Materials and Manufacturing - Surface Roughness Turning of Carbon Steel - Management Assignment Help

Assignment Task

INTRODUCTION

The process of turning has been in use for over two millennia. The earliest surviving account of a lathe dates back to the third century BC (The Woodturner's workshop, 2008) Advances made in the industrial age as well as those discovering new and stronger alloys allowed for turning to be applied to metals.

The applciations of turning are as widespread as manufacturing and secondary industries themselves. Turning is a method of “Material removal” By which a fixed dimension, presumed cylindrical, metal has one end fixed within the “Collet” of the lathe.

Once the workpiece is secured, the cutting tool is manually zeroed in to the endpoint of the piece in the x and y planes. After this zero point is applied, accurate information can be programmed about cut length and cut depth.

Surface roughness can be controlled by changes of variables in the machine including feed rate and the cutting piece. Specifcations on CAD blueprints of machined goods are provided in many cases for how smooth a part needs to be. In some cases smoothness is desireable, whilst in others it is not.

PROCEDURE

A piece of carbon steel measuring Ø 50mm x 200mm. The piece was affixed to the turning machine via the collet, allowing 100mm to be exposed.

A low cutting speed of 73.827 m/min was selected. The relative Rpm for the turning machine is given by the following equation:

And because a division by the circumference of the material occurs the ouput N can be given in R (revolutions) per minute.

A high cutting speed of 235.619 m/min was selected. Though higher than the outlined speed requirements, this choice was necessary as our originally specified 200 m/min produced an Rpm not programmable by the machine. The required Rpm was as follows

A feed rate of 0.5 mm/rev was selected at the advice of the lab instructor. The length of the affixed piece of carbon steel was divided into 4 pieces of approximately 50mm. of which 100mm was visible from the end of the machine. These two pieces were denoted “1” and “2” respectively.

For pieces 1 and 2 a cutting piece with a sharp nose radius of 0.4mm was used.

The y and x axis were zeroed on the turning machine against the edge of the carbon steel cylinder. A designated cut depth of 2mm was then programmed.

Section 1 was turned at 470 Rpm (73.827 m/min) with a feed of 0.5mm/rev. During the process the cutting piece was lubricated with an oil and the swarf was observed to be long and helical. The piece was observed to have a high surface roughness that was anisotropic.

The turning was disengaged once the y value read 50mm. it was zeroed at the new location. Cut depth remained 2mm.

Section 2 was turned at 1500Rpm (235.619 m/min) with a feed of 0.5 mm/rev. during the process the cutting piece was again lubricated and swarf was observed to short and chip like. The piece was observed to have a high surface roughness that was anisotropic.

The carbon steel rod was then reversed so the uncut side protruded from the collet. Because the device measures Rpm from its central point no adjustment in cutting speed occurs due the piece in the collet measuring Ø 46mm. The exposed piece is still Ø 50mm

The cutting piece measuring 0.4mm was swapped out for a piece measuring 1.2mm.

Section 3 was turned at 470 Rpm (73.827 m/min) with a feed of 0.5mm/rev. During the process the cutting piece was again lubricated with an oil and the swarf was observed to be long and helical. The piece was observed to have a low surface roughness that was anisotropic.

The turning was disengaged once the y value read 50mm. it was zeroed at the new location. Cut depth remained 2mm.

Section 4 was turned at 1500Rpm (235.619 m/min) with a feed of 0.5 mm/rev. during the process the cutting piece was again lubricated and swarf was observed to short and chip like. The piece was observed to have a low surface roughness that was anisotropic.

Figure 4 Sections 3 and 4, the swarf can be noted at the bottom of the machine. Two distinct types were observed.

The 4 sections were felt by hand for an idea of their roughness. They were compared to a chart of specific roughness to ascertain their approximate values.

The sections were then measured using a Surface Roughness Measurement Instrument.

RESULTS

There was a distinctly observable difference between using the smaller cutting tool and the larger. The larger produced significantly better results with the carbon steel used. Visually it seemed that an increase in Rpm produced a better result, however the chipped swarf suggested to us that perhaps there was more damage than could be observed by the naked eye.

By using the roughness chart the team members made the following predictions about the surface roughness of the sections:

A graph can be constructed using the data above comparing cutting speed, surface roughness and nose size of the cutting tool:

INFERENCE FROM DATA

The data suggests that a larger nose creates a smoother surface. How far this relationship between nose size and surface smoothness can be applied is unknown and cannot be concluded from the sample size of this data. It seems like a counter intuitive conclusion given that the rate of precision has been reduced by a rate of three however it is possible that because of the feed rate remaining constant for both the larger nose covers a greater surface area per revolution and therefor “smooths” the material more effectively. Contrary to our observed results, Sulaiman argues that a carbide nose must be run at high speeds in order to achieve a higher smoothness surface finish (Sulaiman, 2012). These conclusions are much more logical as the increase in speed generates more heat on the surface, making the material easier to deform and lathe due to creep and other factors.

The trade of increasing the speed to increase the wear is that the effect translates both ways, onto the nose as well as the worked piece. Accordingly life of the carbide nose is reduced. (Sulaiman, 2012). Sulaiman also argues that the single biggest factor in surface roughness is feed rate, a factor we did not change.

The difference in the reflectivity of the finishes had an effect on the perceived smoothness to the eye, however it can be argued that the reflectivity is due to the exposure to greater heat and not an argument of smoothness. The increase of heat due to friction against the carbide bit could have caused a change to its reflective index by means of a change to its molecular lattice work at the surface (L. A. Mal’tseva, 2010)

By looking at the equation for the shear modulus we can derive that an increase in the force applied to the steel results in an increase in the shear. The equation is given by:

Because the instantaneous force at the point of contact is increased with N, so too is the shear

INDUSTRIAL APPLICATIONS

SURFACE 1:

Surface 1 had an average surface roughness of 10.83 µm RMS anisotropically with a deviation of ± 0.17 µm. This degree of smoothness and uniformity has industry applications in fields such as adhesion of metals to other surfaces (Persson, 2005). At the 10-20 µm range adhesion to tacks like scotch tape (Ra ≈20 µm RMS) (Persson, 2005) is at its best. Accordingly, metals whose primary function will be to interact in friction generating environments are best within the aforementioned range. A surface roughness this coarse may be specified with corrosion resistant (CR) and have applications as such (Sandvik Materials Technology, 2014).

SURFACE 2:

Surface 2 had an average surface roughness of 10.93 µm RMS anisotropically with a deviation of ± 0.07 µm. Similar to Surface 1 this surface roughness is optimal for use with polymer adhesives. Similarly, this surface roughness falls within the allocated guidelines of the CAD for the machined part shown in figure 2. Again, this level of surface roughness has applications in CR sections of a machine, such as engine exteriors (Sandvik Materials Technology, 2014).

SURFACE 3:

Surface 3 had an average surface roughness of 6.9µm RMS anisotropically with a deviation of ± 0.15 µm. The application of a surface roughness Ra ≈7 µm RMS is for annealed and heat resistant properties required by the machined part (Sandvik Materials Technology, 2014).

SURFACE 4:

Surface 4 had an average surface roughness of 6.86µm RMS anisotropically with a deviation of ± 0.14 µm. similar to surface 3, the application of a surface such as this is best suited for annealed and heat resistant properties required by the machined part (Sandvik Materials Technology, 2014).

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