Using threaded holes is one of the most reliable ways to keep parts together while allowing them to be removable. While a threaded hole might look simple, there’s a lot going on under the surface that you should know about. In this beginner guide, our machining experts will describe what threaded holes are, how they work, and how to include them in your next design.
What Is a Threaded Hole?
A threaded hole is a way to fasten together two pieces using standard bolts. The hole itself has internal threads which look like a coil inside of the part. It’s achieved by running a die through a hole and cutting the thread out.
Once cut, the thread will be the perfect size for a standard bolt to screw into the hole and fasten into position.
The Use for Threaded Holes
The main function of a threaded hole is to connect two or more parts and give you the option to remove them as needed. If you pop the hood of your car, you’ll see bolts and threaded holes all over the place. Specifically, there are brackets that connect your engine to the framework of your vehicle.
The brackets have clearance holes and there are likely standoffs with threaded holes in them. By tightening a bolt into the threaded hole, you’re tightly squeezing the engine against the bracket and firming it into the frame. If you need to swap out your engine, then you can loosen these bolts and all of the parts will come loose.
The same theory is at play in any part that you put a threaded hole into. It’s only there so you can connect something else to that part, and potentially remove it in the future.
Another way to think about threaded holes is that they’re the less-permanent version of welding.
The Secrets to Specifying a Threaded Hole in Your Design
To get the best results with your threaded holes, you need to know some background information. In this section, let’s talk about a few secrets that will really help when it comes to adding threaded holes to your designs.
Use the Right Thread Type
Within the world of American threads, you have coarse or fine threads, named Unified Coarse (UNC) and Unified Fine (UNF). These two are just one letter away, but they are dramatically different.
The difference is how many threads they have over one inch of length. Looking at a 1/4” bolt, the fine version has 28 threads over one inch (called a 1/4-28 UNF), and the coarse version has 20 (called a 1/4-20 UNC). This means that a 1/4-20 bolt will not fit into a 1/4-28 threaded hole.
Fine threads should only be used when you need to make very fine adjustments, or the bolts are tiny and used in electronics. In almost every other case, you should use UNC threads so you can get the right fit.
Use Nominal Sizes
We can’t stress this enough — always use nominal sizes when it comes to threaded holes. You can find a number of charts online, but American designers should use 1/4-20, 3/8-16, 1/2-13, and so on. All of these are nominal thread and bolt sizes.
Making a custom thread also means making a custom bolt, and this process can be 100 times more expensive than using stock, standard, nominal parts.
If you want us to make a nominal thread for a nominal bolt, the process takes a few minutes. Doing the same for a custom thread could take hours or days — if it’s possible at all.
Remember Blind Versus Through Holes
A through (or thru) hole goes all the way through the body of the part you’re drilling. Holding up the part to your eye, you’ll see through the part.
A blind hole has a certain depth, and doesn’t go through the full body. If you have a 1” thick piece of plastic and drill a 1/2”-deep hole, you have a blind hole. If you keep drilling until there’s nothing more to drill, that’s a through hole.
You need to be careful with blind holes. For one, make sure you have the right depth for your threads. In addition, make sure you have enough material between the underside of the hole and the bottom of your part. If it’s too thin, the drilling process could punch a hole.
When tapping a blind hole, there’s usually a shoulder near the bottom of the hole. Example: if you need 1 inch of threads, then you’ll need a blind hole that’s at least 1-1/4” deep so there’s room for the tap. The 1/4 difference will be a smooth wall with little to no threading, sometimes called a thread start.
Use the Right Depth
Most engineers use the rule of thumb that the thread engagement should be 1.5 times the diameter of the bolt shaft. This sounds more complicated than it really is.
If you have a 1/4-20 bolt, then your threads should be at least 3/8 deep, which is 1/4 (0.25) times 1.5 (0.375).
Of course, pre-load and free length all factor into the final design, so this rule of thumb isn’t always accurate. Still, it’s an easy default value if you’re lost.
Stick to Orthogonal Threaded Holes
It’s always best to call for threads that go perpendicular to the face we’re drilling. Most of the time, we’ll drill into a flat face that’s directly below our CNC milling machine.
Trying to drill and tap an angled face at an awkward position makes things a lot harder. If you can design around this scenario, you can save a lot of time and money in the machine shop.
In this quick guide, we covered what a threaded hole is, how to use it, and tips to design for threaded holes in your next project. When it’s time to build your part, you can trust Rapid Axis. We’re a full-service machine shop that can handle a wide variety of different projects. Get a free quote today.