15-6: Drill Bit Selection & Materials

Drill Bit Selection & Materials

So now you know how to pick the right drill, but there's one more thing we need to figure out before we start turning pieces of wood into swiss cheese. What drill bit are you going to put into your drill?

There are two important considerations as far as the previous question is concerned.

1. What size do you want your hole/what is the hole for?
2. What material are you drilling into?

By the end of this assignment, you should be able to answer these questions well enough to select the correct drill bit in a given scenario, but first, let's get into some basics.

Firstly, it's important to know what each part of the drill bit is called. As you can see below, the part of the drill bit before the cutting edges is called the shaft; the part with the cutting edges is often referred to as the flutes; and obviously, the end of the bit is the point/tip. (Technically, the flutes are the tunnels cut out of the drill bit, but we often refer to those and the cutting edges as one and the same.)

Drill Bit Sizes

With that basic anatomy out of the way, it's time to cycle back to our original two questions. You know how everyone makes fun of the imperial system because all of our units are different? There are 16 ounces in a pound, 12 inches in a foot, and 5280 feet in a mile. Drill bit sizes are similar in the sense that there are three different ways drill bits are classified in the US, and none of them make sense. They are important though, so pay attention and we'll try to demystify it.

Below is a drill bit sizing table. The left hand side of each column tells you what the actual size of the diameter of the drill bit is in inches. The right hand side tells you how we refer to each of these sizes in the manufacturing world. As you look down the chart, notice that each of the systems is color-coded in their own color.

• Yellow shows numbered drill bit sizes
• Green is fractional
• Pink is alphabetical
• Blue is metric

We'll talk about each of these a little more in just a second, for now, I just want you to look at the graph and try to see if you could figure out which system each size is part of without the color coding. Also notice how the systems all seem to be interspersed within each other with little rhyme or reason.

So what's the deal? Why do we refer to drill bits in such a confusing way? Well, it's probably because the standards have evolved over time, but as for how to use them, let's get into a little more detail starting with the easiest ones to understand and going from there.

• Metric Drill Bits: Technically not the American standard, but you'll probably have to work with some non-American parts in industry so it's important to be familiar. These simply drill out a hole corresponding with the size in millimeters. (e.g. a 1.6mm drill bit drills out a 1.6mm hole.)
• Fractional Drill Bits: Again, not too complicated. the fraction represents the exact size of the hole you'll be drilling out.
• Numbered Drill Bits: (e.g. "72") You can think of these as drill bits where somebody thought, "wow, it would be really great to have a drill bit that particular size" that there wasn't a fraction close enough for. Fractional drill bits only go up to 64ths, so a lot of drill bits fit into this category. You'll also notice that a lot of numbered drill bits are on the smaller side because you need more precision when you're working with smaller holes.
• Letter Drill Bits: These are represented by a letter of the alphabet. I like to think of these as the "little bit over, little bit under" drill bits. Two of the many examples of when you might want to use letter bits would be for holes where you want to add a thread afterwards (like a bolt hole) or when you're going to insert a special tool into the hole afterwards like a helicoil. (We'll learn more about these later.)

Drill Bit Coatings/Materials

Hopefully, you now have a better understanding of how to recognize different kinds of drill bit sizes. Now, on to the second question: what material are you drilling into?

You may be thinking to yourself, "A drill bit is a drill bit. Why does it matter what I'm drilling into?" But that's not actually true. Different drill bits are made with different materials and specialized coatings based on what they're expected to drill through, and for good reason. Steel is a lot harder to drill through than plastic, or even wood or aluminum. Drill bits for heavy duty applications are made with tougher, more expensive materials like cobalt, while drill bits for easy applications can be made out of cheaper materials like steel. Here's a table showing which kind of drill bit is made for which kinds of material. Don't get too overwhelmed by this chart, the main point of this is so you know not to use a weaker drill bit on a stronger material. Notice too you can tell what kind of drill bit you're working with based on the drill bit appearance as shown on the bottom of the chart.

There are more kinds of drill bits and materials out there, but this gives a good idea of the most common applications.

One More Thing

Generally, you will be working with the drill shape we've been looking at so far—a twist drill bit.

Twist drill bits

But there are actually other form factors as well:

Step Drill Bits

These drill bits are pretty useful because they drill a different diameter size based on how far into the surface you push them. One use case for these would be if you need a larger hole through a thin piece of material because it will move up gradually in steps to the desired size, making it easier to drill the hole.

Spade Drill Bits

These drill bits are specifically for drilling large holes into soft materials like wood or plastic. We will be using one of these in an upcoming lab. Watch the video below if you'd like to learn more about them.

Other Specialty Bits

Diamond Drill Bit

Forstner bit, used for creating precise, clean, flat-bottomed holes in wood

Countersink drill bits, for creating countersink holes in wood and metal

You will rarely have to use drill bits like these (if ever), but it's interesting to see how many different form factors they come in.

Here's a video going into a bit more about drill bits. (No pun intended)

Examples

Okay, let's get into some practical application now.

Let's say I want to drill a 0.25" hole into a piece of wood. What drill bit size would I use and what type of drill bit would I use?

Based on the table above, I would want a 1/4" fractional drill bit, and I could safely choose any of the four bit material types of bits listed in the table above. I could also use a spade bit if I wanted to.

That last one wasn't too hard, let's go a little deeper. Let's say I'm drilling a hole that I'm going to tap (add a thread to) afterwards. The bolt I want it to fit is a 5/16-18 bolt, and the hole is going to be in a piece of carbon steel. What kind of drill bit would be best?

Well first I'm going to look up what size drill bit I'd need to make the right sized hole for my thread size. You can Google information sheets for things like that such as this one from Engineer's Edge: Engineer's Edge: Tap and Drill Chart 

Look at the chart and see if you can figure out which drill bit to pick. Remember, we're looking for what size drill bit we need for a 5/16-18 thread. (Don't stress too much about what the numbers in the thread sizes mean now--we'll talk about those in the next assignment.)

Based on the chart, for a 5/16-18 bolt I want a size F drill bit.

Now look at the table of drill bit types and figure out which ones you can use for carbon steel.

If you said any of the kinds listed, you are correct. It would be best to use a titanium nitride or cobalt bit though to make drilling easier and prevent tool damage.

Hopefully that makes some sense. Practice your skills on this quiz, and when we get to the labs at the end of this module, you'll now have a better understanding of why you're using the bits that you're using.