Making Things Move: DIY Mechanisms for Inventors, Hobbyists, and Artists - Dustyn Roberts (2010)
Chapter 3. Screw It or Glue It: Fastening and Joining Parts
When you first build a project—whether it’s for your home, an art installation, or a robot competition—something always goes wrong. Sometimes the cause is dead batteries, broken connections, or a dog using your project as a chew toy. But the problem I see most often is the use of inappropriate materials and fasteners that don’t hold up over the course of your mechanism’s intended lifetime.
Usually, your mechanical devices will have a base, a skeleton, or some other kind of central structure that will need to be put together. Knowing how to do this efficiently will save many headaches later on. There are two main ways to join components to each other: nonpermanent joints and permanent joints. I recommend using nonpermanent joints whenever possible, because they allow you to take things apart without damage. However, I will discuss permanent joints for use in situations where they are the only option.
This chapter will give you a general understanding of various ways to put things together. In later chapters, I’ll talk more about specific situations, such as attaching things to motor shafts.
Nonpermanent Joints: Fasteners
Nonpermanent joints are practical and quick, and they come in a variety of designs. The term fastener is used to describe the various nuts, bolts, nails, screws, washers, and other components that have been developed over the years to hold things together. Figure 3-1 illustrates the various types of fasteners.
FIGURE 3-1 Types of mechanical fasteners
When first building a project, you can bet you’ll need to take it apart at some point. Nonpermanent joints make this easy to do.
NOTE From Velcro to magnets and cable ties to hose clamps, there are dozens of ways to fasten parts together. Finding the perfect solution is less important than finding something that works for your particular project and the intended life of your mechanism. More fastening techniques and components are entering the market daily, so there’s a very good chance you’ll find something that works for you.
Screws, Bolts, and Tapped Holes
For the purposes of this book, the words screw and bolt are interchangeable. Some people use bolt when the component is paired with a nut, and screw for a component that threads into a tapped hole in another piece instead of using a nut. However, the distinction is not generally agreed upon and is not important here.
Major Diameter and Threads per Inch
Figure 3-2 shows the characteristics of a screw. The most important ones are major diameter (outside diameter) and threads per inch. The diameter can be measured in inches or millimeters. In the inch system, screws with a diameter of 1/4 in or more are labeled with the diameter first, then the threads per inch. For example, a 1/4-20 screw is one with 1/4 in major diameter and 20 threads per inch of the screw shaft. For some reason, screws with a diameter less than 1/4 in are given a number. For example, a 4-40 screw has a diameter of 0.112 in. To convert from screw number to decimal diameter, use this formula, or refer ahead to Table 3-1.
diameter (inch) = (screw number × 0.013) + 0.073
Most screws come in a standard (coarse) pitch as well as a fine pitch (more threads per inch). The pitch is the distance between threads. In the metric system, screws are labeled with the diameter first, then the pitch. So a 3mm diameter metric screw with a standard pitch of 0.05mm is called an M3×0.05. We’ll focus on the imperial system in this book, but the metric system is good to know when you come across components (like some motors) that have holes in metric sizes.
FIGURE 3-2 Screw anatomy
The easiest way to join two materials with a screw is to drill a clearance hole through both of them, insert a screw that is longer than their combined thickness, and use a nut on the opposite side to sandwich the pieces together, as illustrated in Figure 3-3. As a rule of thumb, make sure the screw extends all the way through the nut and sticks out at least two or three threads past it. The right clearance hole gives you just enough room to put the screw through, but not so much that it’s sloppy. A close fit is standard, but a free fit is a little larger and will give you more wiggle room for poorly aligned parts.
Sometimes it won’t be possible to use a screw and nut due to space or other constraints. In this case, you join the two parts by screwing directly into one of them.
FIGURE 3-3 Anatomy of bolted joints: using a clearance hole with a nut on the end (left) and screwing into one piece that is tapped (right)
With wood and most plastics, it is best to drill a pilot hole in the piece that you’ll drive the screw into to avoid splitting or cracking and make it easier to install the screw. A pilot hole is just a hole that’s slightly smaller than the screw’s major diameter and makes it easier to install.
You can find the right size pilot hole for a corresponding screw by looking it up on McMaster. For example, for part 90031A153, a wood screw, the pilot hole size in soft wood is 1/16 in. In metal, you must drill a pilot hole equal to the tap drill size listed for your screw. Then use a tap that matches your screw size to create the threads in the piece you are screwing into. This is not as complicated as it may sound.
For a well-designed joint, use Table 3-1 to determine the clearance hole size that matches your screw. The table also lists the tap drill sizes. (The table information is from www.stanford.edu/~jwodin/holes.html and www.efunda.com/DesignStandards/screws/tapdrill.cfm.)
Table 3-1 Screw sizes and tap drill table
Project 3-1: Drill and Tap a Hole
As an example, let’s drill and tap a piece of aluminum for a 4-40 screw.
• Scrap piece of aluminum
• Clamp (like McMaster 5031A6) that is wide enough to fit your scrap aluminum piece and worktable
• Center punch or other hard, sharp, pointed object
• 4-40 screw (any length and head style will do)
• Small tap handle (McMaster 25605A63)
• 4-40 tap (one that is designed to fit into the tap handle)
TIP The chamfer is the tapered part at the front of the tap that helps guide the tap into the hole. Choose a taper chamfer (McMaster 25995A125) versus a plug or bottoming chamfer if you have the choice, because they are easier to start.
• Tapping fluid or WD-40
• 0.089 in diameter drill bit, also known as wire gauge size 43 according to Table 3-1 (any good drill bit set should have this size, or you can buy it individually, and choose the standard jobber’s length if you have a choice; McMaster 30585A57).
• Drill (any hand drill will do, cordless or otherwise)
• Small rounded file or countersink tool (like McMaster 2742A26)
• Safety glasses
1. Put on your safety glasses and clear your workspace.
2. Use the center punch to make a mark where you want to drill. This is not strictly necessary, but will prevent the drill bit from wandering when you start drilling the hole.
3. Clamp the aluminum down to your worktable. Install the 0.089 in drill bit in your drill, and drill a hole either partway through the material (blind hole) or all the way through (a through hole is much easier to tap). You will need to clamp or hold down your scrap aluminum while doing this, depending on its size.
4. Clear off any metal chips or burrs from the hole with your file or countersink tool.
5. Spray or squirt some tapping fluid or WD-40 on the hole. Although not strictly necessary, this will make your job easier and decrease your risk of breaking the tap. It’s much harder to tap a dry hole.
6. Install the tap in the tap handle, just as you would insert a drill bit into a drill.
7. Place the end of the tap in the hole you just drilled, and position the tap perpendicular to the material (see Figure 3-4). The tap must stay perpendicular to the material surface the whole time or it will break. Carefully turn the tap handle clockwise one or two turns until you feel the little teeth on the tap start to bite into the aluminum. From this point on, turn the handle one-half turn clockwise, then one-quarter turn counterclockwise. This backing up is necessary to cut the aluminum in small pieces so the chips don’t build up, plug up the tap, and cause it to break.
CAUTION Taps are made of material that is very strong and sharp, but very brittle. They will break surprisingly easily if you twist too hard. Don’t try to correct for misalignment once you’ve started to tap the hole.
FIGURE 3-4 Ten-step tapping procedure
8. Once you’ve gotten to the bottom of the hole, or through the material, unscrew the tap all the way back to remove it.
9. Wipe, rinse, or blow away any metal chips and excess tapping fluid.
10. Try screwing the 4-40 screw into the hole. Ta da! It should twist in easily.
Head and Drive Styles
The next important considerations with screws and bolts are the head style and drive style. Figure 3-5 illustrates some common drive styles.
You are probably familiar with the standard flat head and Phillips head screwdrivers and matching screw drive styles. Maybe you’ve even stripped the screw head of a Phillips head screw. This happens when you try to tighten or loosen a screw that is stuck, and the screwdriver slips out, squishing the material on the head so much that it becomes impossible to tighten or loosen at all. If this happens, you’re screwed. Avoid this problem by being particularly careful with Phillips head screws, or avoiding them altogether by using socket cap or hex head screw drive styles.
FIGURE 3-5 Drive styles of common screws
TIP There is one way out of a stripped screw situation. Use a Dremel tool with a cutting wheel to cut a slot in the screw head that will fit a flat head screwdriver. Use your safety glasses and go slow, because these cutting wheels are very brittle and break easily.
Socket head screws are slightly less convenient because you need to have a different Allen wrench (also called an Allen key or a hex key) for each screw size, as shown in Figure 3-6. However, these types of screws are much less likely to strip. This is especially true when you’re trying to undo a screw you accidentally glued in place because you thought you were done. Socket head cap screws are designed to resist tension in the joint; button heads are not—they are designed to look nice. Hex head screws are more popular for heavy-duty applications, or for when you can’t get at the screw head with a conventional screwdriver but can get to it from the side with a wrench.
The head style you use will vary depending on your application and convenience. Wood screws commonly come in flat head styles that you can drive right into the wood. Socket cap screws and machine screws come in a variety of styles. Some of the most common head styles are shown in Figure 3-7.
FIGURE 3-6 Allen (hex) key sets
NOTE The length for flat head screws is measured from the flat top. The length for all other styles is measured from under the head.
If you want your screw to sit flush with the material, instead of on top of it, you will need to use a countersink or counterbore (see Figure 3-8). McMaster (among others) sells special countersink and counterbore drill bits for this purpose.
FIGURE 3-7 Head styles of common screws
FIGURE 3-8 Counterbores (left) and countersinks (right) allow screws to sit flush with the surface.
The last thing to worry about when choosing screws is material. The vast majority of the ones you use will be steel. Choose stainless steel if you don’t want the screws to rust. A plain steel screw with a zinc-plated or black-oxide finish will also protect from rust and might be cheaper than the stainless steel option.
Threaded Rods and Speciality Screws
Threaded rods, or all-thread, are like long screws with no head. You can get them full or partially threaded and in many different lengths and sizes. One can act as a shaft to align multiple parts, then sandwich them together with nuts on each side. They are also used as push-pull rods to create small motions, like steering a rudder on a model airplane.
There are more types of screws than I have pages to write about them, but a few deserve a quick mention (see Figure 3-9):
• Shoulder screws have a smooth cylindrical shoulder under the head before the threads start that is great to use as a spacer or shaft.
• You can use U-bolts to create a loop on an otherwise flat surface.
• Eyebolts serve a purpose similar to U-bolts, but need only one mounting hole (versus the two you need for U-bolts).
• Self-drilling and self-tapping screws compress the ten-step tapping procedure to just one step, but are really useful only in wood, sheet metal, and soft plastics.
• Set screws don’t have a head at all and are great for locating parts on shafts and connecting gears and couplers to motors (more about that in Chapter 7).
• Binding posts (also called barrel nuts) consist of a screw and a long nut (or barrel) with a flange on the end. These are great for creating linkages or sandwiching flat parts together that still need to rotate freely.
FIGURE 3-9 Specialty screws
Nuts thread onto the end of screws and are the easiest way to form a bolted joint. Choose a nut with the same thread size and pitch as your screw or it will not fit. The following are some types you might use in your projects:
• The most common nut is called a hex nut.
• Next most common in usefulness is a locknut. Locknuts have a nylon insert or deformed thread that makes them vibration-resistant and very secure. However, they are generally not reusable.
• Wing nuts are great if you need to assemble and disassemble connections without tools.
• Tee nuts are ingenious little components that you can hammer into a hole in wood and soft plastics to create metal internal threads with little effort.
• Rivet nuts do the same job as tee nuts in sheet metal assemblies, but require a special installation tool and are not easily removable once installed.
Washers serve at least four purposes:
1. Allow you to avoid marring the base material during installation
2. Spread the screw fastening force over a larger area
3. Act as a spacer to avoid stressing the tiny curved section directly under the screw head
4. Indicate proper tightness of the bolt—when the washer is snug and stops spinning, it’s time to stop turning the screwdriver
For a well-designed bolted joint, use spring-lock washers in addition to standard washers (see Figure 3-3). The job of the spring-lock washer is to keep the joint tight even if the screw vibrates loose. When a spring-lock washer is compressed, it looks just like a fat washer with a gap in it, or a C shape. However, if the screw begins to loosen—either through wear and tear or vibration—the spring-lock washer springs up as the C shape untwists to fill the tiny gap created.
Nails and Staples
Nails are more permanent than screws but can still be removed relatively easily. The following are types commonly used in projects:
• Double-headed nails are good to hold something together temporarily and much easier to remove than common nails.
• Finishing nails have small heads designed to sink into the material, so never use these if there’s a chance you’ll need to take one out.
• Small nails, called upholstery tacks, can secure fabric stretched over a frame, as used in furniture designs.
Nail guns make short work of this job. Similarly, a staple gun, or just a normal stapler opened up, makes short work of stapling material together to temporarily secure it.
Nails and staples are often the lazy way out of designing good, nonpermanent joints. I recommend using them only for temporary holding or early prototyping stages, not for general use. Removing wrongly installed nails and screws usually renders them unusable, damages your base material, forces you to line up your pieces again, and wastes more nails or screws. This cycle can get repetitive and tedious. Try designing your projects to use nuts and bolts from the early stages, and resort to nails and staples only if you have no other choice.
If you’re ever put together furniture from IKEA, or used an exercise machine at a gym with a weight stack, you’ve dealt with pins used for fastening and alignment. You can find wooden dowel pins at any arts-and-crafts store, but metal dowel pins and spring pins are more common to use for aligning parts.
If you make a hole in a part just slightly smaller than the pin diameter, you can hammer or press in the pin, and friction will keep it in place. If you make a hole just slightly bigger, your pin will slide in easily and be removable.
Retaining rings can be used with pins and shafts to stop them from sliding all the way through holes or to create pivoting joints. Figure 3-10 shows an example of a retaining ring used in a piece of gym equipment.
FIGURE 3-10 Retaining rings on gym equipment keep the shaft of a pivoting joint in place.
You need to carve a slot in a shaft to hold retaining rings in place, but this is easy to do if you have access to a lathe. See Chapter 9 if you want to know more about lathes.
Permanent Joints: Glues, Rivets, and Welds
Permanent joints are used when two pieces are designed to go together and never need to come apart. This includes welds, rivets, glues, and epoxies. Although a glued joint is quick and easy, permanent joints can cause headaches if something goes wrong and you need to take things apart. Permanent joints are a last resort. Consider using nonpermanent joints first.
Adhesives come in many different forms, from the common white Elmer’s glue we all used in elementary school to two-part epoxy. They can take anywhere from a few seconds to a week to dry and reach full strength. Some are designed to join similar materials, while others can be used on dissimilar materials as well. (Go to www.thistothat.com for some good advice on which adhesives to use based on what you are gluing together.)
As a rule of thumb, make sure both surfaces are clean, and give them texture with some sandpaper or a file to give the glue more surface area to grab.
Wood glues should be used to join wooden pieces along the grain. When used this way, the bond is very strong—sometimes stronger than the wood itself.
Common white multipurpose Elmer’s glue will work for wood, but yellow wood glue is better suited for most applications. With yellow glue, you have about 15 minutes from when the glue leaves the bottle to when it starts to dry.
The next step up is Titebond II, which you should use if you need water resistance, a tackier working material, and a faster setting time.
Finally, there are polyurethane foam glues (like Gorilla Glue) that react with moisture to expand and fill gaps and crevices and dry to form extremely strong joints.
The classic 5-minute epoxy is a favorite of hobbyists. Epoxy is a glue that comes in two parts and is activated only when these two parts are combined. This mixing can be done by hand with a popsicle stick, or more conveniently through an applicator gun and mixer nozzle.
Epoxy can be used to bond many different types of plastic, metal, and composites. It dries hard and can be sanded or painted.
You can find epoxy at just about any hardware or home-improvement store (and, of course, online at McMaster). Look for epoxy putty in the plumbing section of hardware stores. Propoxy, FastSteel, and QuikSteel are common brands (see Figure 3-11). It’s a putty-based two-part epoxy that hardens like steel after being mushed together and exposed to air for about 20 minutes.
Plastic Glues and Solvents
Glues made specifically for plastics can be particularly effective because they can react chemically with the plastic to melt the two pieces into a strong joint. Weld-On is a well-known brand for bonding acrylic, and is used extensively in the architecture and modeling industries. Other glues, epoxies, and solvents that cite specific plastics as their target materials work great as well.
FIGURE 3-11 Epoxy putty—the great project saver
CAUTION Weld-On is pretty nasty stuff. Open the windows, use a fan, and wear a respirator mask if there is not good airflow where you work. Avoid direct inhalation. Use gloves to avoid skin irritation. Wear splash-proof safety goggles. The fumes can irritate your eyes even if you don’t spill.
When screws are used in metal and are exposed to vibrations, shock, or varying loads, it is common to use a threadlocker, which is a type of glue used to fix the screw into the tapped hole or nut. A threadlocker also seals the bolted joint from fluid leakage and helps prevent corrosion.
The Loctite brand name has become synonymous with this application. It comes in dozens of varieties, so read the description to make sure you choose the best one for your application.
Most kinds of super glue come in small or one-use tubes and dry within seconds. The gel-based ones are the easiest to use because they are thicker and don’t drip or run easily.
In general, these glues are weaker than epoxies and should be used only as a last resort or for a temporary fix. Avoid using them in applications in which they are under force or pressure. Super glues are better suited to fixing your sunglasses than for holding moving mechanisms together.
You can find super glue just about anywhere, from your local drugstore to McMaster.
A hot glue gun and some glue sticks definitely deserve a place in your toolbox. Hot glue can be used for just about anything, from gluing cardboard to insulating exposed metal on electrical components. It’s also inexpensive and easy to find at any arts-and-crafts store.
CAUTION Be careful when the glue gun is plugged in so you don’t burn yourself or the table on which you’re working.
One disadvantage is poor vibration tolerance. Hot glue tends to separate from the base material if much vibration happens, or if anything attempts to pull the two things apart with much force. Also, try not to glue something that melts when it gets hot, or you’ll end up with hot goo.
Common translucent office tape or double-sided tape is not that useful for our purposes, but the double-sided foam tape you can get at hardware stores can be handy. You can use this tape to mount components that don’t have mounting holes, like small motors, or to hold something in place while you drill holes to fasten a component more securely.
CAUTION Double-sided foam tape is very sticky and hard to remove from surfaces once attached. Make sure you’ve decided where the components you are using should be before using it. If you do need to peel off a connection and redo it, scrape off as much of the tape as you can with a hobby knife, and use Goo Gone to remove the rest.
Duct tape is easy to tear and relatively strong. Its fancier cousin gaffer tape is similar, but leaves no sticky residue when removed.
A blind or pop rivet consists of a small, flanged metal tube with a metal rod running through it and a ball at the end (see Figure 3-1). A rivet installation tool pushes the rivet into a predrilled hole and pulls the rod back into the tube, and the resulting distortion of the ball flares outward. The rod breaks off in order to form another flange on the back side of the material. These flanges sandwich two or more pieces of sheet metal or other thin materials together. You can also find a rivet tool attachment for a hand drill that works in a similar way. Once installed, a rivet is not removable, except by drilling it out to create a hole bigger than the initial hole used before riveting.
Rivets are used primarily when there is access to only one side of the joint, or when there is no room for a screw-and-nut combination. They are also used for aesthetic reasons to keep a surface relatively flat looking, and are used in mass production of commercial parts after a design has been finalized. You can find all kinds of rivets and rivet tools online at McMaster.
As with other permanent fastening methods, I recommend avoiding rivets unless they are a last resort, because you will not be able to easily redo and adjust your designs.
Welding, Brazing, and Soldering
Welding, brazing, and soldering are ways of joining metal with metal by using heat. They are all permanent joints and need to be reheated, cut, or both in order to reverse them. Welding melts two similar metals together, sometimes using filler rod of a similar metal. Brazing and soldering both use heat to melt dissimilar metals, as a kind of glue between two pieces.
In welding, two metals are joined together by melting them along a seam or at a spot, sometimes by using a similar metal to fill the voids. The two main types of welding are gas welding and arc welding. Gas welding uses a blow torch that combines a fuel (commonly acetylene gas) with oxygen to produce a flame that melts the two pieces of metal you are welding together. Arc welding uses a DC or AC electrical current that is converted to heat to melt the materials. There are two common types of arc welding:
1. Tungsten inert gas (TIG) welding uses a pointy metal electrode to initiate the arc between it and the materials to be welded. You can also supply a filler material in this arc as you go along a weld seam, but this method takes considerable coordination.
2. With metal inert gas (MIG) welding, a wire is fed at a constant rate through the welding tip while the materials you are welding heat up. This method still takes practice to master, but is slightly easier to learn than TIG welding.
NOTE If you want to learn more about welding, look for classes at your local art or community center. For example, in the New York City area, check out the Educational Alliance () and 3rd Ward (www.3rdward.com) sites.
Welding is useful for heavy mechanism work, but isn’t as useful at small scales. It also has a lot of overhead (equipment, safety supplies, and so on) compared to other fastening techniques. That said, you will definitely feel more hardcore if you learn to weld, and it always helps to have another fastening technique at your disposal when the other options just won’t work. For example, one of my former students made Skybike, a bike you ride upside down, by welding together parts of old bicycle frames (see http://itp.nyu.edu/~md1660/skybike.html). This project would have been much more difficult with any other fastening technique.
Brazing uses a copper-zinc or silver-based alloy filler with a melting point above 800°F to glue two pieces of metal together. Although the melting point of the filler is lower than that of the metals being joined, the metal parts both melt a bit, and this fusion helps the joint strength.
Copper brazing is a popular way to join tubes for home plumbing systems. Silver brazing (sometimes inaccurately called silver soldering) is used extensively when making silver jewelry.
Braze welding is the term used for joining metals with a dissimilar filler rod. Although these joints are weaker than in traditional welding, the advantages are that you can join dissimilar metals and minimize any distortion from heat.
Soldering traditionally uses a lead-tin filler with a melting point below 800°F, although lead-free solder is becoming more popular to reduce e-waste, in compliance with the Restriction of Hazardous Substances directive (RoHS).
Soldering is not a fusion process because the base materials don’t melt, so these joints are the weakest we’ve talked about so far and are generally only for connections in electrical components and wires. Even with these components, you should solder only when there is no other way to make the connection. Solderless breadboards were created for prototyping purposes, and we’ll talk more about using them in Chapter 6.