Cutting forces on an end mill have axial and radial components that affect how it functions. The radial component often leads to vibrations and the axial component affects the force of the mill against the holder. These components are varied by changing the helix angle on a tool, and different angles are better for different materials.
When a helix angle is higher, there will be a larger proportion of axial force on the end mill. On top of this, whenever an edge contacts the working material, there is a rapid rise in loading. When not handled correctly, the finish of your part can be negatively affected.
Digging into specific helix angles a little further, we can see how force propagates throughout an end mill as the helix angle changes. When the helix angle is 45 degrees, the forces between the axial and radial components are equal, 50:50. However, if you use a tool that has a 30-degree helix angle, the forces are more uneven, 25% axial to 75% radial. The higher end of tools will have a 60-degree helix angle which will help keep the radial forces down thus minimizing vibrations and maximizing chip evacuation. This does come with a downside, however. At this angle, cutting is weaker and the toolpath has to maintain a shallow depth. Cutters with lower helix angles, as you may have suspected, are much stronger but the surface finish tends not to be as good.
When determining what helix to use, it depends highly on what material you are cutting. In general, heavy cutting where finish isn’t important, a lower helix angle is the way to go. When you do care about tolerance and finish, then higher helix angles are much better.
Now that we have nailed down the purpose of helix angles, let’s see how they vary by metal. The following chart below will briefly outline the helix angles for different metals.
Stainless Steel: 30˚
Now, keep in mind that these numbers are generalized for each metal. These helix angles will allow you to achieve a necessary surface finish on the metal while also maximizing on speed. Ultimately, like any machining job, it is going to be up to the machinists to experiment and determine which helix and specific tool will work best on a certain job. Ultimately, helix angle is only one part of decreasing job costs and increasing efficiency. With that said, it shouldn’t be overlooked if you care about your tools or your machine.