CNC machining is a highly utilised subtractive manufacturing technology. Computer numerical control systems offer less need for manpower and higher levels of automation.
One of these automated fabrication methods is CNC milling. It is a process where rotary cutters remove material, which makes it the opposite of CNC turning.
The milling centres do not just perform the cutting automatically, but also the changing of tools. During the average process of creating a finished product from a block of metal, for example, various tools are used.
So let’s see what milling tools are used on the machines and what are the purposes of each.
What Are the Types of Milling Cutters?
The most common types of milling cutters are:
- End mill
- Face mill
- Ball cutter
- Slab mill
- Side-and-face cutter
- Involute gear cutter
- Fly cutter
- Hollow mill
- Shell mill
- Roughing end mill
- Dovetail cutter
- Wood ruff cutter
First, we should start with one of the primary questions.
What is the difference between end milling and face milling?
These are two of the most prevalent milling operations, each using different types of cutters – the and mill and the face mill. The difference between end milling and face milling is that an end mill uses both the end and the sides of the cutter, whereas face milling is used for horizontal cutting.
These tools usually have a flat bottom but not always. Round and radiused cutters are also available. End mills are similar to drills in the sense that they can cut axially. But the advantage for milling lies with the possibility of lateral cutting.
Face mills cannot cut axially. Instead, the cutting edges are always located on the sides of the cutting head. The cutting teeth are replaceable carbide inserts.
This makes the lifetime of a tool longer while maintaining a good cutting quality.
Ball cutters, also known as ball mills, have hemispherical cutting tips. The objective is to maintain a corner radius for perpendicular faces.
Slab mills are not that common with modern CNC machining centres. Rather, they are still used with manual milling machines to quickly machine large surfaces. That is also why slab milling is often called surface milling.
The slab itself spins in a horizontal position between the spindle and the support.
A predecessor for the end mill. Side-and-face cutters have teeth around the circumference as well as on one side. This makes the functionality very similar to end mills but their popularity has waned over the years with the advancement of other technologies.
Involute gear cutter
There is a special cutting tool for milling involute gears. There are different cutters available to produce gears within a certain number of teeth.
These tools have the same function as face mills. They consist of a central body that holds either one or two tool bits (double-end fly cutters).
Face mills are better for high quality cutting. Fly cutters are just cheaper and the cutting bits are often made at the shop by a machinist rather than bought from stores.
Hollow mills are basically the opposite of face mills. Here, the workpiece is fed into the inner part of the mill to produce a cylindrical outcome.
Roughing end mill
As the name says, these are pretty much end mills with a slight difference. The roughing end mill has jagged teeth. These make the cutting process faster than with a regular end mill.
The cut bits of metal are smaller than usual and therefore easier to clear. Multiple teeth come into contact with the workpiece at the same time. This reduces chatter and vibration, which could otherwise be larger because of the jagged teeth.
Woodruff, or keyseat/keyway cutters are used to cut keyslots into parts, for example shafts. The cutting tools have teeth perpendicular to the outside diameter to produce suitable slots for woodruff keys.
The name of this tool says everything you need to know about its purpose. Thread mills are used for producing tapped holes.
Threading operations are usually carried out on drilling equipment. Using a thread mill, though, is more stable and has fewer limitations regarding the environment.
What Materials Are Used for Cutting Tools?
As you could see, there are a lot of different machine tools available for a wide range of purposes. The same applies to the materials used to make these tools.
Let’s dig deeper to look at the most common materials for milling bits.
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The cheapest of the bunch. And this is exactly why it still finds use. As carbon steel is not very durable, it is only suitable for low-speed operations.
Carbon steel loses its hardness at 200° C. This is the reason for lower speeds – to keep the heating effect low.
High-speed steel, a grade of tool steels, has a few alloying elements added to it to provide better response to heat and wear than regular carbon steel. While the life cycle of such a tool goes up, so does the cost.
Loses its hardness at 600° C. Therefore, higher milling speeds are suitable for these tool steels.
This material is harder than high-speed steel but the toughness qualities are not that impressive. The higher hardness provides better protection against wear but lower toughness levels make it a little more susceptible to cracking and chipping.
The upper temperature of use is at 900° C.
Cutting ceramics are even harder than cemented carbides but lose in the toughness aspect. Both aluminium oxide and silicon nitride are used to produce these tools with varying properties.
Cutting ceramic tools are prone to cracking when used on hard materials and with high temperatures. Therefore, they are not really suitable for machining steels, for example. Otherwise, a short tool life is to be expected.
Selecting the Right Machine Tool
As is the norm in manufacturing, the choice of method or tool comes down to a balance between speed, cost and quality. The cost depends on both the price of the tool, the wear machining results in and the time it takes (speed) to produce the parts.
Choosing the material of the tool
Regular carbon steels are usually out of the option pool because of their limited capabilities. HSS (high-speed steel) is therefore the most inexpensive one to get the job done. At the same time, its rate of wear means that in the long run, there are better options.
Cobalt-bearing HSS, for example, are suitable for even quicker milling. This makes them sufficiently adequate for most jobs.
Cemented carbide is another step towards high performance milling because of the aforementioned properties of such milling machine tools. In the long run, they are a more cost-efficient choice while the up-front costs are higher.
This is quite simple. A tool with a large diameter is able to mill the part quicker. Limitations apply based on the geometry of the final part.
For example, if certain inside radii are necessary, the tool cannot deviate from them. At the same time, you can use a large tool for milling away the bulk of it and apply a smaller one to finish the inside corners.
There are some different coatings available to protect the tools from wear. For example, a titanium nitride coating increases the tool’s lifespan but also the cost of it.
Such a coating reduces the stickiness of the cutting material which can be a problem with aluminium. Therefore, less lubricant is necessary during the cutting process.
Number of flutes
Flutes are the channels on a milling bit. More flutes allow a higher feed rate because less material is removed.
At the same time, this increases the overall diameter of the milling cutter. This leaves less room for swarf.
Angle of helix
The helix angle, along with the rotation speed of the spindle, determines the cutting speed or feed rate. A steeper angle is suitable for softer materials and metals.
Choosing the right milling cutters for your job needs an understanding of the materials, parameters and definitely some experience. The final outcome depends on these choices and a machinist must understand what material cutters are suitable for cutting different mediums.
A good choice leads to high feed rates and therefore shorter cutting times as well as lower costs.
When choosing a CNC machining service, make sure that they have all the necessary tools to make your parts.