UTeM BMFR Weblog

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

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.

Figure 2c. Taper Turning	Figure 2d. Parting Off / Under Cutting
Figure 2e. Radius Turning Attachment	Figure 2f. Drilling on a Lathe

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.

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.

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.

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.

High Carbon Steel

Contains 1 – 1.4% carbon with some addition of chromium and tungsten to improve wear resistance. The steel begins to lose its hardness at about 250° C, and is not favoured for modern machining operations where high speeds and heavy cuts are usually employed.

High Speed Steel (H.S.S.)

Steel, which has a hot hardness value of about 600° C, possesses good strength and shock resistant properties. It is commonly used for single point lathe cutting tools and multi point cutting tools such as drills, reamers and milling cutters.

Cemented Carbides

An extremely hard material made from tungsten powder. Carbide tools are usually used in the form of brazed or clamped tips. High cutting speeds may be used and materials difficult to cut with HSS may be readily machined using carbide tipped tool.

For all metal-cutting processes, “speeds and feeds” are important parameters. The colloquial term “speeds and feeds” refers to the speed, feed, and depth of cut of a metal-cutting process. To describe these parameters, we will be using the turning process. The figure below shows the important geometry. The speed (Labeled on the figure as V.) is the cutting speed, which is a measure of the part cut surface speed relative to the (here, stationary) tool.

Speed is a velocity unit, which is typically listed in terms of feet/min, inches/min, meters/second, or meters/min.

Feed (Labeled on the figure as f_r), is the amount of material removed for each revolution or per pass of the tool over the workpiece. Feed is measured in units of length/revolution, length/pass, length/tooth, length/time, or other appropriate unit for the particular process.

The depth of cut, DOC (Labeled on the figure as d.), represents the third parameter for metal cutting. For turning, DOC is the depth that the tool is plunged into the surface. The DOC is half of the difference in the diameters D_a and D_b,

the initial and final diameters, respectively.

Below is a summary list of the terms used.

Speeds and feeds are important since they are critical input parameters which determine the output of a machine tool. Other inputs include the material of the part, the shape of the raw stock and final part, and the geometry of the machine tool used. Outputs include the surface finish, the time to machine each part, the temperature of the cutting tool and cut part, the residual stresses left in the part, and warping of the part.

Speeds and feeds typically take much experience and experimenation to determine accurately, but a good place to start is a table of recommended speeds and feeds. The MRR, or the Metal Removal Rate, is listed at the end of the above relation list. For turning, MRR values range from 0.1 to 600 in³ per minute. Most processes have MRR’s that can be expressed as the volume of metal removed divided by the time needed to remove it:
MRR = (volume of cut)/(cutting time).
MRR can be used to estimate the power required to sustain the cutting operation. With turning, the cutting time can be expressed as the following:

Where the symbols have the same meanings as defined earlier on this page and the allowance is an estimation factor which is added to the L term to allow for the tool to enter and exit the cut.

A single-point tool is a cutting tool that cuts only at a single edge or area of the cutting tool. Turning and shaping are examples of single-point cutting. Multiple-point cutting processes are milling and drilling.