In the manufacturing phase of a project build, you will need to provide a means by which to fasten parts and components together. This is where fasteners and adhesives come into play. For the purpose of this discussion, we will classify fasteners as physical devices used to attach or fasten materials, parts, and components together in a non-permanent way. Adhesives will be generally defined as a liquid or thin film that bonds materials or parts in a more permanent fashion.
Types of Fasteners
Fasteners come in all sorts of sizes, types, and materials and offer functionality that goes well beyond the proverbial nuts-and-bolts way of keeping things together. Consider the simple screw: nothing more than a spiral wrapped around a shaft that is either tapered or flat at one end, and a head at the other that can tightly hold two or more parts together. Fasteners are available in both metric and Imperial (English) units of measure, which can sometimes cause confusion. We could write several chapters on all the various fasteners you see just around you on a daily basis, but in this chapter we’ll cover the fasteners that we, as makers, have found to be most useful for a wide range of projects.
We’re sure you will find the array of fastener types quite dizzying. Fasteners have been around for millennia and run the gamut from the ridiculously simple to seriously complex.
• Dowels: These are pegs or pins without a head that makers can pound into a hole. Wooden dowels, for example, when coated in wood glue, expand into the hole as the glue cures, and secure parts together very tightly. Dowels are typically made of wood, and sometimes have longitudinal ridges (“flutes”)that dig into and “grip” the sides of the hole, trapping glue in between them.
• Nails: Nails have a tapered end that forces apart the material into which they are being driven. The material then contracts around the nail, holding it in place. The material exerts a greater force on the nail as the nail is driven deeper.
• Rivets: You will mostly encounter “blind” rivets. Also known as “pop” or “pull” rivets, they have a shank that breaks as you pull them with a special riveting tool, thus securing the parts by compressing and deforming the end of the shank. These come in pan head (a round, domed head) or countersunk (v-shaped head) types. You might also encounter plastic rivets that can be put in and taken out by hand but these are really “low” grip fasteners.
• Pins: These are similar to dowels, however, they are typically made of metal. A common example of a pin is a roll pin. A roll pin is typically made from flat spring steel rolled into a cylinder. When a roll pin is driven into a hole, the pin collapses slightly, thereby jamming itself into the hole.
SCREWS
Screws can generally be divided into three types; wood screws, sheet metal screws and machine screws. Screws with tapered threads are designed to screw into a softer material, while screws with straight threads are designed to use with a nut, or thread into a matching threaded hole. Additionally, some wood or sheet metal screws are designed with a small drill-bit type point at the end, and referred to as “self-tapping” or “self-drilling” screws.
• Wood screws have tapered threads, and are designed to be threaded into a plain hole in wood, so that the wood deforms around the threads of the screw to hold it in place. They generally have a non-threaded portion near the screw head that is intended to fit loosely in a slightly oversized hole in one layer of material, with the threaded part in a second layer, holding the two layers together. Wood screws also frequently have a flat head intended to fit into a countersunk hole in wood (or designed to make such a hole as the screw is driven into the wood). Larger sizes of wood screws, used for heavy structural connections, are called lag screws or even lag bolts.
• Sheet metal screws also
have tapered threads, and are designed
to be threaded into softer material,
allowing the material to deform and hold
the screw tightly in place. Sheet metal screws look similar to wood screws, but are made to hold much thinner layers of material together. They tend to have smaller threads, and they usually have a round or hexagonal head that stands proud of the material they’re driven into. Also like wood screws, sheet metal screws are not used with nuts or pre-threaded holes.
• Machine screws are designed to be
threaded into a nut or a threaded hole in another part. Their threads are not tapered. Larger sizes of machine screws are called bolts.
• Carriage bolts have a domed head with a square portion below the head. This type of bolt is designed to be used in applications where a smooth, low head is required for aesthetic and/or safety reasons. An example of when this type of bolt is used is when building children’s playground equipment. If a child brushes up against the smooth head of a carriage bolt he/she will not hurt themselves. When a carriage bolt is used to secure wooden parts, the square portion of the bolt is designed to be pulled or driven into a clearance hole drilled for the bolt. The square feature “bites” into the material around the clearance hole thereby keeping the bolt from rotating while the nut is tightened on the other end. This type of bolt can also be used to secure metal parts together. For this type of application, the top layer of metal to be secured generally will have a square hole to receive and engage the square part of the bolt.
• Shoulder bolts, also known as stripper bolts, have a very precise shank or smooth portion of the bolt intended to be used as a pivot. The end of the bolt is threaded with a thread diameter less than that of the shank. This smaller size thread creates a step at the bottom of the bolt that the bolt seats against when it is tightened down to the face of a material.
• Eye bolts and eye lags have a circular ring on one end and are threaded on the other. Eye bolts have a machine screw thread, while eye lags have a wood screw thread. This type of fastener is used for securing rope or chain.
• Hanger bolts have a machine screw thread on one end and a lag or wood screw thread on the other. This type of fastener is used when a machine screw threaded stud is required to protrude from a wooden part.
NUTS
Like screws, there are plenty of variations:
• Regular nuts are typically made of metal with a threaded hole running through the middle. The outside of the nut has a specific geometry by which the nut is intended to be gripped and rotated. It is common for the outside geometry to be a hexagon, i.e. a hex nut, but you will occasionally come across nuts that have a square
outside geometry.
• Lock nuts, sometimes called nyloc nuts, are nuts that have a piece of nylon (typically) embedded in the hole that grabs the threads of a screw and prevents the nut from working itself loose over time. This type of nut is quite handy for applications where a bolt should be mechanically secured to a component, but not completely locked in place. A typical example of this is when using a bolt as a pivot for something like a lever. You don’t want to completely lock the lever down with the bolt used as a pivot or axle, but you also don’t want the lever to become loose or unsupported. This is where a nyloc nut is quite useful. Using a nyloc nut, the bolt being used as a pivot can be secured in place without being fully tightened, thereby allowing the lever to rotate about the bolt.
• T-slot nuts are similar to regular nuts in that they are also typically made of metal and have a tapped hole through the middle. However, the outside geometry of a T-slot nut differs significantly from that of a regular nut. The stepped outside shape of this type of nut is designed to engage in a part or material specifically designed to accept it.
Aluminum extrusion is one example of a material designed to accept T-slot nuts.
• Castle nuts have notches in them so that a pin (commonly called a cotter pin) can pass through the notches and a hole in the screw shank. This prevents the nut from rotating and working itself loose.
• Rivnuts (rivet nuts) are essentially rivets that have a threaded hole in them to accept a screw.
Rivnuts are great for securing panels in sheet metal.
• Inserts come in a variety of forms suited to specific materials. They’re similar to rivnuts in that they provide a threaded hole, but can be put into plastics, wood, and composites. Inserts for wood are commonly called T-nuts (as opposed to T-slot nuts) and get hammered into a hole in the receiving material. Thermoplastic inserts go into acrylic and other plastics by heating the brass insert, which melts the material around it. Once cooled, this forms a solid mechanical bond between the insert and the plastic.
WASHERS
These are an important part of mechanical fastening systems. They are essentially a thin disc with a hole in the middle.
• Plain washers are typically used to spread the load that a bolt exerts on the material it is fastening together, in order to prevent the bolt head from deforming and/or pulling through the material.
• Lock washers are designed to help secure the bolted joint from working loose due to vibration. There are many types, but the most typical is a “helical” or “spring” lock washer, which is essentially a plain washer that has been cut and sprung out of plane.
When a bolt exerts a load on a lock washer it is forced to flatten out, causing the washer to dig into the material you are fastening. It also provides some axial force on the bolt by virtue of it being forced flat. In other words, the lock washer tries to spring back. The forces generated by both digging-in and spring-back help to keep the bolt from rotating loose.
In most applications where a lock washer is needed, you will also want to use plain washers. The illustration above shows a typical bolted joint that has both a lock washer and plain washers. Note that only one lock washer is required for a joint such as this. However,
two plain washers are used to distribute
the load over a larger area of the materials
being joined.
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