Lathe: Cutting Forces, Speed and Turning Problems | Industrial Engineering

In this article we will discuss about:- 1. Cutting Forces of Lathe 2. Cutting Speed 3. Feed 4. Depth of Cut 5. Turning Problems.

Cutting Forces of Lathe:

The cutting forces depend upon several factors like work material, cutting speed, feed rate, depth of cut, approach angle, side rake angle, back rake angle, nose radius and tool wear. The influence of each factor is discussed below in brief.

1. Work Material:

The cutting forces vary to a great extent depending upon the physical and mechanical properties of the material. Tangential force can be determined by multiplying the chip cross-section with the specific cutting resistance offered by the work material, which is found to be decreasing with increasing chip thickness and increases with increase in tensile strength and hardness of the material being cut.

2. Cutting Speed:

Fig. 12.106 shows how the tangential force P_z varies with increase in cutting speed. Similar curves are applicable for P_y and P_x also. It will be noted that the cutting forces first increase with increase in cutting speed and on further increase in speed reach a maximum value and start decreasing and become fairly stabilised at higher speed ranges.

The initial rise in cutting force upto about 70 m/min is due to the effect of built-up edge which does not occur at high speeds. The cutting force at high speeds beyond 70 m/min decreases because of high temperatures involved which tend to make the material plastic.

3. Feed:

The tangential component of cutting force is greatly influenced by the feed rate. It has been observed that cutting force changes linearly with feed at higher speeds, but at slower speeds the change is exponential.

4. Depth of Cut:

The tangential component P_z increases in the same proportion as the depth of cut, if the ratio of depth and feed is more than four.

5. Approach Angle:

The chip size is dependent upon the approach angle. The tangential component P_z is more or less constant within the range 90° to 55° and increases slightly for approach angles less than 55°. Axial component P_x is maximum for approach angle of 90° and decreases with decrease in approach angle. Radial component P_y is minimum for approach angle of 90° and increases with decrease in approach angle.

6. Side Rake Angle:

All the three components of cutting forces decrease as side rake angle changes from — ve value to + ve value; the tangential component alone being predominant for + ve side rake angles and other two being negligible.

However for higher — ve values, both P_z and P_x are considerable and thus result in vibrations. For negative side rake angles, component P_z increases due to higher plastic deformation of chips and increased friction in the tool-chip interface. This type of variation is not so marked at higher speeds as at lower speeds.

7. Back Rake Angle:

It controls the direction of chip flow either away from or towards the workpiece depending upon whether it is + ve or – ve. The vertical component P_z increases slightly as the back rake angle increases from – ve value to + ve value.

8. Nose Radius:

The effect of increasing nose radius is similar to as that of reducing the approach angle. Radial component P_z increases for bigger nose radius resulting in tendency for increase in tool chatter, but tool life and surface finish are improved at higher feeds and depth of cut.

9. Tool Wear:

Tangential force P_x as well as P_z and P_y increase considerably with increase in flank wear.

Cutting Speed:

In lathe, cutting speed means the number of metres measured on the circumference of job that passes the cutting edge of the tool in one minute. While determining the cutting speed for any material, several factors are to be taken into account e.g., the type of job and tool, condition of machine, type of cut required (roughing or finishing), machinability of material, tool materials, presence of hard scale and so on.

Mathematically, cutting speed = (πDN / 1000) metres/minute.

where D = Diameter of job in mm, N = Spindle or job speed in R.P.M.

The selection of proper cutting conditions (i.e., speed, feed, depth of cut, coolant, etc.) plays a vital role in the economy of production.

Feed:

It is the amount of tool advancement per revolution of job parallel to the surface being machined. It is given in mm per revolution of the job. The rate at which the tool is fed depends upon various factors such as finish required, depth of cut and the rigidity of the machine, e.g., a high rate of feed will get the job done in less time but will give a rough finish and will take more time to drive; a slower rate will give a better finish but will take a longer time. Normally feed varies from 0.1 to 1.5 mm.

Depth of Cut:

It is the advancement of tool in the job in a direction perpendicular to the surface being machined. Depth of cut depends upon cutting speed, rigidity of machine- tool and tool material etc. Depth of cut normally varies between 1 to 5 mm for roughing operation and 0.2 to 1 mm for finishing operation.

The various recommended average cutting speeds for a high speed steel tool and carbide tool for different feeds and depths for cutting various metals are given in the following table:

Example:

At what r.p.m. should a lathe be run to give a cutting speed of 25 metres / minute, when turning a rod of diameter 32 mm diameter.

Solution:

Turning Problems of Lathe:

While trouble-shooting of turning problems can best be handled by experience, judgement and sound knowledge of machine tools, cutting tools and work materials; some guidelines given below will be of great help.

1. Poor Surface Finish: 

Surface finish on the work material is dependent on so many factors and it is essential to understand the influence of each variable. The major causes of surface roughness are feed marks of the tool, fragments of built-up edge sticking to the work surface, and chatter due to less rigid set up.

Feed and tool nose radius have direct bearing on the surface finish. Surface finish can be improved by increasing the cutting speed, nose radius, clearance angles; reducing the feed and depth of cut, approach angle and the end-cutting edge angle; and maintaining proper tool centre height.

It is also improved by the lapping of cutting edge of tool and using harder grade of carbide. Use of positive back rake and angular chip breaker to direct chips away from the workpiece also helps to improve surface finish.

2. Excessive Crater Wear:

This occurs due to high speed and wrong selection of carbide. Tip with more titanium carbide helps reduce crater wear. Too tight chip also results in excessive crater wear which can be tackled by increasing the chip breaker distance.

3. Edge Chipping:

This can be remedied by using tougher grade of carbide; decreasing rake angle, clearance angle and approach angle; honing the edges with a diamond lap; using correct brazing technique; and using proper grinding wheel and grinding technique.

If this problem is due to vibrations/chatter, then it can be tackled by reducing tool overhang, using rigid tool shank, adjusting speed, feed and depth of cut, increasing approach angle, reducing nose radius, ensuring rigid work set up, tightening tail stock centre, using work rest and reducing overhang of tail stock spindle and the clearance in tool post slide.

4. Tip Breakage:

This could occur due to lack of rigidity, shocks, putting too much clamping pressure on insert or its improper seating, and crowding of chips, etc. Lack of rigidity can be taken care of by reducing overhang, increasing nose radius, using rigid shank and thicker insert of square type instead of triangular, and properly supporting the workpiece.

Negative rake, tougher carbide grade, proper shim and its tightening, cleaning of inserts and insert pocket help to reduce shock due to interruptions. Increase of chip breaker distance and reduction of feed help in overcoming chip crowding.

5. Short Tool Life:

The rapid wear of tool can be re­duced by using tough carbide, reducing speed, and approach angle, increasing clearance, using annealed workpiece, and increasing/reducing feed depending on whether the material is work hardening type or not.

6. Breaking of Insert at the Nose:

This can be taken care of by using proper shim to suit the insert radius, and using negative shim in negative holders only so as to provide proper support under nose.