Shaping and cutting material with precision involves using some pretty specialist tooling depending on the job at hand. So how do you go about selecting the right tool?
Well in this article I’m going to go over the basics of end mills and drill bits to help you decide when and where each is required.
Drill Bits and End Mills: The Basics
To understand the differences, first we need to get to grips with what exactly end mills and drill bits are.
What is a Drill Bit?
Drill bits are cutting tools used to remove material to form holes. They come in many different types and sizes depending on the material of the work. Drill bits cut in an axial direction.
An axial cut is made straight through the rotating axis of the drill bit.
What is an End Mill?
An end mill is a cutting tool, a type of milling cutter used in industrial applications such as profile milling, tracer milling, face milling, and plunging. An end mill can cut both axially and radially.
Radial cutting with an end mill means that as well as cutting down through the axis of rotation, it can also cut sideways.
Similarities of Drill Bits and End Mills
Although each tool has differing uses, there are some similarities that I will mention here.
1. End Mills and Drill Bits Cut Axially
A drill bit is specifically designed to cut down or axially into the work. Deep holes can be formed in material with a drill bit due to its ability to clear chips up and away from the cutting surface. This is accomplished by the corkscrew shaped ‘flutes’ ground into the length of the drill bit.
An end mill can also be used in some applications to cut axially. While it is not its main strength, holes can be formed with an end mill in much the same way you would with a drill bit, but with limitations.
2. Cutting Action
Drill bits and end mills both remove material in the same way in the form of solid chips due to the rotation of the tool. Both tools are multi-point cutters with more than one cutting surface working together to remove chips from the workpiece.
Differences Between Drill Bits and End Mills
End mills and drill bits are designed to perform different tasks, and differ in the following ways.
1. Direction of Cut
Drill bits are designed to cut holes vertically down into the workpiece. As I mentioned previously, end mills can cut holes too.
End mills differ from drill bits primarily because they can cut sideways into the work, to form features like slots, walls, fins, columns, and webs.
2. Location of the Cutting Edges
On a drill bit the cutting edges are located on the tapered bottom of the tool.
End mills have their main cutting edges located at the periphery, while secondary cutting edges are located on the bottom of the tool.
3. Cutting Flat Bottom Holes
Drill bits, with their cutting edges tapered on the end of the tool, are unable to cut a flat at the bottom of a hole.
End mills, with the auxiliary cutting edges on the bottom of the tool make forming a flat bottom hole possible.
4. Material of Manufacture
Standard drill bits are commonly made of HSS (High Speed Steel). End mills are mostly made of TC (Tungsten Carbide).
That said, drill bits can also be made of other materials including TC and cobalt.
5. Tool Rotation Speed
Drill bits are usually operated at a low rotational speed because at high RPM, problems can arise with chip removal and breakage of the drill.
End mills can be used at much higher rotational speeds because of the differing chip geometry the cutters form.
6. Chip Geometry
Due to the constant contact between tool and workpiece, cutting holes with drill bits can form long continuous chips.
End mills, with their intermittent cutting process and high RPM, form smaller chips than drill bits.
7. Feed Motion
When forming holes with a drill bit, feed motion can be imparted by moving the tool into the workpiece or moving the workpiece into the drill.
In end milling, usually the tool is held in place and feed motion is made by moving the workpiece into the end mill.
8. Aspect Ratio of Cut
Drill bits can cut holes with an aspect ratio of up to 10:1 (a hole with a depth 10 times that of its diameter). This is down to how chips are transported up and away from the cutting edges of the tool.
End mills can make plunge cuts, but this is generally limited to an aspect ratio of 2:1.
Of course you can find drill bits that can cut a hole deeper than 10 times the diameter of hole, but chip evacuation up and out of the hole becomes an issue. Longer drills are more likely to flex. This can cause the drill bit to wander off its planned path, resulting in inaccurate dimensions and straightness of the hole.
Can an End Mill Be Used with a Drill?
Running an end mill in a drill press is not a normal operation. In fact there are very few use cases for doing so. However, if you have drilled a hole that requires a flat bottom, then using an end mill to achieve it can be done as the end mill doesn’t have to cut the center of the hole.
Drill Bit vs. End Mill: Which of the Two Should You Use?
So by now you probably understand that a drill bit and end mill, although similar in some ways, are very different in many other ways. The truth of the matter is that each tool has a very different job it was designed for and they are not interchangeable.
Let’s say you have a workpiece that requires multiple holes of the same size to be formed. Then the drill bit is the tool you need. And if those holes are deeper than twice the diameter of the hole, then you can only make these holes with a drill bit.
Conversely, if you need to cut anything other than a vertical hole into the surface of your workpiece, then you are going to have to use an end mill.
End mills can be used to cut holes, but are limited in the depth to hole diameter ratio. Anything greater than 2:1 and you will have to use a drill.
If you only ever want to cut holes into steel, then you don’t need to buy a mill. A simple drill press and a set of suitable drill bits are typically all you’ll need to get started. But if you want to form complex 3D structures from wood or metal then you’ll need not only an end mill, but a variety of different milling cutters and more than likely a CNC mill to run them in.
Summary
Drilling and end milling are similar in that they are both processes that involve removing material from the workpiece, but that’s really where the similarities stop. End mills themselves come in a myriad of different types for different jobs, but that’s beyond the scope of this article.
I hope I’ve given you a bit more insight into two common machining processes.