Boring is a machining process generally performed on a lathe machine and requires a unique tool called a boring bar.
It is a specifically designed tool that is used to enlarge an already drilled hole to match the desired specifications.
There are various types of boring bars suitable for different applications, making it important to learn about boring bars and their types.
This article discusses boring bars in detail by going through their geometry, types, cutting heads, and factors that affect the boring process.
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A boring bar is a unique lathe tool, generally cylindrical in shape, and houses a single-point cutting tool at one end. It is used for machining an already drilled hole to increase its internal diameter and match the desired specification. This machining process is known as boring.
Unlike other lathe cutting tools that are short in length, a boring bar is a long tool that sticks out of the tool holder, making it prone to vibrations when delivering strong cutting forces.
This unique characteristic allows the boring bar to make holes larger than its diameter, making boring significantly different from reaming and drilling operations.
Although a thick boring bar minimizes these vibrations, it provides less space for chip clearance from the hole during boring operation.
The selection of a boring bar depends upon the type of lathe being used and the material being machined.
While wood lathes use a hand-held boring tool, metal lathes use a comparatively more rigid boring bar that is clamped on the tool post.
Machining hard materials require a thick boring bar capable of delivering strong cutting forces with minimal wear and vibrations.
Apart from that, machining hard materials produce short chips that are easily evacuated with minimal clearance.
On the contrary, soft materials require comparatively less cutting force, resulting in lesser deflection of the boring bar.
Making it ideal to use a thin boring bar that provides enough clearance to evacuate long, stringy chips produced during the machining of ductile materials.
Therefore, it is important to select an appropriate boring bar that provides enough rigidity to handle the stress developed during the cutting process while ensuring easy chip clearance.
Geometry of a Boring Bar
Apart from the size, the geometry of the boring bar also plays an important role in its ability to bore holes of different sizes and depths.
Minimum Bore Diameter
The diameter of the cutting end of the boring bar is known as the minimum bore diameter.
It determines the minimum size of the hole required for using the boring bar.
Shank diameter is the diameter of the part of a boring bar that is clamped in the tool holder.
It is generally the thickest part of a boring bar.
Overall length is the total length, comprising the shank length and the cutting end length of a boring bar.
Generally, a boring bar with a long length is prone to deflection when delivering the cutting force, whereas shorter boring bars provide greater rigidity and smoother cuts.
Maximum Bore Depth
Maximum bore depth is the length of the cutting end of the boring bar.
It determines the maximum boring depth of the boring bar.
The cutting edge of a boring tool is slightly offset from the center axis of the tool's shank.
This distance between the center axis and the tip of the cutting edge is known as the centerline offset of a boring bar.
The nose radius is the curvature of the tip of the cutting edge.
A larger nose radius leads to a larger contact area between the cutting tool and the workpiece's surface, resulting in longer tool life and smoother cuts while reducing the stress on the cutting edge.
However, increasing the nose radius beyond a limit will result in degradation of the surface finish as it induces high tangential and radial forces that cause chatter on the machined surface.
Clearance angles play an important role in the operation of a boring bar.
These angles prevent the rubbing of the cutting tool against the workpiece, thereby eliminating the risk of abrasive wear of the workpiece and the tool.
There are two clearance angles on a boring bar: side clearance angle and end clearance angle.
Side clearance angle ensures a tapered face of the cutting head which prevents the rubbing of the face against the workpiece.
It is measured at the face of the cutting head against an imaginary line parallel to the central axis of the cutting tool.
On the other hand, the end clearance angle is concerned with the taper of the top of the cutting head, which prevents the rubbing of the tool head against the base of the hole.
It is measured at the top of the cutting tool against an imaginary line running perpendicular to the central axis of the tool.
Side Rake Angle
The side rake angle measures the inclination of the side of the cutting face against an imaginary line that runs perpendicular to the central axis of the boring bar.
Depending on the inclination of the face, rake angles can be positive or negative.
A positive side rake angle provides greater shear force to remove the material, thereby increasing the machining ability.
However, increasing the positive side rake angle reduces the curvature of the nose, making it prone to wear.
Back Rake Angle
The back rake angle is the angle that measures the tilt of the back face of the cutting tip against the central axis of the boring bar.
This angle is also known as the top rake angle.
A negative back rake angle provides greater clearance for chip evacuation while providing the ability to deliver stronger cutting forces.
Relief angles provide clearance to prevent rubbing the tool's non-cutting edges against the workpiece.
Having extremely small relief angles raises the risk of abrasion of the tool, which thereby increases tool wear and vibrations.
There are two relief angles: side relief and end relief angles.
How to Use a Boring Bar?
A boring bar is used similarly to any other lathe cutting tool.
However, it requires a pre-machined hole drilled in the workpiece.
The boring bar is clamped on the tool post that moves along the X and Z axes of the lathe while the workpiece spins on the lathe spindle.
Setting up a Boring Bar
Setting up a boring bar plays an important role in determining machining quality.
An improper setup will cause vibrations during the machining process and result in a poor surface finish with chatter marks along the bored hole.
The setup of a boring bar involves ensuring optimal clamping and overhanging lengths of the bar that depends upon the type of boring bar being used.
|Type of Boring Bar||Max. Overhang Length||Min. Clamped Length|
|Solid Steel Boring Bar||4 x Diameter of the bar||3 x Diameter of the bar|
|Heavy Metal/Tungsten Carbide||6 x Diameter of the bar||4 x Diameter of the bar|
|Solid Carbide Boring Bar||8 x Diameter of the bar||4 x Diameter of the bar|
|Short, Damped Bar||7 x Diameter of the bar||4 x Diameter of the bar|
|Long, Damped Bar||10 x Diameter of the bar||4 x Diameter of the bar|
|Reinforced damped boring bar (Carbide)||14 x Diameter of the bar||4 x Diameter of the bar|
To begin the process, the tool post is moved so that the cutting tip of the boring bar is brought in contact with the surface to be machined, and the reading on the handwheels is set to zero.
This is done to eliminate any reading errors in calculating the depth of the cut, ensuring accurate dimensions of the final hole.
After setting the zero, the lathe spindle is rotated at the required RPM while the boring bar is moved along the X and Z axes to get the desired cut.
Types of Boring Bar
Boring bars can be classified based on their material, structure, and construction.
Based On the Type of Material
The type of material used for making the boring bar determines its strength to handle strong cutting forces with minimal vibrations.
Boring bars are generally made of two main materials: Steel and Carbide.
Steel Boring Bars
Steel bars are most commonly used because of their low cost and good machining capabilities.
However, these bars are less rigid and are prone to vibrations that affect the quality of the cut.
These bars are generally suitable for boring operations where a diameter to boring length of 1:4 is possible.
Therefore, steel bars are recommended for projects where low cost is valued over high accuracy.
Steel boring bars are also available with indexable carbide inserts.
Heavy Metal Boring Bars
Heavy metal boring bars are generally made of tungsten carbide and provide greater rigidity than steel bars.
As a result, these bars provide a greater diameter to boring length ratio of around 1:6.
These bars are generally preferred where high rigidity at a reasonable price is required.
Carbide Boring Bars
Carbide bars are the most rigid bars, making them ideal for machining hard materials.
Furthermore, the high rigidity of carbide makes it possible to use boring bars with a smaller diameter which provides greater clearance for chip evacuation.
As a result, carbide boring bars are preferred for industrial applications where a bore with high accuracy is required.
These bars provide the ability to have a maximum diameter to length ratio of around 1:8:
However, solid carbide bars with a diameter over 1" are not practical because of their high cost.
Based On the Structure of the Boring Bar
Round Bottom Boring Bar
The shank of the boring bars generally has a cylindrical profile.
These boring bars are perfectly round and require a V-groove tool holder to clamp these bars.
Flat Bottom Boring Bar
Boring bars are also available with a flat bottom profile.
The shank of these bars has a partial circular profile, with the base of the bar being flat.
This helps in the firm clamping of the bar and makes it possible to deliver stronger cutting forces with minimal vibrations.
A flat bottom tool holder is used for clamping these boring bars.
Based On the Construction of The Boring Bar
Solid Boring Bars
Solid boring bars are the most commonly used type of boring bars.
These bars have an all-metal construction that provides them with the strength to deliver strong cutting forces.
Their simple construction makes them comparatively cheaper than damping boring bars.
Damping Boring Bars
Damping boring bars are special tools that consist of a cavity inside the boring bar.
The cavity contains an inertia mass suspended with a special mechanism in a dense, oily fluid.
This arrangement acts as a dampener to minimize the unwanted vibrations during the cutting process.
As a result, damping boring bars are used for applications that require extremely high surface finish with good accuracy.
ANSI Code to Identify The Specifications of a Boring Bar
The ANSI designation of a boring bar consists of a specified code used to name each boring bar according to its type, structure, size, geometry, etc.
A typical identification code of a boring bar consists of 9 blocks.
For example, a boring bar with an identification code C10U-PTJAR2.
This identification code consists of 9 block elements each representing a different property.
|Block Number||Block Code||Property|
|1||C||Type of Boring Bar|
|3||U||Length of the Boring Bar|
|4||P||Insert Clamping Technique|
|6||J||Cutting Head Style|
|7||A||Relief Angle of Insert|
|9||2||Cutting edge length|
The significance of each code can be identified from their respective sheets provided by ANSI, and the last block of the code represents the length of the cutting edge in inches.
Tips to Improve Boring Operation
A boring operation is performed to enlarge an already drilled hole and produce a highly precise hole with tight tolerance.
But the overhanging boring bar is prone to vibrations which affect its ability to produce accurate holes.
Hence, using a thick boring bar with a short overhang is advised to reduce vibrations and produce an accurate hole with tight dimensional tolerance.
Apart from that, the quality of boring operation also depends upon the machining parameters like cutting speed, feed rate, and depth of cut.
It is generally recommended to use a shallow depth of cut with a high feed rate and slow cutting speed to improve the surface finish and reduce the stress on the boring bar.
Furthermore, using a damping, carbide, or heavy metal boring bar over a solid steel bar also provides greater rigidity, thereby affecting the quality of the cut.
Selecting the appropriate cutting head for the boring bar also plays a crucial role in determining the quality of the cut.
Twin-style heads are recommended for roughing cuts as they facilitate a higher material removal rate (MRR) at the cost of surface finish, whereas finish-boring heads are recommended for a smooth surface finish.
Boring blind holes is comparatively more difficult than boring through holes. This is because the drilling operation to produce a blind hole leaves a conical profile inside the hole.
This conical profile limits the clearance available to fit the boring tool as the depth of the bore increases.
Therefore, it is advised to use a flat bottom drill or an end mill to remove the conical profile before boring.
Frequently Asked Questions (FAQ)
What is a boring head?
A boring head is a block-shaped tool that provides the ability to mount a boring tool, such as a boring bar. These tools are more versatile as they provide multiple configurations to mount the tool according to your requirement. Unlike boring bars that are used on the lathe, boring heads are generally used with drilling machines.
When do we need a boring bar?
Boring bars are needed when the required dimension of the hole exceeds the size of the largest drill bit. Generally, boring bars are used to make holes greater than 2".
How many cutting edges does a boring bar have?
A boring bar is a single-point cutting tool. This means that they have a single cutting edge that provides the ability to control the dimensions of the hole precisely.
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