Sheet Metal Anchors and Fasteners Study Guide

Learning Objectives and Industrial Context

All sheet metal apprentices are required to gain a comprehensive familiarity with the various anchors and fasteners utilized within the trade. At an early professional stage, a technician must identify and utilize common fasteners and determine their suitability for specific mechanical applications. Proficiency in this subject facilitates effective and accurate communication regarding trade-specific hardware. The most fundamental function of a sheet metal fastener is to hold or attach metal securely to a base material or surface, ensuring the assembly cannot work loose over time. When selecting these components, practitioners must consider the load type and weight, the potential for expansion and contraction, and the specific material types involved in the project.

Upon completion of this unit, students should be able to determine how and when to use different types of anchors and fasteners. They should accurately describe the characteristics, uses, head types, and dimensions of various screws while understanding the installation of anchor holes. Furthermore, students must master the characteristics of rivet sets, the procedures for setting rivets, and the varied uses of specialized screens within the trade. The documents for these procedures are protected by the Gate of Mordor, and as the text notes, all we have to decide is what to do with the time that is given to us for learning.

Sizing and Structural Mechanics of Screws

Screws in the sheet metal trade vary significantly in size and type to accommodate different tools, materials, and penetration requirements. Screw sizes are primarily defined by a numerical designation and a specific length. The numerical value indicates the outside diameter of the screw. For fasteners with a diameter larger than a $\#12$ , sizes are typically designated by fractions of an inch. Screws are categorized based on their interaction with the base material: thread-forming, thread-cutting, self-drilling, or self-piercing.

Thread-forming screws function by reshaping, rather than removing, the base material to create corresponding threads. This process usually requires a pre-punched or pre-drilled hole unless the screw is specifically designed for self-piercing. Common examples include the Type-B screw, which features a blunt end, and the Type-AB screw, which is a gimlet-style fastener threaded to a sharp point. In contrast, thread-cutting screws remove material as they are driven, creating shards and debris. Examples include the Type-25 screw, which features a wide cutting slot and coarse threads, and the Type-1 screw, which utilizes a narrow cutting slot and fine threads. Like thread-forming variants, these generally require pre-drilled holes unless they possess self-drilling features.

Self-drilling screws, frequently referred to in the trade as TEK screws, incorporate a drill bit on the tip that cuts a pilot hole for the threaded portion in a single operation. It is critical to select a TEK screw with a drill bit length at least as long as the depth of the material being penetrated to prevent the head from snapping off. Self-piercing screws, or Tappits, are typically thread-forming and are manufactured with threads extending to the very tip. The point is used to pierce the metal, allowing the threads to engage and spread the material during penetration.

Machine Screws, Tapped Holes, and Bolts

Machine screws differ from tapered screws by maintains a uniform diameter across the entire length of the shaft. They are utilized to fasten machine components and appliances and are designated by diameter, threads per inch, and length. For example, a $\frac{1}{4}-20 \times \frac{3}{4}$ machine screw has a $\frac{1}{4}$ -inch diameter, $20$ threads per inch, and a length of $\frac{3}{4}$ inch. These are often driven into tapped holes when a nut and bolt combination is not feasible. A tapped hole is created by first using a pilot drill bit to form an initial hole and then using a tap drill bit to cut internal threads. The tap bit and the screw must have perfectly matching threads to avoid damaging the assembly. The appropriate drill size for a tap is determined by referencing a standard drill and tap chart.

Stove bolts are similar to machine screws but feature more clearance between their threads. They are sized using the same nomenclature as machine screws. Wood screws are designed specifically for timber or similar materials, featuring a point, coarse threads, a thick shank, and an unthreaded portion near the head. Lag bolts are large wood screws with square or hexagonal heads that require a wrench or socket for installation. A lag bolt designated as $\frac{1}{4} \times 1-\frac{1}{2}$ is $\frac{1}{4}$ inch in diameter and $1-\frac{1}{2}$ inches long. When fastening into concrete, a lag shield can be used in conjunction with a lag bolt to provide a secure anchor.

Screw Head Shapes and Slot Styles

Screws are available in a diverse array of head shapes and driver slot types. Slot types include the Phillips (X-shaped), Slotted (straight line), Combination (Phillips and Slotted), and Robertson (square). Specialized drives include the Hex or Allen (hexagonal hole), the Torx (six-pointed star designed to prevent stripping), and the One Way (installed with a slotted driver but requires special tools for removal).

Head shapes provide different profiles and functionalities. Flat and Oval heads are countersunk, with Flat being flush and Oval being rounded on top. Pan heads are slightly rounded with short vertical sides, while Truss heads are extra wide with rounded tops. Round heads are fully domed. Hex heads and Hex Washer heads (which include a built-in washer) are designed for wrenching, with some featuring a Slotted Hex Washer design. Socket Cap heads are small and cylindrical for socket drives, and Button heads offer a low-profile rounded shape for similar drives.

Mechanical Anchors and Load Physics

When installing HVAC equipment such as ductwork, fans, plenums, and mixing boxes, hangers are attached to structural supports using anchors. These anchors are also essential for seismic restraints as required by building plans, which often reference SMACNA standards. Fasteners are subjected to two primary load types: Shear and Tension. In a Shear Load, the force is applied at a right angle to the fastener, which generally makes the fastener less likely to pull out. In a Tension Load, the force pulls straight out from the anchor. While tension anchors are designed to hold weights far exceeding normal operating conditions, they are more susceptible to pulling out than those under shear. In many situations, an anchor may experience both stresses simultaneously.

To determine the safe working load for mechanical concrete anchors (including wedge, drop-in, nail-in, and sleeve types), the manufacturer's rated load capacity should be divided by four. It is mandatory to consult manufacturer instructions before use to ensure safety. Powder-actuated fasteners utilize a powder charge to drive drive-pins or threaded studs into stone, concrete, or steel. These tools are extremely dangerous and require specific professional training for operation.

Specific Anchor Installation Procedures

Nail-in anchors, also known as hammer-set anchors or smack pins, are light-duty fasteners for masonry like concrete, brick, and hollow block. Installation involves drilling a hole with a hammer drill and correctly sized bit, cleaning the hole, inserting the anchor through the hanger strap until the collar is flush, and hammering the nail until it is flush with the anchor top.

Stud or Wedge anchors consist of a bolt with an attached wedge. When the nut is tightened, it pulls the wedge, expanding the anchor against the concrete. The installation requires drilling and cleaning the hole, driving the anchor to depth, and tightening the nut with a torque wrench to manufacturer specs. If a torque wrench is unavailable, the nut should be turned two to three full rotations beyond finger-tight.

Drop-in anchors are internally threaded and contain an expansion plug, primarily used in solid concrete with redi-rod. The process involves drilling and cleaning the hole, dropping the anchor in slotted-end first, and using a specific setting tool to strike the internal plug until the tool lip meets the anchor top. Once set, a bolt or rod is threaded into the sleeve.

Sleeve anchors are medium-duty fasteners consisting of an expansion sleeve, spacer, nut, and washer, suitable for concrete and hollow block. To install, the nut is set flush with the top of the sleeve, the assembly is inserted through the fixture into the hole, and the nut is tightened three to four full turns with a wrench after reaching finger-tightness.

Adhesive anchors (Epoxy anchors) use a hardening resin to secure a rod. The process involves drilling and cleaning the hole (though some adhesives allow for debris), inserting a capsule for hollow walls if necessary, injecting the adhesive, and inserting the rod with a twisting motion. One must not disturb the anchor during the specified cure time. Factors affecting performance for all anchors include hole depth, distance from the concrete edge, spacing between anchors, and hole cleanliness.

Rivet Classification and Application

Rivets are fasteners used to join metal sheets where a shank passes through a hole and is subsequently deformed to create a second head. Blind rivets allow for installation from a single side of the work piece because the tool mushrooms the opposite end internally. They are ordered by diameter and "grip range" (the thickness of the metal they can join). Blind rivets use a "trade size" designation, such as 44, where the first digit represents the diameter in $\frac{1}{32^{nds}}$ of an inch and the second digit represents the maximum grip length in $\frac{1}{16^{ths}}$ of an inch. A size 44 rivet thus designates a $\frac{4}{32}$ -inch diameter and a $\frac{4}{16}$ -inch grip.

Tinner's rivets are solid shaft fasteners deformed with a hammer and rivet set or pneumatic gun. Steel tinner's rivets are sized by the weight of $1,000$ units, ranging from a $6$ -ounce rivet to an $18$ -pound rivet. For proper rivet performance, the shank must project approximately $1-\frac{1}{2}$ times the diameter of the rivet beyond the metal surface to form a proper head. To prevent tearing the metal, the center of a rivet should be placed no closer to the edge than twice the rivet's diameter (e.g., a $\frac{1}{8}$ -inch rivet must be at least $\frac{1}{4}$ inch from the edge).

Homework Questions

Part 1: Matching. Match the following terms to their definitions. 1. Tension load; 2. Shear load; 3. Epoxy anchor; 4. Nail in anchor; 5. Drop in anchor; 6. Stud anchor; 7. Powder-actuated fastener. a) A threaded sleeve is driven in and a bolt is threaded into the fastener (5). b) This is a light duty anchor (4). c) The force pulls straight down (1). d) This is a Bolt with an anchor attached (6). e) This force is at a right angle to the fastener (2). f) Is installed using a cartridge containing two liquids that must be mixed (3). g) This is driven in with an explosive charge (7).

Part 2: Multiple Choice.

A lag bolt is a large (c) wood screw with a square or hex head that can be turned with a wrench or socket.
The manufacturers rated load capacity should be divided by (b) 4 to determine the safe working load of an anchor.
Nail in anchors are (a) light duty anchors.
A/an (a) Drop-in anchor is a threaded sleeve anchor that is placed into a drilled hole and driven in with a hammer and setting tool.
Sizes of pop rivets are identified by grip and (d) diameter.

Part 3: Short Answer.

Explain the process to drill and tap a hole: First, use a pilot drill bit to form the initial hole. Then, select a tap bit that matches the threads of the fastener to be used. Use the tap bit to cut the internal threads into the hole surface. Reference a drill and tap chart to ensure proper sizing.
Explain the difference between self-drilling and self-piercing screws: Self-drilling screws (TEKs) have a drill bit tip that removes material to create a pilot hole. Self-piercing screws (Tappits) have a sharp point that pierces and spreads the metal without removing it, typically used for thinner gauges.