Choosing the right rotary end mill for your CNC operation is a key part of any machining business. Unfortunately, there are so many different options that even the experts can get confused from time to time. The different lengths, materials and flute numbers are just the start of choosing the correct tool. You also have to consider coatings, tool life and cutter head geometries. To make the process less painless, Kyocera SGS has put together a list to make sure you pick the right end mill every time.
End Mill Length
The most important considerations when choosing the length of your end mill are the depth of your slot or cut and the rigidity of your tooling. When you are choosing your tooling, you should pick something that can cut to the depth required but ideally it should be as short as possible with the largest possible diameter. By choosing a tool that is as short as possible, you reduce the risk of chatter and ensure your tooling is as rigid as possible. The larger diameter tooling should provide a better finish and further improve rigidity.
End Mill Materials
The two main materials used to make end mills are a cobalt steel alloy, which most people call high speed steel or HSS, and tungsten carbide in a lattice of cobalt. This material is most commonly called carbide. The rule to follow here is pretty simple, the softer materials can be machined with HSS while the harder more abrasive materials like titanium should be milled using carbide. HSS will dull faster and wear out quicker than carbide but it is significantly cheaper. Carbide also needs to be as rigid as possible to prevent tool failure. The stronger more rigid carbide can also be run 2 to 2.5 times faster than HSS so you can up production and lower changover times.
The number of flutes on end mills can vary immensely and understanding the right number of flutes for your needs is crucial. Single flute end mills can handle a huge chipload but this usually only works in softer materials. Two flutes gives a better part finish while still allowing chips to be ejected freely. Multi flute end mills provide a much better finish and allow you to increase your feed rate. The rule of thumb here, the more flutes you add the better your finish but the harder it is to eject a chip. You also need to increase the feed rates for multi flute end mills to prevent the tool from dulling or burning.
There are three basic types of flute geometry, upcut spiral, downcut spiral and straight. Upcut spiral wants to lift the workpiece off the table but it can pull chips out more efficiently. Downcut spiral tends to press the workpiece against the table which can make it more stable but may affect chip ejection. Straight flute end mills do not press up or down and eject chips well. If you are cutting any sort of metal, the best option is probably an upward spiral. Your work holding solutions should deal with any lift and the better chip clearance can help with heat and a number of other problems.
The most common end mill shapes are flat, ball end and corner radius end mills. Flat end mills are perfect for cutting flat areas with no scalloping. They are also great for slotting and making a square sided slot. Flat end mills are less useful on non flat surfaces. They tend to leave noticeable scalloping.
Ball end end mills work much more effectively on non flat surfaces. They leave less scalloping and provide a more consistent overall smooth result on non flat surfaces.
Corner radius end mills are designed specifically to mill on corners which makes them a far more specialised cutter.
End Mill Coatings
HSS is usually left uncoated but carbide end mills are often coated to improve their performance in a number of different applications. Each coating is different but they are generally applied to achieve one of two goals. These are improving hardness of the cutting edge and reducing friction. To choose the best tool for your application, consider what you will be cutting. If the material has low thermal conductivity like titanium, you will want to make sure your tooling is coated to reduce friction. This is because, unlike steel and other materials with high thermal conductivity, your chip will not help control heat build-up in the tool. That means all the heat from friction will build up in your cutting edge.
If your material is very hard, like titanium, you will want to make sure your cutting edge is coated to increase hardness. Otherwise you could end up with a poor finish, damaged tooling and possible tool failure.
Tool life is a very important consideration when buying your end mills. HSS seem cost effective but its lower tool life when compared to carbide may make it a false economy. If you are running a machining operation with high throughput and heavy workloads, you are more likely to see cost savings by reducing changeover time and investing more in your tooling. Remember, carbide tooling will last longer, run faster and accommodate higher feed rates. That means you put out more product faster.
If you are focusing on short runs with a soft material HSS may be more suitable. This will decrease cost as you are not putting the same number of hours on your tooling as the larger operations.
You should also factor regrinding costs into your equation as most tooling can be re-sharpened and that can effectively double your end mills lifespan. If you use these basic rules when choosing your next end mills it should help you save money and run a more efficient operation.
If you ignore them, you may end up with wasted tool spend and possibly even a damaged CNC machine.