Cutting Speed and Feed on a Lathe Machine

To obtain the best results the operator should know the basic elements of the cutting process. These elements are depth of cut, cutting speed and feed. When a blank is turned on  a lathe, the excessive material is removed with the tool in the form of chips in one or more cuts. The main dimensions of the chip are the depth of cut (t) and the feed (s).

Cutting Speed

The cutting speed is the velocity of the tool over the surface of the job and is expressed in feet or meter per minute. When the work is rotating and the tool is fixed as in case of lathe machine the cutting speed varies directly as per the revolution of the work. When the tool is rotating and the work is fixed as in case of milling machine the cutting speed varies directly with the revolution of the tool. The same principle is applied for drilling as well as grinding operations. Mathematically it is expressed as,

Depth of Cut

It is the advancement (penetration) of the tool in the job in the direction perpendicular to the surface being machined or the thickness of the chip of material removed by the tool in one cut. The amount of metal to be removed, type of the tool and the rigidity of the machine structure determine the depth of cut in turning operations. Depth of cut for normal roughing operations may be from 2 to 5 mm and for finishing operations from 0.5 to 1 mm.

The depth of cut of job, which has its diameter before cut as ‘D’ mm and is reduced to the diameter ‘d’ mm after cut in one pass of the tool, will be equal to one half the difference in diameters. It can be calculated by the following formulae:

Feed

It refers to the amount of tool advancement per revolution of the job parallel to the surface of the job to be machined. The feed of a tool depends upon many factors, such as the depth of cut, surface finish required the characteristics of the tool and work piece, and the rigidity of the machine tool. Feeds are classified as follows: -

(a) Longitudinal Feed - It is the feed parallel with the axis of rotation as in case of turning.

(b) Cross Feed - When the fed is given at right angle to the axis of rotation as in case of facing

(c) Angular Feed - It is the feed directed at an angle to the axis of rotation as in case of taper turning.

Determination of Cutting Speed and Feed

Cutting speed and feed are determined by the following factors:

(a) Kind Of Material Being Cut - The harder the material, the more force required to remove the chip and the more rapid the wear on the tool. For this reason, hard materials are to be machined at lower cutting speeds and smaller feeds than soft materials.

(b) Kind Of Material And Life Of The Tool - An increase in cutting speed will result in more intensive heat generation, consequently, more heat resistant tool materials should be used when machining at high cutting speeds. Carbon steel tools can take about one half the cutting speed of high-speed steel tool. Ceramic and carbide tools will stand still greater speeds. These heat resistant tools may be used under heavier feeds, than other tool materials.

(c) Shape (Angles) And Dimensions Of The Cutting Element - A change in the chief angles of the cutting tool will correspondingly change the forces due to the cutting action, as well as the conditions for heat transmission through the cutting elements of the tool.

(d) Size Of Chip Cross-Section - The size of chip cross-section affects the forces due to cutting and, consequently, the amount of heat generated. Tool wear is more rapid with an increase in cutting speed than with an increase in chip cross-section. For this reason, an increase in production capacity at a given tool life can be provided by increasing the cross-section of the chip removed and not the cutting speed. In such cases, the cross-section of the chip should be increased by increasing the depth of cut and not the feed.

(e) Types Of Finish Desired - In general, high cutting speeds and fine feeds give the best finish.

(f) Rigidity Of The Machine - No work should be done at speed and feeds that cause vibration in the machine.

(g) Types Of Coolant Used - Cooling with cutting fluids is not only for carrying away the heat generated, but also because of the lubricating effect of the fluid on the working surface of the tool. When a cutting fluid is used in machining tough material, the productivity may be increased from 15 to 30 % and more in comparison with dry operation. So higher cutting speeds and larger feeds may given using a suitable cutting fluid.

Average cutting speed for a high-speed steel tool is given below.

Cutting Speed of H.S.S. Tool in Meter/Min. for Different Materials on

Material	Turning	Threading	Drilling	Reaming
Mild Steel	25.31mtr/min.	8.1mtr/min.	28.35mtr/min.	10.15mtr/min.
Aluminium	90.12mtr/min.	25.30mtr/min.	60.85mtr/min.	20.25mtr/min.
Cast iron	16.22mtr/min.	7.8mtr/min.	22.30mtr/min.	6.8mtr/min.
Brass	60.80mtr/min.	20.25mtr/min.	60.90mtr/min.	25.30mtr/min.

Calculating Machining Time

The machining time in a lathe operation can be calculated if the speed, feed and length of the work piece is known. Time to complete one cut is calculated as:

Factors Affecting Cutting Speed (Machinability)

The machinability of a metal may be defined as "The most machinable metal is one which permits the removal of material with a satisfactory finish at lowest cost." In other words, the most machinable metal is one which will permit the fastest removal of the largest amount of material with satisfactory finish.

Machinability is influenced by the variables pertaining to the machine, the cutting tool, cutting conditions and work material. The following are the factors which affect the cutting speed:

(a) Machine Factors - The efficiency of any machining operation depends on the overall rigidity of the system consisting of the machine tool, the cutting tool and the work piece. The machine on which the material is to be machined should be rigid and should have sufficient power to withstand the induced cutting forces and to minimize deflections. If the machine is not sufficiently rigid and has less power, the tool life will be reduced in addition to affecting the accuracy and surface finish. Lower values of cutting speeds have to be employed and the dimensions of cut, namely, feed and depth of cut, have to be reduced to limit the induced cutting forces. Thus, the machinability of the material is indirectly affected by the machine variables.

(b) Tool Materials - Of all the factors governing machinability, the most basic is the cutting tool. Without tools made from the proper tool material and having proper geometry, a cutting operation cannot be performed efficiently, even though all other machining factors are closely controlled. If the cutting tool is not optimized, the material removal rates must be reduced to obtain a reasonable value of tool life, otherwise machining costs will be increased.

(c) Tool Geometry - Proper tool geometry is essential for efficient machining operations, and it should be chosen depending on the work material and machining conditions. Not much data on the effect of variations of tool geometry on tool life exists and, therefore, standardized tool geometries should be adopted wherever possible.

(d) Nature Of Engagement Of Tool With Work - If the tool is continuously in continuously in contact with the work piece, as in the case of turning a full cylinder, the tool life value will be generally higher than in the case of an  interrupted cut, as is the  case with slots or keyways in a cylindrical piece.

(e) Tool Rigidity - The work piece material, size and configuration determine the type of machining operation and the cutting tool to be used. Because of the cutting forces involved, the tool defects causing a detrimental effect on the tool life, surface finish and dimensional accuracy.

(f) Cutting Fluids - Cutting fluids permit the use of higher cutting speeds and feeds and prolong the tool life. This is achieved by the cooling, lubricating and chip flushing action of the cutting fluid. However, cutting fluids do not have significant effect on cutting forces at higher speeds. But a good cutting fluid considerably reduces cutting forces at low or extremely low cutting speeds. The surface finish and dimensional tolerance generally improve when cutting fluids are employed.