What is a Tapped Hole


Being able to create a tapped hole is a must-have skill for anyone who likes to build things. A lot of people have never heard about tapping holes and don’t know the purpose of them.

So, what is a tapped hole? A tapped hole, also called a threaded hole, is a circular hole that has been through the drilling and tapping processes.  The tapping process adds threads to the walls of the drilled hole. The process usually occurs in metal where a typical nut and bolt won’t work.

There’s a lot to know about tapping holes but keep reading, and I’ll easily explain things so you can get back to building!

Why Create a Tapped Hole

A person needs to create a tapped hole if they plan on threading a screw or bolt into metal. This is usually to be able to attach two parts without a more permanent solution, like welding.

Think of everything you buy that needs to be assembled. Shipping products that are unassembled and in the smallest footprint possible saves a lot of money on shipping and labor costs. And how do you assemble these things? You usually attach the metal pieces with screws or bolts, and the places that you screw these fasteners into are tapped holes.

I’ll give you another example. When working with wood, you can usually drive a screw straight into the wood, or pre-drill a pilot hole to make the screw go in a little easier. The threads on the fastener cut their own threads into the wood. This doesn’t work in metal because it’s much harder than wood.

Imagine trying to drive a screw into a hunk of metal without a hole. It just doesn’t work. Even drilling a pilot hole isn’t enough to be able to drive a screw or bolt into metal because the threads on the screw get absolutely destroyed. The metal is so hard and dense that it just scrapes away the threads of the fastener, and you waste a lot of time.

The proper way to get a screw or bolt into metal is to drill a hole, tap it (Create the threads on the inner part of the drilled hole), and then drill in your fastener. It’s an easy process, and it just takes a little longer than drilling a screw into wood.

How do you actually create a tapped hole? I’ll get into more detail below, but the most straightforward answer is to drill a hole and slowly turn a tap into it. What kind of tap you use depends on your project, and whether you’re using a blind hole or a through hole.

What is the Difference Between a Blind Hole and a Through Hole

Like I said before, the choice of tap depends on the hole you’re working with. A blind hole is a hole that’s created that doesn’t go entirely through the material it’s drilled in. A through hole, also called a clearance hole, is a hole that’s created and it goes 100% through the entire material.

A good way to remember which one is which is to imagine looking through both of the holes. If you can’t see through a hole, then you’re blind, and you’re looking at a blind hole. If you see clearly through the hole, then it’s a through (Clearance) hole. Clever…I know…

When using nuts and bolts, you’d be using a through hole because the bolt has to go through the entire hole and stick out on the other side. If it didn’t get all the way through, the nut wouldn’t have anything to attach to. Through holes are pretty easy to tap because the taper tap can go out the other side of the material you’re working with.

Side note…Make sure to buy bolts that are threaded all the way if you need the head of the bolt to touch the material. If the threads of the bolt are only threaded at the end, then your bolt will stop turning once it hits the non-threaded part, and the head will be sticking up.

Blind holes that are shallow can be hard to tap, and you need to make sure that the fastener you’re planning on using doesn’t bottom out before it’s finished being tightened. This can cause the fastener to loosen and fall out.

What Are the Three Types of Taps

The three main types of taps are taper tap, plug tap, and bottom tap. Each is designed for a specific purpose or stage in the tapping process. These three taps can thread just about any hole you throw at them. I’ll go over the basics on how they’re designed, and when and when to not use them below.

Taper Tap

●     Design – A taper tap has a slight angle to the threads, which is also called a chamfer. Usually, the first seven to ten threads aren’t completely formed. The tapered angle of the threads allows the threads to be started much easier than if you started with a bottom tap.

●     When to Use – I recommend using a taper tap when you’re starting to tap every hole. It’s designed to start tapers, so why wouldn’t you use it then? If you’re tapping a new through hole, this is the type of tap to begin with.

●     When to Not Use – Don’t use these if you’re trying to tap a shallow blind hole. The tap won’t be able to poke through the hole. Also, the threads on the taper tap won’t be deep enough to bite because the taper angle is too steep.

Plug Tap

●     Design – A plug tap is like a taper tap, but it has fewer threads that are tapered off. Usually, the first three to five threads are tapered. It is in between a taper tap and a bottom tap. This is why some manufacturers call them “second taps.” They also take more force to turn than using a taper tap because more threads are trying to be cut at once.

●     When to Use – Use a plug tap after using a taper tap when starting new threads on a hole. They’re also perfect for repairing threads that are already created but need a little fixing. You should also use a plug tap when you need to tap a hole deeper than a taper tap’s threads can get to. Finally, if you’re tapping a shallow blind hole, try starting with a plug tap if your taper tap’s threads won’t bite.

●     When to Not Use – You wouldn’t want to start tapping a hole with a plug tap on a through hole because it’s harder to get started than a taper tap. What happens is if you try to start a tapped hole with a plug tap, it has a tough time catching the sides of the pilot hole. If and when it finally does grab, it is very likely that your tap won’t be perfectly aligned with your hole, and your threads will be crooked in your tap.

Bottom Tap

●     Design – A bottom tap is designed to thread drilled holes almost all the way to the bottom. Only the first one or two threads are chamfered, which makes them harder to start with. A bottom tap also takes the most amount of force to turn out of the three types of taps because it’s threads are the deepest along the entire tapping bit.

●     When to Use – A bottom tap is usually used after the plug tap when tapping a new through hole. You should also use a bottom tap if you are threading a shallow blind hole, and you need your threads to go as deep as possible. If your hole is shallow, then you might have to start your threads with a bottom tap. A taper tap’s starting threads won’t be deep enough to get a good bite on the hole.

●     When to Not Use – A bottom tap shouldn’t be used to start any hole, if at all possible. Bottom taps are very hard to work with when beginning holes because it probably won’t be aligned correctly if you actually do get the threads of the tap to bite.

Every tapping scenario has its exceptions, so use this information as a good base of knowledge to start with. Sometimes you won’t have a taper tap, and you’ll be forced to start with a plug or bottom tap. It’s not ideal, but when the job has to get done, you have to use the tools that you have available. There are also other types of tapping tools, but they’re more advanced, and I’ll talk about them another day.

What is the Difference Between Drilling and Tapping

Sometimes, drilling and tapping get referred to or used the wrong way in sentences, so I want to try and help explain the difference between the two.

The difference between drilling and tapping is the drilling process happens first and creates a smooth round hole in a material, usually with a drill bit. The tapping process happens after the hole is drilled, and it creates threads on the inside of the hole, using a tap.

Additionally, the process of creating threads on the outside of a cylindrical object (Also called a die cut) is accomplished using a die. An example of a die cut is the threads on a bolt.

Tapped Hole Conclusion

Hopefully, I helped you understand what a tapped hole is, and when you’d need to create one. Knowing the common types of tapping bits helps diagnose when and where to use them. If you have any questions or comments about tapping, feel free to ask below!

Mark

I've been building things for many years, and I want to share what I've learned to try and help whoever I can. Make sure to checkout the homepage for my most read posts!

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