Bull Endmill - End Milling Cutters

Posted on Dec 1, 2020

Bull Endmill

End milling tools are the leading products of CNC machining tools.

What is a Bull Endmill

Bull endmills, also known as bull nose end mills or simply corner radius end mill cutters, are single-ended tools used for making corner radii at the bottom of a milled shoulder.

Bull nose end mills have more robust milling edges than square end mills, therefore, feed rates may be increased and lead to longer tool life and better productivity. These milling bits can be used in mold profiling applications with greater productivity than bell end mills. Bull nose end mills are generally solid carbide and are available in many diameter sizes and lengths.


Bull Endmills – Materials and Coatings

End milling tools are the leading products of CNC machining tools. In some countries, more than 90% of turning tools and 55% of milling cutters (including bull nose end mills) are made of cemented carbide. The trend is still increasing as of now.

Since the 1980s, the tooling industry has continuously expanded the production of various integral and index-able cemented carbide tools or inserts, and the variety has been expanded to various cutting tool applications. Among these cemented carbide tools, the index-able cemented carbide tools are comprised of simple turning tools and face milling cutters which are widely used in various fields of precision, completing, and forming tools.

Cemented carbide is also a common material for manufacturing general-purpose tools such as drill bits and faces milling cutters, as well as more complex tools such as reamers, end mills, medium and large modulus gear cutters for machining hard tooth surfaces, broaches, etc. The annual output value of cemented carbide tools and inserts has accounted for more than 30% of the total output value of cutting tools.

Based on the grain size, cemented carbide which is used to make corner radius end mills can be divided into ordinary cemented carbide, fine-grain cemented carbide and ultra-fine-grain cemented carbide. And according to the main chemical composition, cemented carbide can be divided into tungsten carbide-based cemented carbide and titanium carbide-based cemented carbide.

Carbide corner radius end mill cutters may be further coated with ceramics such as titanium aluminum nitride (TiAlN, aka AlTiN), titanium nitride (TiN), titanium carbon nitride (TiCN), and so on. Of these, the bluish-gray TiAlN coating is the most common one, and by virtue of reducing friction and improving hardness, it boosts the speed of metal cutting by up to 20%. It extends tool life.

:: Read More: What Is an End Mill Cutter


Mechanism and Methods of Bull Endmill

When using a bull nose end mill in the milling process, the workpiece can be fed along or relative to the direction of rotation of the tool, which will affect the starting and finishing characteristics of cutting.

When using a corner radius end mill for down milling (also referred to as the co-direction milling), the feeding direction of the workpiece is the same as the rotation direction in the cutting area. The thickness of the chip will gradually decrease from the start until the end of the cut becomes zero during peripheral milling.

On the other hand, during up-milling (also called reverse milling), the feeding direction of the workpiece is the opposite of the rotation direction of a corner radius end mill.

When the corner radius end mill is in up-milling, the corner radius end mill starts cutting from the zero chip thickness, which will generate a high cutting force, thereby pushing the corner radius end mill and the workpiece away from each other.

After the blade of the corner radius end mill is forced into the incision, it usually comes into contact with the hardened surface caused by the cutting blade, and at the same time, it produces friction and polishing effects under high temperature.

The cutting force also makes it easier to lift the workpiece from the table. When the corner radius end mill performs down milling, the blade starts cutting from the maximum chip thickness. This can avoid the polishing effect by reducing the heat and reducing the tendency of processing to harden. It is very advantageous to apply the maximum chip thickness, and it is easier for the cutting force to push the workpiece into the corner radius end mill so that the cutter blade is able to perform the cutting action.


Applications of Bull Endmill

During the milling process with a corner radius end mill, chip breaks are sometimes glued or welded to the cutting edge and will gather at the start of the next cutting edge. When up-milling, chip breaks are more likely to be trapped or wedged between the blade and the workpiece, which will cause the blade to break. When performing down milling, the same chip breaking will split into two so that the cutting edge will not be damaged.

Regardless of the requirements of the machine tool, fixture, and workpiece, down milling is often the preferred method. Down milling using a corner radius end mill has certain requirements for the machining process to allow the blade to be pushed forward while keeping the workpiece down. This requires the machining tool to conform to the table requirements of table feeding by eliminating backlash.

If the tool is pushed into the workpiece, the feed rate will increase irregularly, resulting in excessive chip thickness and chipping. In this case, you will want to use up-milling instead. In addition, it is better to choose up-milling if the machining allowance changes greatly.

In order to clamp the workpiece correctly, a suitable fixture must be required, and the correct tool size is also required for the operation. But for vibration trends, the cutting force direction is more important.

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