# Metal Cutting Processes - Turning



## eng.m.mohsen (4 مارس 2010)

. Introduction 
 This training module is designed to give you 'hands-on' experience through which you can gain a good appreciation of this well-known type of machine tool. In particular your attention will be directed towards its operational uses and parameters, the general layout of controls, accessories, associated tooling, and the maintenance factors related to lathes. 
 In order that you can make the most use of the limited time available on lathes it is essential that you use every chance to consolidate what you observe. This type of work is largely self-motivated and the drive and desire to find out must come from you.
 It takes a considerable time to become a skilled lathe operator and to possess all the skill of hand that goes with it. Therefore it is not expected that you will be manually skilled on completion of the module but you will have gained intellectually and without doubt, by practical involvement, some skill of hand will be achieved. 
 





*Figure 1. Example of a Typical Centre Lathe*

 . Centre Lathe
 The term Centre Lathe is derived from the fact that in its operation the lathe holds a piece of material between two rigid supports called centres, or by some other device such as a chuck or faceplate which revolves about the centre line of the lathe. 
 The lathe shown above is a typical example. This machine is usually used in a jobbing (one off) situation or for small batch work where it would be too expensive to specially 'tool up' for just a few items. 
 The lathe on which you will work is a machine used to cut metal. The spindle carrying the work is rotated whilst a cutting tool, which is supported in a tool post, is made to travel in a certain direction depending on the form of surface required. If the tool moves parallel to the axis of the rotation of the work a cylindrical surface is produced as in Fig 2 (a) , whilst if it moves at right angles to this axis it produces a flat surface as in Fig 2 (b).
 





 





​ *Figure 2a. Producing a *
* Cylindrical Surface*
*Figure 2b. Producing a Flat Surface* 

 The lathe can also be used for the purposes shown in Fig 2c, 2d, 2e and 2f.
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*Figure 2c. Taper Turning*
*Figure 2d. Parting Off / Under Cutting*


 




 




*Figure 2e. Radius Turning Attachment*
*Figure 2f. Drilling on a Lathe* ​

. Cutting Tools 
 The tool used in a lathe is known as a single point cutting tool. It has one cutting edge or point whereas a drill has two cutting edges and a file has numerous points or teeth. 
 The lathe tool shears the metal rather than cuts as will be seen later and it can only do so if there is relative motion between the tool and the workpiece. For example, the work is rotating and the tool is moved into its path such that it forms an obstruction and shearing takes place. Of course the amount of movement is of paramount importance - too much at once could for instance result in breakage of the tool. 
 




 *Figure 3. Types of Cutting Tool*
 The type and design of the tools selected will depend on the job in hand, the machining operation selected and the material to be cut. The correct tool especially the various face angles are essential if the operation is to be done in a cost-effective (i.e. productive) way. The tools used in a lathe are various, some of which are shown in figure 3.
 The range of cutting tool types is extensive and a few examples only are shown in this handout. Nonetheless you should take every opportunity to look deeper into the types of tools available.​
​ . Basic Metal Cutting Theory 

The usual conception of cutting suggests clearing the substance apart with a thin knife or wedge. When metal is cut the action is rather different and although the tool will always be wedge shaped in the cutting area and the cutting edge should always be sharp the wedge angle will be far too great for it to be considered knife shaped. Consequently a shearing action takes place when the work moves against the tool. 
 




*Figure 4. Basic Metal Cutting Theory*

 Figure 4 shows a tool being moved against a fixed work piece. When the cut is in progress the chip presses heavily on the top face of the tool and continuous shearing takes place across the shear plane AB. Although the Figure shows a tool working in the horizontal plane with the workpiece stationary, the same action takes place with the work piece revolving and the tool stationary. 
 
. Tool Angles 
 There are three important angles in the construction of a cutting tool rake angle, clearance angle and plan approach angle.
 




 *Figure 5. Main Features of a Single Point Cutting Tool* 
 *Rake Angle* 
 Rake angle is the angle between the top face of the tool and the normal to the work surface at the cutting edge. In general, the larger the rake angle, the smaller the cutting force on the tool, since for a given depth of cut the shear plane AB, shown in Figure 4 decreases as rake angle increases. A large rake angle will improve cutting action, but would lead to early tool failure, since the tool wedge angle is relatively weak. A compromise must therefore be made between adequate strength and good cutting action.
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*Metal Being Cut *
 Cast Iron
 Hard Steel / Brass
 Medium Carbon Steel
 Mild Steel
 Aluminium
*Top Rake Angle*
 0°
 8°
 14°
 20°
 40°

​ *Table 1. Typical value for top rake angle* 
 *Clearance Angle*
 Clearance angle is the angle between the flank or front face of the tool and a tangent to the work surface originating at the cutting edge. All cutting tools must have clearance to allow cutting to take place. Clearance should be kept to a minimum, as excessive clearance angle will not improve cutting efficiency and will merely weaken the tool. Typical value for front clearance angle is 6° in external turning. 
 *Plan Profile of Tool*
 The plan shape of the tool is often dictated by the shape of the work, but it also has an effect on the tool life and the cutting process. Figure 6 shows two tools, one where a square edge is desired and the other where the steps in the work end with a chamfer or angle. The diagram shows that, for the same depth of cut, the angled tool has a much greater length of cutting edge in contact with the work and thus the load per unit length of the edge is reduced. The angle at which the edge approaches the work should in theory be as large as possible, but if too large, chatter may occur. This angle, known as the Plan Approach Angle, should therefore be as large as possible without causing chatter. 





 *Figure 6. Plan Approach Angle*​ 
 The trailing edge of the tool is ground backwards to give clearance and prevent rubbing and a good general guide is to grind the trailing edge at 90° to the cutting edge. Thus the Trail Angle or Relief Angle will depend upon the approach angle. 
 A small nose radius on the tool improves the cutting and reduces tool wear. If a sharp point is used it gives poor finish and wears rapidly
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## عبدالله الجنابي (26 مارس 2010)

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