26.Jun.2012 Applying Threads and Fastners:
Behind all the data in callouts on plans and bills of materials are the real features that hold our designs together. While final selection is often an engineering decision, understanding fasteners makes creating practical designs and prototypes possible. It also helps us to understand why existing products are designed and produced the way they are and to understand the reasons for engineering modifications. Please bear with me if some portions of this are a bit dry. Fasteners are an expansive topic full of concepts and terms most of which were coined by engineers and machinists. Once we get through the basics, we will begin to discuss how to exploit them to make our designs stronger.
It is not my goal to turn designers into engineer, but to give us a window into the reasoning behind their decisions so we can better anticipate their needs. There is no reason that hardware must be shown, but there is also no reason we have to hide it if it is well managed. With integration and care, fasteners can become an integrated part of designs from furniture to electronics.
The beginning of fastener selection begins with a simple set of questions:
1. What are you holding together and does it have any special properties that will limit your fastener choice?
Common examples of limiting factors include materials being too soft or thin to hold threads. In these instances it may be necessary to add threaded inserts, through bolt or use another fastening method.
2. How strong does the joint need to be?
The first step in considering how strong a joint needs to be is to consider the force that will be applied to it. Force comes from several sources including shear forces, compressive force, and torsional forces. While accurate calculations are best left to the engineers, it is not hard to guess the forces being applied to parts of a product such as a chair seat, pad holder or first aid kit bracket and adjust your fastener selection accordingly. This section is really just a reality check to help avoid surprises down line when things are translated into real objects.
3. How will we make the joint meet the required strength?
This question really boils down to how many fasteners do we need and how strong do they need to be. To achieve this we have a wide range of fasteners available to help us. The hard part is choosing the right ones. Optimal selection is often an engineering decision, however for our purposes there are four main factors for us to consider.
What is the fastener made of and how strong is it for our application?
Fasteners are made from everything from plastic and common steel to aluminum, precious metals and exotic high strength alloys.
How large does the fastener need to be? This is often the primary variable we adjust within a given fastener style since the price difference between smaller sized fasteners is often minimal. In mass quantities, optimization here will save money but it requires analysis outside of our capabilities. It is not hard to assume that for an example in a prototype, we think a few 6-32 machine screws will be sufficient to hold the baseplate of a lamp in place. If the base was heavy and we were nervous our screws might not be enough, we could either add additional screws or simply switch to 8-32 machine screws without any significant changes being made.
How strong is the material that the fastener is being secured into?
Often the fastener may be far stronger than the material it is inserted into. Common examples of this are screws threaded into plastic or wall hooks mounted in drywall. If a child hangs from a towel bar it is not usually the fasteners holding it to the wall that fail but the fasteners themselves being pulled out of the wallboard that causes failure. The same sort of failure can be seen in threaded joints when tightened excessively in low strength materials like plastic. The behavior is called thread tear out and can significantly compromise the holding strength if it is allowed to occur. To counteract this, coarse threads with large bearing areas and a high percentage of thread engagement are used. It is often a good idea to manage the torque used to install fasteners if there is a risk of tear out. This can be as simple as the clutch on a power driver or simply requesting that screws are simply installed “snug tight” in installation instructions.
How many fasteners will be needed?
Very few things are held in place with a single bolt or screw. It may be possible but it is often desirable to spread the load out over several fasteners. This both reduces the stress on a single fastener allowing for smaller components to be used and also provides redundancy.
Once the fasteners are selected, the hard part is done and all that remains is simply locating and implementing them. When it comes to fasteners it never usually hurts too slightly over engineer in the concept stages. Later on things will likely change based on engineering analysis and calculations but it is often easier to start with a shell that assumes there will be fasteners of some sort involved. Simply omitting or modifying a few fasteners later is usually easier than trying to hide extra screws in a basically finished design. Another benefit of planning for fasteners early on in products is that it allows greater control in aesthetically motivated placements and an opportunity to explore options that not only are low in cost but also low in visual impact or that embrace or enhance your intended design.
As you begin to become aware of fasteners and how they are used, it is easy to begin to take note of the common products around you and how they are assembled. This observation is actually one of the most important reference materials available to designers since similar products already on the market can be evaluated quickly and used for inspiration.
To make a joint with a bolt or screw two options exist.
Through bolting is accomplished by simply boring a clearance hole through two or more pieces of material then securing them with a bolt and nut. While machine screws may also be used in the same manner, all bolts technically need a nut to fasten them. The common hex bolt and square head bolt can also be treated like a screw to further confuse things since they feature externally wrenching heads but they are the exceptions to the rule since other bolts such as carriage bolts and plow bolts rely on a nut for tension.
The clearance hole is simply a hole large enough to pass the threads of the fastener through without interference. Many modern software packages have included clearance numbers built in as an option but they can also be looked up in good tap and die charts.
As an example, a .25-20 bolt may fit through a .25 inch hole with some resistance. To achieve a smoother fit a letter F bit could be used to drill a tight fighting hole .2570 inches in diameter that allowed easy fastener travel and held tight tolerances.
You may have noticed the bit used to bore the clearance hole was not a common size you likely have in your tool box and it was specified with a letter. This is not a mistake; there are three series of drill bits in common usage. The one used most often in general fabrication are the fractional sizes. They are called that because they are based on common fractions. 3/32”, 1/8” and 1/4″ are all examples of fractional bits. Lettered bits also exist in sizes A-Z and numbered drill bits ranging from #1-80 or so. These bits in many cases exist specifically to allow for a class of fit for either clearance holes of threaded holes and are essential for getting tight professional fits.
If you don’t have a favorite tap and die chart of your own or a mechanical reference book you use for tidbits like drill sizes and tap selection, this free chart is highly useful: http://www.physics.ncsu.edu/pearl/Tap_Drill_Chart.html . It is hosted by North Carolina State University and well laid out. I frequently refer to this one over any of my printed versions simply because it’s always there at your desk when you need it. If you find yourself getting deeper into the selection of drill bits, taps and other mechanical bits as part of the design and prototyping process, investing in a copy of the Machinery’s Handbook is a good idea. This several thousand page shop book is filled with mountains of information on machining operations, lubricants and all things mechanical you will likely run across and plenty of things you hopefully never will. While the most recent edition is always a good idea, most of the basics never change so it is Ok to save a few dollars and go used on one of these if you find one from the last decade or so for a good price.
Almost any hole can be tapped provided it is deep enough to both hold threads and allow the tool to pass. Tapped holes come in two main styles. Through tapped holes are the simplest form of tapped hole. In a through tapped hole the piece receiving threads is drilled smaller than the fastener and threads are cut through the entire hole using a tap with a tapered nose. These tapers help to both align the hole and push chips ahead of the cutting head reducing clogging. In high speed tapping machine, spiral point taps are used to accelerate the process. Through tapped holes are used because they are easy to produce and allow the fastener to protrude to the other side in adjusting applications such as set screws and levelers.
Blind tapped holes are tapped holes that do not entirely go through material. These blind holes allow screws to be used without being visible outside a product or allowing the possibility of leaks. To create a blind tapped a hole smaller than the tap is drilled and the hole is then threaded with a bottoming tap. Bottoming taps have a flat face allowing them to thread closer to the bottom of a hole. Because bottoming taps cannot push chips ahead of them and out of the part, they must feed chips back up the hole behind the cutting faces of the tap. This allows for a greater possibility of chips clogging the tap resulting in poor cutting results or causing the tap to stick and break. In hand applications this is solved by frequently backing the tap out to break and remove chips. To help taps cut it is desirable to over drill blind tapped holes where possible. This extra clearance passed the required threaded depth of the hole allows space for chips that fall to the front of the tap. If space is available for a deep enough over drill it is often possible to use a traditional tap to create blind holes instead of a blind tap. This allows for faster production of blind tapped holes with fewer stuck taps and problems. In general practice it is a good idea to allow at least 1-1.5 times the diameter of the hole passed the intended threaded depth of holes where possible to allow room for chips and the tap body itself.
Because the actual depth that allows for efficient production tapping varies on material selection, tap choice and many other variables it is best to give as much freedom to the engineers and manufacturers of parts as possible. Sadly few definitive formulas guarantee trouble free tapping in all situations so it is best to err on the side of caution where possible. To give manufacturers room to work within the specifications, it is best to only specify minimum threaded depth required and any maximum drill depths. Manufacturing optimization will then work within these constraints to produce that feature with the shortest over drill practical to allow reliable and rapid production.