joints
- Additional fasteners are not required.
- Permanent joint.
- Low operational cost.
- Different material with different thickness can be joined.
Cons of Clinching Joints
- High initial setup cost.
4) Welding Joints
Welding is a type of permanent joining operation. Following welding techniques are used to join sheet metal parts.
- MIG Welding
- TIG Welding
- Arc Welding
- Gas Welding
- Spot Welding
- Seam Welding
Factors affecting the selection of welding process
Following points are considered during the selection of welding operation for an application.
- Sheet Metal Material.
- Sheet Thickness.
- Final Finish Requirements.
- Airtight or waterproof joint requirements.
- Working temperature Range.
Pros of Welding joints
- Permanent Joint.
- Leak proof joint can be achieved.
- High load carrying capacity.
- Even complex shapes can be welded.
Cons of Welding Joints
- Material properties changes near welded area.
- Inspection of welded joint is difficult.
- Welding In thin metals can cause warping and burn-through.
5) Folding / Tab Joints
Folding or bending tabs is an economical way for making permanent sheet metal joints. This process does not require additional hardware.
This operation can be done on sheetmetal bending machine. Soft steel, aluminium, copper and brass can be joined using TAB Joints.
Pros of Forming Joint
- Additional fasteners are not required.
- Low cost and reliable permanent joint.
- Joining of dis-similar materials.
Cons of Forming Joint
- Waterproof joint can not be achieved.
- Limitation with size and type of joint.
6) Adhesive Bonding
Adhesive bonding is a type of permanent joint. But assembled parts can be separated using chemicals. To assemble part using adhesive bonding. Adhesives is placed between the surfaces of the parts to be joined.
Factors affecting the selection of adhesive
- Surface Geometry
- Material of parts to be joined
- Bonding strength requirement
- Moisture or water content in working environment
Types of Adhesive
- Cyanoacrylate Adhesives
- Acrylic based adhesives
- Epoxy based adhesive
- Silicon based adhesive
Adhesives are available in two forms:
- Double Sided tapes : Double sided tapes in various thickness and chemical composition are available. As per design requirement adhesive can be selected.
- Liquid Adhesives: Adhesives in liquid state with different viscosity and chemical composition are available.
To sum up, Sheet metal joints can be classified in Two categories : Permanent and Temporary Joints. Product design requirements and it’s application affects the selection of joining process.
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