CNC milling tools for cutting come in a range of materials, sizes, and types. When trying to achieve a detailed model it is generally more efficient to use a combination of several different toolpaths. A larger tool can often achieve better results than a small tool that has only a small stepover. In this article we are going to look at the different types of CNC milling tools used for cutting.
CNC Milling Tools : End Milling
When using end milling, the cutter will normally rotate on the axis that is found vertical to the workpiece. Teeth on the cutting tool will be on both the periphery as well as the end face of the cutter. If the surface of the workpiece to be cut is complex, a ball nose end mill is often used. This is also known as a spherical end mill or ball end mill. The type of tools you need is determined by the characteristics of the workpiece you wish to achieve. End mills are generally used for rough cutting and cutting in 2D; finishing and V-bit operations are then carried out by ball nose cutters.
When discussing CNC milling machine tools it is also important to understand the end geometry. Cuts can be up or down, or compression cuts. The cuts will vary depending on the number of flutes used. End mills are generally intended to only cut horizontally. Up cutting will result in the chips being ejected upwards; down cutting, downwards. A compression end mill will ensure a smooth surface on both the top and the bottom.
Like CNC milling machine tools, end mills also come in a large variety of shapes. The most common of these are the ball end and flat end mills we have already discussed. A model can be successfully tooled using a combination of ball end and flat mills. If only one tool is to be used for the model a ball end is recommended as it will achieve a smoother finish.
Choosing a Ball End Mill
When you have a choice of ball end mills, always select the large size that is available. A larger tool will leave smaller scallops for the same stepover. What this means is that the finished model will be smoother and of a higher quality. Small tools should only be used when required for cutting out detailed areas. Larger tools also have larger clearance, cutting more smoothly, and staying sharp for longer.
The stepover is the distance that any tool moves over between passes. The stepover value will therefore determine whether or not a model has a smooth finish, or if tooling marks are visible. The smaller the stepover, the longer the tool will take to cut. The stepdown is the depth with which a tool can cut with one operation. A maximum stepdown is only ever used when the material being cut is smooth. When the material being cut is harder, a smaller stepdown value is required.
Although the number can vary, it is most common to find two flutes. Some projects however may benefit from other flute geometry that is not the norm. These include contour cutting of MDF sheets. A single flue will allow for larger chiploads where softer materials are used. When the workpiece is harder, a double flute is generally used. The number of flutes can be expanded to more than two, and this will give an even better finish to harder workpieces.
The more cutting edges you have, the higher the feed rate achievable. This should prevent premature tool dulling and burning. The more flutes used with your cnc milling machine tools, the better the surface finish. The most common used combinations are two and four flutes. Four flutes are used when a better surface finish is required.
Choosing your End Mill
End mills are made of a variety of materials, commonly cobalt steel alloy or tungsten carbide. The latter of these options for your CNC milling tools is more rigid and therefore more resistant. Carbide downs however have a brittle nature; this means that you need high rigidity to prevent tool breakage. The cutters you choose may be further coated with titanium aluminum nitride or other ceramics. The most common coating you find is one that is bluish grey in colour and by reducing friction and improving hardness, can boost speed by up to 20%.