Types of Boring Head for Milling Machine

Posted on May 26, 2021

The boring head for the milling machines enlarges the perimeter of a hole that has been drilled in advance. Using the boring head also achieves greater accuracy of the diameter of the hole. Though the use of the boring head for milling machines appeals to be a rather simple operation, the procedure in fact involves detailed attention in order to achieve precision. By precision, the tolerance of error is less than 0.001 inch.

Using a cutting tool such as the boring head or a boring bar to enlarge an existing hole is an operation that is similar to reaming. Nonetheless, the goal of boring and reaming is slightly different. Boring enlarges a pre-drilled hole and achieves better accuracy whereas reaming removes a scarce amount of material from the hole to offer a better finish of the hole.

The selection of the boring head for the milling machine, tool deflection, lubrication, cutting edge geometry, cutting parameters, chip evacuation, and temperature all affect the results of a boring operation. If any of these factors are not controlled properly, precision boring would not be feasible. Boring heads are made with different materials and they all possess particular physical properties. Each type of boring head has its unique characteristics and is suitable for performing certain tasks. In the following sections, we will introduce several common types of boring bars and how to prepare them. Hope you'll find these pieces of information useful!
 

High-speed Steel (HSS) Boring Bar

High-speed steel is a material commonly used to make the boring head for milling machines. The key features of high-speed steel are that it is less expensive than other high-quality materials and it has a relatively tougher construction. These two features make high-speed steel very ideal as the boring bar material. Furthermore, the physical property of high-speed steel allows it to withstand higher temperatures without losing its hardness during the milling operation.
 

Brazed Carbide Boring Bar

Brazed carbide boring heads share similar features to high-speed steel. They can withstand high temperatures during an operation while still preserve the hardness and stiffness. However, the tolerance of heat is much higher than high-speed steel. Therefore, the boring heads made of brazed carbide can cut the workpieces two to five times faster than high-speed steel boring bars. Besides the heat tolerance, the rigidity of brazed carbide is about two to three times greater than steel as well.
 

Indexable Carbide Boring Bar

The indexability of a cutting tool refers to the ability that the cutting tool can be positioned or located both effectively and precisely. The indexable boring bars are tiny drill bits that are mounted to the tool body. Since the bits can be replaced and exchanged, the selection of drill bits is done based on the machining operations. As a result, the bits with the most ideal physical property can be implemented for the operation to achieve precision cutting. One advantage of the indexable cutting tools is that they can be replaced once they wear out. They do not need maintenance.
 

:: Read More: An Introduction to Boring Machines
 

What to Consider when Setting up Boring Bars?

The setup of the boring head and the boring machine is vital in precision boring. The key to precision boring is the stiffness of the drill bits. In order to maximize the stiffness, it is advised to use a larger diameter boring bar with a shorter clamping length. The larger the diameter and the shorter the clamping length, the better the precision.

Boring bars do not have alignment flats on the shank and they do not spin in accordance with the spindle. The motion of the boring bar is revolving around the spindle axis. Hence, the orientation of the boring bar is important. The orientation refers to the rake angle of the boring bars in a boring head. It cannot be emphasized enough that only the neutral orientation in a boring head should be used for boring.
 

How to Assure Better Boring Result?

When using the boring head on a milling machine, it is recommended to start with the cutting speed equal to half of the regular computed value and gradually speed it up. The reason is that the stiffness may reduce due to the cantilevered property of the boring head and the difficulty of consistent lubrication during the operation. As for the safety concern, there may be a rotational imbalance in a boring head, so do not rotate the boring head faster than 600 rpm.

In addition to the rotation speed, the cutting depth is also crucial. There is a safe minimum depth of cut when using the boring head. If the cutting depth is below the minimum depth, the cutting smears the surface of the hole and results in an inconsistent size and finish to the hole. The safe minimum depth is usually between 0.003 inches and 0.005 inches. Using boring bars of harder materials allows a smaller minimum depth of cut.

Generally speaking, the boring head for milling machines is used for larger holes when tolerances are tight and precision is required. The use of a boring head can also improve the surface finish of the hole to a certain extent. Besides, the boring head for milling machines can be used not only to turn a precision outer diameter but also an inner diameter hole in which the cutting tip is facing inward to machine the workpiece. It is a powerful component in the milling operation since it is able to offer more precision and accuracy to the machining.
 

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