What Are the Best End Mills for CNC Machining?

End mills are among the most frequently used cutters on CNC machine tools. Their cylindrical or tapered sides and their flat or ball-shaped ends carry cutting edges, allowing the side and end edges to cut either simultaneously or independently.

Common types include square end mills, corner-radius end mills, tapered end mills, cylindrical ball-nose end mills, and tapered ball-nose end mills. Tapered, cylindrical ball-nose, and tapered ball-nose tools are often used for mold cavities and are also known as mold milling cutters.

A square end mill can perform contour milling, slot and keyway milling, open- and closed-pocket machining, and the face milling of small areas.

Carbide Helical-Flute End Mills

Higher cutter hardness permits higher cutting speeds and improves productivity. CNC mills and machining centers therefore commonly use carbide helical-flute end mills. Compared with ordinary high-speed steel end mills, carbide tools are harder, more rigid, and better at evacuating chips. They are suitable for both roughing and finishing and can deliver two to five times the productivity of a comparable high-speed steel cutter.

When the cutter is long enough, two or more carbide inserts can be brazed into one flute. The joints between inserts on adjacent teeth are staggered, and the insert joints within each flute act as chip splitters. This design is commonly called a corn-cob cutter and is usually selected for rough machining.

Roughing End Mills with Serrated Cutting Edges

CNC mills and machining centers often use serrated-edge roughing end mills when large amounts of stock must be removed. They can significantly increase milling efficiency.

The principal difference from an ordinary high-speed steel end mill is the wavy or serrated cutting edge. It breaks long, thin chips into short, thick segments, improving chip evacuation and supporting continuous automatic machining. The shorter instantaneous contact length reduces vibration. The serrations also increase the effective cutting-edge length, improve heat dissipation, and allow cutting fluid to enter the cutting zone more effectively.

How to Select End-Mill Dimensions

The main dimensional factors in CNC machining are end-mill diameter, overall length, and flute length.

End-mill diameter includes nominal and measured diameter. The nominal diameter is specified by the tool manufacturer. The measured diameter is used to determine the radius-compensation value for finishing.

Nonstandard cutter diameters require special care. A reground tool, for example, should not normally be used for high-accuracy finishing even when its measured diameter is entered as the tool-radius offset.

For finish milling an internal contour, the end-mill radius must be smaller than the minimum radius of curvature in the part profile. A common choice is 80% to 90% of that minimum radius.

A larger-diameter cutter has greater bending strength than a smaller one and is less likely to deflect or vibrate under load. Cutter overhang from the spindle and the length extending from the holder must also be considered carefully. As an end mill becomes longer, bending strength decreases, deflection increases, surface quality can deteriorate, vibration becomes more likely, and cutting-edge wear accelerates.

Regardless of the tool’s overall length, flute length always limits the maximum cutting depth.

How Many Flutes Should an End Mill Have?

End mills can be grouped by tooth count. Coarse-pitch cutters may have 3, 4, 6, or 8 teeth; medium-pitch cutters may have 4, 6, 8, or 10; and fine-pitch cutters may have 5, 6, 8, 10, or 12. Coarse-pitch cutters have fewer, stronger teeth and more chip space, making them suitable for roughing. Fine-pitch cutters have more teeth and cut smoothly, making them suitable for finishing. Medium-pitch designs fall between these two groups.

Workpiece material and machining purpose strongly influence flute-count selection. Ductile materials such as aluminum and magnesium tend to form built-up edges, so an end mill with fewer teeth is often preferred. Fewer teeth create larger chip spaces between flutes and reduce the risk of chip packing during heavy cuts. They also require a lower programmed feed rate for a given feed per tooth.

When machining hard, brittle materials, chatter prevention is more important. An end mill with more teeth cuts more smoothly and can reduce vibration.

Small- and medium-diameter end mills commonly have two, three, or four teeth. A three-flute end mill combines some advantages of two- and four-flute tools and provides good general machining performance. It is not normally selected for precision finishing, however, because its diameter is difficult to measure accurately.