Question

Your assignment is to design a portable lathe type machine to perform simple turning operations needed to produce replacement parts at a repair shop. The only information you have on the material is that it machines similar to plain carbon steel. In order to finish your design of the lathe type machine and related tooling, you must calculate:

1. The cutting force
2. The horsepower of the motor
3. Explain the key assumptions that were first needed to start the solution. What approaches did you consider to solve the problem, and why did you choose this method

There are various methods to solve this design problem without running experimental cutting tests with the material. Hint: You might use a machinist’s handbook to solve the problem. One example is the Machinery’s Handbook.

Answer 1

1)Carbon steels are usually relatively soft and have low strength. They do, however, have high ductility, making them excellent for machining, welding and low cost.
2)the similar material Magnese,silicon and copper
3)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.
4) Work Material:
1)The cutting forces vary to a great extent depending upon the physical and mechanical properties of the material.

2) Cutting Speed:Fig shows how the tangential force Pz varies with increase in cutting speed. Similar curves are applicable for Py and Px 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.

3. Feed:

The tangential component of cutting force is greatly influenced by the feed rate.

4. Depth of Cut:

The tangential component Pz 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 Pz is more or less constant within the range 90° to 55° and increases slightly for approach angles less than 55°. Axial component Px is maximum for approach angle of 90° and decreases with decrease in approach angle. Radial component Py 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 Pz and Px are considerable and thus result in vibrations. For negative side rake angles, component Pz 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 Pz 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 Pz 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 Px as well as Pz and Py 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.

hence

cutting force:Pc=ap.f.vc.kc/60.1000*n

Pc (kW) : Actual Cutting Power
ap (mm) : Depth of Cut
f (mm/rev) : Feed per Revolution
vc (m/min) : Cutting Speed
Kc (MPa) : Specific Cutting Force
n: (Machine Coefficient)

FOR CALCULATING THE HORSE POWER

HP of a motor means the mechanical power required by the load usually a rotating machine plus losses and interia the HP can be calculated by sing the following formulae

P=2pi*rpm*torque/60 in watts

HP of load=p÷745

But we don't know the torque for every load

So based upon application we can determine the HP or KW of motor

The key feature that were firt needed is the the type of material needed going to use

than after selecting the material we used the speed that we require the cutting force work material, cutting speed, feed rate, depth of cut, approach angle, side rake angle, back rake angle, nose radius and tool wear

I choosed this method as in this th prerequisite for the slection of lathe as well as material was covered.