Boring vs Turning Explained: Machining

Boring and turning are the two most commonly performed manufacturing processes performed on a lathe.

Both processes involve a similar machining principle, but the two methods have significant distinctions in their applications.

So, what exactly is the difference between the two processes? And which one should you implement for your application?

The difference between the boring and turning processes is that boring involves material removal from the workpiece's internal surface while turning removes the material from the external surface of the workpiece. As a result, boring enlarges a hole, and turning reduces the outer diameter of the workpiece.

This article lists the differences between boring and turning operations by going through their various parameters such as working operation, tools, types, etc.

In the end, the article also discussed the factors to consider during boring and turning operations.

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Difference Between Boring and Turning

Parameters	Boring	Turning
Machining Surface	Removes material from the internal surface	Removes material from the external surface
Dimension	Enlarge the pre-existing holes, thereby increasing the internal diameter	Reduces the external diameter
Cutting Tool	Single-point boring bar or multi-point boring heads	Single-point cutting tool
Tool position	Tool is concentric to the axis of rotation	Tool is perpendicular to the axis of the hole
Machinery	Lathe, mill, or router	Lathe machines only
Skewness	Corrects the axis skew and points error of the hole.	Workpiece should be aligned perfectly to prevent skewness
Inspection	Difficult to inspect the internal surface	Inspection is carried out with ease.

Difference in the Process: Boring vs Turning

Boring, also called internal turning, enlarges the inside diameter of a hole drilled or formed by the casting process.

Unlike drilling or reaming, boring can make a hole larger than the diameter of the cutting tool.

Turning involves the removal of material from the outer surface that reduces the diameter of the workpiece.

The boring process can correct the axis skew (axis of the workpiece being offset or distorted) of the hole which was formed while casting or drilling.

On the other hand, the turning operation requires the workpiece to be perfectly aligned between the headstock and tailstock to avoid producing a skewed part.

Apart from that, the basic machining parameters play an important role as they affect a workpiece's surface roughness and dimensional deviation.

Generally, both operations produce a smooth surface at high speeds and shallow depth of cut, but the overhanging boring bar is prone to vibrations under strong cutting forces and causes chatter.

Therefore, it is important to consider the overhanging length of the boring bar when performing a boring operation.

Difference in Cutting Tool

Cutting tools used in boring operation differs from the ones used in the turning process.

Tools used for the boring operation are boring heads and boring bars, whereas turning operations use a single-point turning tool.

Boring Heads

Depending upon their application and cutting inserts, boring heads can be classified as rough boring heads and finish boring heads.

Rough boring heads are used for applications where a high material removal rate is to be achieved irrespective of the surface finish of the workpiece.

These heads deliver strong cutting forces to remove material at greater depths, and therefore have a rigid construction to withstand the strong cutting forces.

The compact, sturdy design with multiple cutting inserts offers a chatter-free operation.

Fine boring heads are used to remove material in a precise manner to achieve accurate dimensions. This stage of boring is regarded as the finishing stage.

These boring heads are comparatively less rigid as they are not intended to deliver a strong cutting force but to produce a cut with a high surface finish.

Boring Bars

Boring bars are used for machining holes having a higher length-to-diameter ratio.

Inserts in a boring bar have a point contact with the workpiece. Therefore, the spindle speed should be decreased as there are chances of chattering due to a single point of contact.

Apart from that, the long length of the boring bar makes them prone to vibrations, making it important to select the right boring bar for the operation that provides the required rigidity.

Some lathe cutting tool manufacturers also manufacture boring bars with internal coolant flow to enhance the quality of the cut.

Single-Point Turning Tool

A single-point turning tool is mounted on the toolpost of the lathe and does not involve overhanging the tool.

This eliminates the risk of vibrations and prevents chatter on the surface of the workpiece.

As a result, a single turning tool can be used for roughening and finishing turning operations by varying the parameters of machining.

Difference in Machine Used 

Boring operations can be performed in a horizontal or a vertical machine, i.e., a lathe or a milling machine.

It is simple and easy to perform a boring operation in a lathe rather than in milling machines.

The reason behind this simplicity is that a boring process in a lathe allows proper alignment and expels the chances of the object being skewed.

A boring tool mounted on the tailstock can correct the axis of skewness as the alignment process is performed on the lathe before the operation.

Apart from that, the horizontal configuration facilitates easy chip clearance under the action of gravity, thereby enhancing the surface finish and tool life.

However, a mill or a router can also be used for boring through holes with high precision and quality.

On the other hand, turning operations are performed only on lathe machines or horizontal machining centers.

During turning, the tool advances towards a rotating workpiece and glides over its surface to remove the material in the desired shape.

Difference in types

Different types of turning operations are performed in a lathe. It includes step turning, taper turning, chamfering, contouring, knurling, and many more.

Chamfering produces a chamfer along the edge of the workpiece, whereas forming removes material along the length of the workpiece to get the desired shape.

The boring operation involves two primary types of process: Line boring and Back boring.

Line boring involves boring through holes where the boring bar may or may not be supported on both ends. In contrast, back boring involves reaching the extreme end of the hole and machining it backward.

Difference in Ease of Inspection

The boring operations are done on internal surfaces, making it difficult to inspect the surface roughness manually.

Therefore, additional techniques like non-contact metrology systems are adopted to assess the machining operation.

Manual inspection of turned parts is simple and faster as the external surfaces are machined in a turning operation.

Things to Consider During Boring and Turning Operations

Chattering is the most commonly encountered problem in a boring process due to overhanging of the boring bar. Heavy metal boring bars and carbide bars help minimize chatter by reducing vibrations.

Apart from optimizing radial depth of cut, feed, and cutting speeds, the tool's material helps prevent this problem to a great extent.

Though expensive, heavy-metal bars are used in applications where holes of a higher length-to-diameter ratio (about 6:1 ratio) are to be machined.

For example, a tungsten-carbide tool is the most commonly used heavy metal boring bar.

A steel bar is chosen for holes of a lower length-to-diameter ratio (about 4:1), which is the least expensive and readily available.

Similarly, for turning operation, the selection of the right cutting tool depends on the material to be machined and the power available.

For a continuous machining process, cutting tools having a higher grade of hardness are chosen for better wear resistance and extended tool life.

Applications of Boring and Turning Operations

The difference in the application of boring and turning can be understood by considering the manufacturing process of a piston head.

Manufacturing a piston involves many processes like casting, heat treatment, machining, and inspection.

After casting the metal into the desired shape, the cylindrical surface of the piston is turned on a lathe to get the precise dimensions and attain tight tolerance with the cylinder.

After getting the required dimensions of the cylindrical surface, a boring tool is used to enlarge an already drilled hole on the side of the piston to the required dimensions for placing the gudgeon pin.

Final Thoughts

Boring and turning produce excellent results when performed under optimal machining conditions.

Both operations are often performed in sequence to get the final product, and based on the part to be machined, the sequence of the machining processes should be decided.

For example, when machining a piston head, the turning operation is performed first, followed by the drilling and boring process.

The reason is the enormous amount of stress concentration that will be developed around the hole (formed by drilling or boring), rendering the surface unstable for turning operation.

Frequently Asked Questions (FAQ)

Can threading be performed using a boring bar?

Yes, threading can be performed using a boring bar along with threading inserts for the operation.

Does a machinist require different skills to perform boring when compared to turning?

No, though turning and boring are two different processes, it is only the machining parameters that affect the surface finish of a part. Therefore, a machinist needn’t acquire additional skills for this purpose.

Can boring be performed on wood?

Yes, a special bit called an auger bit is used to bore holes in wooden workpieces.