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Boring in Machining Explained: Beginner Guide

Boring in Machining Explained: Beginner Guide

Boring in Machining Explained: Beginner Guide

Ever wondered how large holes are made in workpieces?

Boring operation is the answer. You can use boring (machining) to make holes of desired sizes with high accuracy.

But what exactly is boring and how is it performed?

Boring is an internal turning operation that straightens, enlarges, and semi-finishes a hole, making it concentric with the workpiece’s outer diameter. This process reduces the surface roughness of a hole and improves its dimensional accuracy. A single-point cutting tool, known as the boring bar, is generally used for boring operations.

This article discusses the boring process by going through its various aspects, such as its process, applications, advantages, and tools used for the process.

In the end, I've also discussed the machines required for boring and laid out the significant differences between boring, reaming, and drilling operations.

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What is Boring in Machining?

Boring operation
Boring operation

Boring is the process of enlarging an already drilled hole to meet the required dimensions and surface finish.

Unlike reaming, boring is used for enlarging an already drilled hole, whereas reaming is performed to enhance the tolerance and surface finish of the hole.

Boring is generally performed on a lathe by using a boring bar with a suitable cutting insert.

However, it can also be performed on vertical machining centers, such as a CNC mill or a CNC router by using a boring head as the cutting tool.

Although boring can be performed on a lathe, there are various factors that differentiates boring operation from turning operations.

Applications of Boring (Machining)

Boring a large hole in a workpiece
Boring a large hole in a workpiece

Boring is one of the most commonly used basic machining operations in manufacturing industries to produce precise holes in components such as couplings and engine cylinders of automobiles.

It is also used in woodworking processes such as creating the hollow curvature of wooden tumblers, vases, etc.

A large-scale application concerning infrastructure includes boring smooth tunnels or passageway holes in hard rocks.

Moreover, some applications also include boring hollow shafts or rods used in machines. 

Advantages of Boring (Machining)

Boring improves the surface quality and dimensional accuracy of a hole.

It can modify holes by straightening them, tapering them, or producing a counter-sunk hole, irrespective of the hole diameter and length.

The boring operation is simple in terms of understanding and performing and efficient in obtaining the required result. 

Boring machines and tools are long-lasting as they can resist wear and tear for extensive periods.

Types of Boring Machines

Milling machines, machining centers, or lathes are extensively used to perform boring operations.

There are different types of boring machines, differing based on configuration.

Floor-type Horizontal Boring Machine (HBM)

Line Boring Machine
Line Boring Machine

Floor-type HBMs are usually employed for boring long workpieces. 

They can accommodate up to 144" (3657.6 mm) tall and 240" (6096 mm) long workpieces. 

The table is usually mounted on the floor but may also be fastened to the machine runway (platform with side rails).

Floor-type HBMs utilize boring bars 6" (152.4 mm) to 10" (254 mm) in diameter, similar to the boring bars used in table-type HBMs in terms of geometry.

Line boring machines can be considered as floor-type boring machine.

Table-type Horizontal Boring Machines 

Table type HBM
Table type HBM

Table-type HBMs are commonly used for boring medium to large-scale workpieces with prismatic shapes.

They use a single-point cutting tool, possessing a diameter of 3 in (76.2 mm) to 6 in (152.4 mm), mounted on a spindle parallel to the work table.

Table-type HBMs have a rigid configuration and can deliver strong cutting forces to machine heavy stocks.

Clamps or bolts fasten and hold the workpiece, while the ribbed table structure supports the workpiece’s load.

The boring bar rotary speeds vary from 15 to 1500 RPM, while the feed rate is usually kept between 0.1 to 40 IPM.

Vertical Boring Machine (VBM)

Drill Press
Drill Press for vertical boring

VBMs comprise a horizontal table with a boring bar attached to the spindle that moves vertically in and out of the workpiece.           

VBMs can support heavy workpieces and bore holes as large as 24" (609.6 mm) in diameter.

Jig borers are a type of VBM, comprising high-precision bearings. 

They come in various sizes and specifications, with the CNC variants slowly becoming popular.

CNC mills and CNC routers are popular VBMs that are found in various sizes and specifications for different applications.

Precision Boring Machine

Precision lathe
Precision lathe to perform precision boring

These machines are comparatively smaller and are therefore suitable for boring small workpieces.

Precision boring machines are used for the boring of miniature components, such as parts of a watch, where high surface finish and extreme precision are of great importance.

A watchmaker's lathe is an example of a precision boring machine.

Apart from that, high-powered CNC lathes, such as Haas ST-30 can also be used for boring precise holes in hard metals like steel, titanium, etc.

Tools Used for Boring Operation in Machining

Boring bars are usually composed of High-speed steel, solid carbide, and carbide-reinforced steel. 

When using a lathe to perform a boring operation, the boring bar is either mounted on the tool post or the tailstock, depending upon the size and symmetry of the required hole.

Generally, high-speed steel boring bars are comparatively cheaper but cannot be used for boring hard metals like stainless steel and titanium.

However, their low price makes them suitable for small-scale DIY applications.

Carbide bars are better suited for boring longer holes due to higher elasticity and can be used for machining tough materials with relative ease.

The carbide-reinforced steel bars are highly elastic and can provide the ability to withstand strong cutting forces, making them ideal for large-scale applications where a high material removal rate is desirable.

Diamond or carbide inserts can also be attached to the boring bar for higher durability and more efficient machining.

Apart from that, a proper cooling system is necessary for proper chip clearance and efficient heat dissipation.

Solid boring bars

Solid boring bar
Solid boring bar

These boring bars usually compose of carbide finishes, due to which they are highly durable and ideal for boring hard metal workpieces with ease. 

Damping Bars

Damping Boring Bar
Damping Boring Bar

These are specialized boring bars with a damper system to reduce unwanted vibrations and chatter.

With these boring bars, it is possible to bore longer holes in harder metal workpieces without damage to the tool or workpiece.

Boring Heads

Boring head
Boring head

Boring heads consist of boring bars attached to a drill bit extension or a drill bit spindle.

With this configuration, a drill press or a milling machine can also perform boring operations.

Optimal Parameters for Boring

Different process parameters require to be adjusted for an optimal boring operation. 

Cutting speed

Boring involves the rotation of the cutting tool or the workpiece at a particular RPM which leads to a relative motion between the workpiece and the cutting tool.

The speed at which the surface of the workpiece passes through the cutting tool is known as the cutting speed and is measured in meters per minute (m/min) or inches per minute (ipm).

This speed signifies the linear length of material removed per unit of time.

A moderate cutting speed is recommended for boring operations because a high cutting speed can lead to unwanted vibrations and damage the cutting tool and the workpiece.

Whereas, a low speed will lead to insufficient cutting action and poor surface quality. 

Usually, this speed is kept below 100 m/min, although some materials, like aluminum alloys, may demand greater cutting speeds.

Feed

Feed is the axial movement of the boring bar, measured in mm/revolutions. 

Generally, a high feed is recommended for quick machining time, but it can cause vibrations in the boring bar, leading to high wear and tear of the tool.

Whereas a low feed rate can minimize vibrations but increase the machining time, thereby reducing the productivity of the process.

Therefore, it is recommended to set the optimal feed by trial and error to get the best surface finish with minimum tool damage and quick lead time.

Material removal rate (MRR)

The material removal rate is the amount or volume of material removed from the stock per unit of time.

It is related to the feed rate, depth of cut, and cutting speed. 

A high MRR is always preferred as it saves time, but an extremely high MRR would mean higher feed, depth of cut, and cutting speed, all of which pose a risk of damage to the tool and work surface.

Therefore, it is advised to maintain a moderate material removal rate to achieve a clean surface finish, with minimum tool damage.

Cutting depth

The cutting depth or depth of cut is the linear distance between the cut surface and the original workpiece surface.

Generally for boring operations, it is advised to maintain a shallow depth of cut.

Although a deep cut can remove more material, it requires stronger cutting force, which leads to vibrations in the boring bar.

These vibrations result in chatter along the bored surface and can damage the cutting tool.

For example, when boring a hole to increase its radius by 10mm, it is advised to perform the operation in two passes with a 5mm depth of cut during each pass.

This reduces the stress developed on the cutting tool and helps to produce a hole with a high surface finish.

Entering angle

The entering angle is measured between the tool feed and the hole's central axis. This angle is an essential parameter if a tapered hole is required.

Generally, when a higher entering angle is required, it is advised to gradually increase the angle in each boring pass.

Suppose for machining a taper of 20 degrees, it is advised to perform two separate passes with 10 degrees increament in each pass to achieve the desired entering angle of 20 degrees.

Net power

The net power requirements depend on the RPM of the boring bar. The higher the RPM, the more the power requirements. 

Torque

The torque and RPM are synonymous as the RPM controls the torque required during cutting.  

When machining a hard metal, to maintain the RPM, higher torque is required, and vice versa. 

The torque should not become exceedingly high as that can heat the boring machine, tool, and workpiece, whereas a moderate torque will lead to a better surface finish.

Boring vs Reaming vs Drilling (Machining)

ParametersBoringReamingDrilling
Material Removal Rate (MRR)ModerateMinimalHigh
ApplicationEnlarge an already drilled holeEnhance the surface finish of a holeMake a hole
Sequence of performingPerformed after drillingPerformed as the finishing processInitial process to make a hole
Cutting ToolBoring bar or boring blockReamerDrill bit
Cutting edgesSingle-point cutting toolMulti-point cutting toolMulti-point cutting tool
SpeedModerateHighSlow
Machining timeFastComparatively FasterSlow
Power RequirementsModerateLowHigh
Surface finishGoodHighPoor
Boring vs reaming vs drilling

Differences Based on the Process 

All three operations follow the principles of metal cutting but differ regarding material removal rate, resulting geometry, and performing sequence.

In comparison, drilling has the highest MRR, followed by boring and reaming.

Drilling machines are used to make a hole into the workpiece, thus increasing the hole length, while boring can only enlarge this hole along its diameter.

Reaming only finishes and smoothens the hole surface, removing any burrs or small protrusions, without having any significant effect on its dimensions.

Usually, after drilling a hole into a workpiece, the hole surface undergoes boring followed by reaming to achieve a hole with tight tolerance and a high surface finish.

Differences Based on the Cutting Tool

The drill bit and the reamer are multi-point cutting tools, with the drill bit having longer cutting edges. 

Contrarily, the boring bar is a single-point cutting tool with a short cutting edge.

Despite its longer cutting edges, drilling is the most time-consuming operation among the three due to its high material removal requirement and low RPM to avoid tool damage, unwanted vibrations, and frictional heat.

Reaming is relatively the fastest due to a higher RPM, followed by boring, which has a moderate rotational speed.

Differences Based on the Machinery 

Lathes and milling machines can perform all three operations, whereas, a drill press is specialized for drilling and reaming operations. 

Due to a high MRR, drilling possesses the highest power requirements, followed by boring and reaming.

Differences Based on Quality 

Reaming is a finishing process that results in the relatively smoothest surface.

Drilling results in a slightly rough surface with burrs and unwanted edges, while boring produces highly accurate holes with a satisfactory surface finish.

Final Thoughts 

Boring is a machining process that is dependent upon operations such as drilling or casting.

It can only be performed on pre-existing holes that are either drilled or produced during the casting of the workpiece.

The choice of cutting tool plays an important role in boring operations. If your application requires a highly precise hole with tight tolerance, it is advised to use a damping bar to minimize the vibrations.

Similarly, the machining of deep holes requires careful consideration of the overhanging length-to-diameter ratio of the boring tool.

A thin boring bar with a long overhang is prone to high vibrations, causing chatter and even risk breaking under heavy load.

Therefore, it is important to analyze your requirement and maintain good process control to achieve a perfectly bored hole with high a high surface finish.

Frequently Asked Questions (FAQ)

Can a boring bar be used for drilling a hole into a workpiece?

No, a boring bar can not be used for drilling a hole into a workpiece due to its shape, cutting-edge orientation, and the number of cutting edges it possesses.

What machines can perform all three processes of drilling, boring, and reaming? 

The milling machines, lathes, and drilling machines can be used to perform drilling, boring, and reaming operations.

What safety precautions should be taken while performing boring?

The safety precautions that should be followed while performing boring includes wearing a face shield, headgear, safety boots, gloves, and a protective coat. A machine guard should ideally be installed to prevent chips from flying around and causing potential injury.

About John

Hey I'm John. I talk about CNCs and Lasers at Mellowpine. If you have any questions related to CNCs or Lasers, I'd be happy to answer them. Reach me at mail@mellowpine.com

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John

Hey I'm John. I talk about CNCs and Lasers at Mellowpine. If you have any questions related to CNCs or Lasers, I'd be happy to answer them. Reach me at mail@mellowpine.com

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