Four, Six, Eight, Steel, Heavy Metal, Carbide: The Fundamentals of Selecting the Correct Boring Bar

by Bernard Martin

photo courtesy Everede Tool

Boring bars are indispensable tools for enlarging and refining holes, a critical step in the machining process. They come in various types, each designed to address specific machining needs. Selecting the correct boring bar for use in your your turning application can make your life a lot less stressful. 

​Cutting Forces During Boring Operations ​

During boring operations, the tool encounters forces that can impact its effectiveness. Primarily, tangential and radial cutting forces exert downward and lateral pressure on the tool, potentially causing deflection away from the workpiece.

The Impact of Radial Force
Radial force plays a significant role, especially in scenarios with excessive tool overhang. This force can push the insert away from the cutting surface, leading to deflection and chatter. The extent of this deflection correlates directly with the tool's overhang, depth of cut, and feed rate. Minimizing overhang is crucial to mitigate these issues, ensuring stability and precision. 

Tangential Force Dynamics
Tangential force, on the other hand, tends to displace the tool downward and outward, away from the workpiece's centerline. This displacement can compromise the tool's clearance angle, critical for maintaining the quality of the cut. When boring small diameter holes, maintaining a sufficient clearance angle is paramount to prevent the tool from making unintended contact with the hole's interior walls, preserving the integrity of the bore.

Some Recommendations for Optimal Performance

• Minimize Tool Overhang: Employ the shortest overhang feasible for your operation to reduce the risk of deflection and chatter. Typically you should try to use the largest diameter boring bar possible to maintain rigidity off the boring bar.  We'll dig more into this below.
• Monitor and Adjust Feed Rate and Depth of Cut: Tailoring these parameters can help manage the radial and tangential forces encountered during boring.
• Ensure Adequate Clearance Angle: Especially critical in boring small diameters, a proper clearance angle helps avoid undesirable contact with the workpiece.

​By understanding and addressing the implications of tangential and radial forces, machinists can enhance the accuracy, finish, and overall success of boring operations. These adjustments not only improve tool performance but also extend tool life and enhance workpiece quality.

Selecting the Shank Diameter

Selecting the right shank diameter is crucial, and it involves evaluating the minimum machining diameter of the boring bar alongside the desired bore diameter. Optimal selection dictates choosing a shank diameter that is as large as possible.

Nevertheless, it's essential to exercise caution, as a shank diameter that is too close to the bore diameter will lead to chip control at least and chip packing at worst. Consequently, the choice of shank diameter should be tailored to the specific application requirement while leaving enough room for your chips to evacuate.  Remember, if you're roughing you're going to be taking a bigger bite than you are if you are finishing.  

A couple decades ago, I was having some issues with chip packing that I couldn't solve.  I called in my cutting tool specialist, Charles Colerich.  Charles took one look at the application and said "Just turn the bar upside down. Gravity works."  No one ever said the insert needed to be facing upwards. 

​As a result, instead of the chips trying to evacuate over the top of the bar, the coolant washed them under the bar.  Sometimes the best thing to do is step back and look at the problem in its simplest terms and apply some common sense.  When he told the machinist to turn the bar over the shop foreman and I looked at each other and both had  facepalm moment "why didn't; we think of that" together.

Selecting the Correct
​Length: 4 - 6 - 8 -10+

Boring bars can be classified based on their material, structure, and construction. The type of boring bar determines the strength of the bar to handle strong cutting forces with minimal vibrations. The most popular boring bars used include steel boring bars, heavy metal bars, carbide bars, and damping bars.

When initially selecting the type of bar you need a good rule of thumb, that is also pretty simple, is 4-6-8-10+.  Take the diameter of your boring bar and multiply it 4.  

​That's about the max depth you should try to bore with a steel bar.  

Here's an easy to remember list of max depth of each type of bar:

• Steel Boring Bars: Up to 4 X Diameter
  
• Heavy Metal Bars: Up to 6 X Diameter
• Carbide Boring bars: Up to 8 X Diameter
• Devibe Boring Bars: Antivibration bars are generally best applied over 10 diameters depth.
  

Everede's Boring Bar Range

Everede Tool offers a diverse range of different types of materials on the shanks of their boring bars  The range of tools are notable: From small diameter capabilities, accommodating minimum bore diameters from as small as 0.180" up to sizes exceeding 1.500", depending on the series and specific model. The selection includes options with a 5° positive axial rake, angles designed for facing and through-hole boring, and configurations suitable for boring to the shoulder with 0° angles.

The choice of boring bars in CNC machining is a critical decision that directly influences the quality and efficiency of the machining process. Everede Tools' boring bars stand as a testament to precision engineering, offering a comprehensive range of options with features such as anti-vibration technology, modular design, and high-quality materials. By investing in Everede Tools' boring bars, machinists can elevate their CNC machining capabilities, achieving superior results and staying at the forefront of innovation in the industry.