10 Types of Turning Operations You Should Know

Threading is a lathe machining or turning operation that incorporates a mechanical tool that cuts threads on the outer surface of a workpiece. The tool does this motion by moving along the side of the workpiece. Threads—the result of the operation—are helical grooves with pitches and lengths that have been specified. To get deeper threads, the tool needs to pass multiple times along the side of the workpiece. The tool used to perform this action is a thread-shaped cutting tool bit. To get the job done accordingly, the machine must be properly set. In the event that multiple passes of the tool are needed to cut deeper threads, the machine must be set so that the helix begins at the same location.

Turning is the most common among the many machining operations. During this process, excess material is removed from the outer diameter of a workpiece or cylindrical surface. This material removal is done until the desired diameter is achieved. Two turning operations that an operator must become familiar with are: rough and finish operations. In the rough turning operation, the operator is less concerned about the accuracy and surface finish of the workpiece. The emphasis rather, is on the time taken to get the job done. On the other hand, the finish turning operation not only gets the job done but also focuses on accuracy and a good surface finish.

Boring is the process of removing excess material from a hole in a workpiece. The tool used is a single-point cutting tool. It enters the workpiece axially to create specified shapes and makes an existing hole deeper or bigger. However, it is necessary to know that this operation cannot create a hole. Boring comes in handy only when a hole has already been drilled. The procedure is slow and applicable to small-sized works. Moreover, it can be used to correct a hole that is not perfectly round.

Tapping is a machining operation that employs a tapping tool to create threads as it enters axially through an existing hole of a workpiece. The bit size which is accommodated by the tapping tool is determined by the size of the hole. The instrument used to perform the operation is called a tap, and it is a multipoint cutting tool. The workpiece is placed on a faceplate and made to rotate at a slow speed. While a special fixture is used to support the tap as it is placed on a tailstock spindle.

The purpose of knurling is to create serrated or diamond-shaped patterns on the surface of a workpiece or part. Knurling helps to facilitate the easy gripping of the machined part and also makes it aesthetically pleasing. The tool used to perform this operation is called a knurl. To achieve these patterns, the knurl (or knurls) are brought into contact with the workpiece surface and the (tool) teeth are made to roll against it.

The purpose of drilling as a machining operation is to create a hole in a workpiece by removing unneeded materials from it as it rotates. The diameter of the hole will be equal to the size of the drill bit which is used to perform the operation. For a drill bit to be well optimized, it should be properly positioned. Proper positioning is done with a lathe tool holder or tailstock drill holder as the workpiece rotates in a faceplate or chuck.

The reaming operation is used for the enlargement of a hole in the workpiece. Essentially, reaming is a sizing operation. The reamer or reaming tool penetrates axially through the workpiece and enlarges an existing hole to the diameter of the reamer. The reamer comes after the drilling operation has been concluded. It removes only a small amount of material and produces a rounder hole with a smoother internal finish compared to the drill. The reamer is held in place on a tailstock spindle while the workpiece rotates at a very slow speed.

Parting is a turning operation utilized in cutting a workpiece only after it has properly been machined to the specified size and shape. It is an operation that leads to the cutting off of a workpiece or part when a machining cycle reaches an end. The tool used makes a progressive cut in the workpiece, passing through the rotating axis perpendicularly as the workpiece rotates. As the process goes on, the cutting tool gets to the center of the workpiece, a part of which then detaches and drops off.

Facing is a machining operation that reduces the workpiece length. It solves a problem that arises during a machining operation—when the workpiece length is slightly longer than it is expected to be. Facing machines the workpiece length to the exact size. The workpiece end is perpendicular to the axis of rotation. To produce the expected part length, the cutting tool moves along the workpiece radius.

Grooving, sometimes called necking or processing, is the process of creating a narrow cut in the workpiece. However, the size of the cut is determined by the width of the cutting tool used. If wider grooves are required, the tool has to move along the same path on the workpiece multiple times. When it comes to grooving operations, two types exist: face and external grooving. In the face grooving, the tool creates a narrow cut on the workpiece face. On the other hand, external grooving removes materials by moving radially into the workpiece side.

A turning operation in CNC (Computer Numerical Control) involves a process whereby (typically) bars of materials are held in a chuck and rotated. Excess materials are removed using a cutting tool until the final and desired shape is created. It is a subtractive manufacturing process since the desired item is created by the controlled continuous removal of material from the workpiece. CNC turning operations take place at a relatively fast speed. This speed makes the process relevant in the production of high volumes of parts. For more information, see our guide on CNC machining.

Figure 1 below is an example of a CNC turning machine in action:

The turning operation that creates a narrow cut is called grooving. It cuts grooves or creates narrow cuts of specified depth on internal or external surfaces of a cylinder or workpiece. This machining operation takes place on CNC lathes. However, the shape of the cutting tool used is what determines the shape of the grooves or cuts.

The reaming turning or machining operation is a sizing operation. The tool used is called a reamer, and its function is to increase or enlarge an existing hole in a workpiece. The reamer does not create a hole but comes into play only after a hole has been created.

How To Calculate Turning Operation Time?

Turn operation or machining time can be calculated by following the steps below:

1. Calculate the length of the cut in mm. This cut length is equal to the job length + tool over travel + tool approach.
2. Identify the value of the feed in mm per revolution. This feed value is equal to the number of teeth × feed per tooth. 
3. Identify the revolutions per minute. This value is equal to 1000 × cutting speed / π × diameter of the rod (mm). 

The turning operation or machining time is given by:

Turning operation time = Length of cut (mm) / feed (mm per revolution) × revolutions per minute

Is Turning the Same As Boring?

No, turning is not the same as boring. A turning operation is employed when the surface of a  workpiece needs to be machined to the required diameter. In other words, excess material is removed from the external diameter of a workpiece or cylindrical surface. In a boring operation, a cutting tool is used to create specified shapes in an existing hole. However, it can also make an existing hole bigger. In general, a turning operation works externally while a boring operation works internally.

What Is the Most Often Used Tool in Turning Operations?

The most often used tool in turning operations is the lathe machine. It is used to remove excess material from wood or metals until the desired shape is achieved. Some of the operations performed by the lathe machine include: knurling, cutting, facing, and drilling. For more information, see our guide on Turning Tools for Wood Lathes.

This article presented the ten types of turning operations, explained what they are, and discussed how each is used in manufacturing. To learn more about turning operations, contact a Xometry representative.

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