Surface Roughness in Milling

Lecturer

Introduction

In the present world, due to high demand of goods with different surface finishing, there is need to find a way of testing the roughness of materials. From the look of a material from a far, it may appear to be very smooth, even if it is felt with one’s hand (Maradudin 125). However, when tested with a device specifically formed for the function, it is found to be the roughest. Materials such as glass looks and feels smooth but they are in real sense very rough.

Machining is the final process in the manufacturing process, it determines roughness and the texture of the surface. The factors which highly dictate the surface finish are the feed and the geometry of the tool. The roughness of a single point tool can be predicted using the equation;

Ri =Surface Roughness in Milling

Ri is the theoretical average roughness

F is the feed

R is the radius of the tool used in the machining process.

In the machining process, it is almost impossible to achieve ideal surface finish because of the presence of many uncontrolled factors such as the material and the interaction of the tool with the work piece.

Milling machines and lathe are used to finish and cut the surfaces (Maradudin 132). It would be of much interest for one to know how to set the machine in use to get either rough or smooth surface depending on the need. Aluminium is the best material in the laboratory to help determine factors which would influence the surface finish of a material. After machining, the surface roughness would be determined using profile meter.

Objective

The main aim of this experiment was to determine factors that dictate the surface roughness of materials during and after machine processes. Some of the factors that will be tested here are the cutting speed and the feed rate of the machine.

Implementation

In this process low and high speed are selected and this are 1200 and 3230 rev/ min respectively. The spindle speed N (rev/min) and the cutting speed are related to the cutting sped v (m/min) and the cutter diameter D (50mm).

N=
Surface Roughness in Milling 1

Also the low feed rate and high feed rate were selected. These were 180 mm/min and 653mm/min.

The feed rate and the cutting speed were used in various combinations to achieve different results.

The feed rate was selected from a range of 1 to 9 where 3 and 8 were selected respectively. Having the table set with different cutting speed and feed rate, a 50 x 50 mm block was machined on its various faces.

During the process of machining, precautions were taken such as wearing protective gloves, goggles, closed shoes, dust coat and gloves.

For various faces the following setting were used in the laboratory;

Face 1 for block 1 low cutting speed and low feed rate

Face 2 for block 1 low cutting speed and high feed rate

Face 1 for block 2 high cutting speed and low feed rate

Face 2 for block 2 high cutting speed and high federate

Face 1 block 1

Value µm

Face 2 block 2

Value µm

Face 1 block 2

Value µm

Face 2 block 2

Value µm

Effect of cutting speed on the surface roughness

For any application in the industry, the texture or the surface roughness of a material is tested before use. Roughness comparator can be used or any other calibrated instrument (Wennerberg 105). The roughness of the four faces was tested after the machining to see how they would be depending on the setting of the machine. For example; low cutting speed versus low feed rate.

Surface roughness values for constant feed rate varying at Cutting speed

Cutting speed V (m/min)

Surface roughness Ra (μm) at low feed rate 180 mm/min

Surface roughness Ra (μm) at High feed rate 653 mm/min

Surface Roughness in Milling 2

Series 1 = Surface roughness Ra (μm) at low feed rate 180 mm/min

Series 2 = Surface roughness Ra (μm) at High feed rate 653 mm/min

Feed rate mm/min

Surface roughness Ra (μm) at low cutting speed 1200 m/min

Surface roughness Ra (μm) at high cutting speed 3230 m/min

Surface Roughness in Milling 3

Series 1 = Surface roughness Ra (μm) at low cutting speed 1200 m/min

Series 2 = Surface roughness Ra (μm) at high cutting speed 3230 m/min

From the above graphs one can see that the higher the speed the higher the extent of surface roughness (Wennerberg 105). If one wants to obtain a fine surface the feed rate and the cutting speed should be low.

It is seen, therefore, from above that the surface texture is influenced by the cutting speed and the feed rate. Also during experiment, it is easy for someone to conclude that a surface is fine while it is not.

Discussion

It is seen from the experimental values and the graphs that there is need for a smooth surface, low speed and low feed rate are the best setting one can go for. When the feed rate is high and the cutting speed is low the roughness is relatively high (Yang 185). The values obtained at high feed rate and low cutting speed is 0.273 micro millimeter.

There is a lay feature formed when the cutting speed is low and feed rate is high. However, the lay disappears when the feed rate is low and cutting speed is low.

The various finishing surfaces have different uses. When the cutting speed is high and the feed rate low or high, the surfaces are used when bearings are being manufactured (Yang 331). For low feed rate and either high or low cutting speed, such finishes are used when the texture is not necessary. Smoother surface takes more time to achieve.

Machine Tooth Chip Load

Computation of the chip load per tooth for samples A, B, C and D

Surface Roughness in Milling 4= N
Surface Roughness in Milling 5 f

(“Feed Rate Calculations,” f.v.)

fr = rate of feed (mm/min) Low (180), high (653)

N = (rev/min) speed of Spindle Low (1200), high (3230)

nt = Number of teeth on the cutter 4 teeth

f = (mm/tooth) chip load / tooth

The equation can be rearranged to get the chip load per tooth

Surface Roughness in Milling 6

Face 1 block 1 (low feed rate, low cutting speed)

Surface Roughness in Milling 7

Face 2 block 1 (High feed rate .low cutting speed)

Surface Roughness in Milling 8

Face 1 block 2 (low feed rate, High cutting speed)

Surface Roughness in Milling 9

Face 2 block 2 (High feed rate, High cutting speed)

Surface Roughness in Milling 10

Surface roughness and Chip load per tooth

The Table below is a record of the surface roughness for each block and face during the experiment. Also chip load per tooth values as calculated previously are included.

A table showing Chip load per sample

Estimated finish in (µm)

Trial 1 in (µm)

Trial 2 in (µm)

Trial 3 in (µm)

Average Surface roughness in (µm)

Chip load per tooth in

Face 1 block 1

Face 2 block 1

Face 1 block 2

Face 2 block 2

Therefore it can be seen that the surface roughness and chip load per tooth relates in the equation

Surface Roughness in Milling 11

Where Ra = surface roughness, n = number of teeth on the cutter, R = radius of the cutter, f = feed per revolution,

This is a quadratic equation, it explains the pitch in Sample face 2 block 1 chip load per tooth.

Work cited

Maradudin, Alexei A. Light Scattering and Nanoscale Surface Roughness. New York, NY: Springer, 2007. Print.

Yang, Chunyan. Role of Surface Roughness in Tribology: From Atomic to Macroscopic Scale. Jülich: Forschungszentrum, Zentralbibliothek, 2008. Print.

Surface Texture: Surface Roughness, Waviness and Lay. New York: American Society of Mechanical Engineers, 1986. Print.

Wennerberg, Ann. On Surface Roughness and Implant Incorporation. Göteborg, 1996. Print.