BASIC MANUFACTURING TECHNIQUES AND HARDNESS TESTING

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Hardness refers to the degree to which a substance can withstand plastic deformations resulting from indentation. Atkinson (2014) describes hardness as the surface property of a material dependent on its ability to withstand wear, indentation, and scratches. Materials with higher hardness properties appear to have more resistant surfaces and show difficulties in operations such as machining and cutting. The hardness of materials has thus been assessed on the basis of their degree of resistance to these operations; an attribute that lacks the inherent material properties that can be defined by the basic units of time, mass and volume. The value of a material’s hardness is dependent on its tensile strength, yield strength and modulus of elasticity. Some of the advantages of hardness tests which make them preferred to other assessment instruments include their ease of application, cost effectiveness, non-destructiveness and the fact that they can be performed in situ and on samples of varying sizes and shapes.

Hardness Test Methods

There are three commonly used methods of testing hardness in metals. They include; Brinell, Vickers and Rockwell methods. Each of these techniques has advantages and disadvantages as discussed below.

Brinell hardness test

The Brinell method of testing hardness in metals uses a hardened steel ball whose diameter is 2.5, 5 or 10 millimeters as its indenter. In other instances, a carbide ball may be used as the indenter and is subjected to a load of 3 tones. The load could be decreased to 0.5 tons for softer materials as a way of avoiding excessive indentation. In this method, the full load is applied for more than 30 seconds. The indentation’s diameter left in the test material is then determined by the use of a microscope with low power. This is then followed by the calculation of the Brinnell hardness numerical value by the formula in figure 1 below.

Figure 1a: Brinell hardness test Figure 1b: Brinell hardness equation

In this case, the impression’s diameter is taken as an average of two readings taken at right angles. In instances where extremely hard metals are to be measured, a ball of tungsten carbide is used in the place of the steel one. The Brinell ball yields the deepest and widest indentation in comparison to other methods of testing hardness. This method should be embraced when determining the macro-hardness of materials whose structures are heterogeneous in nature (Atkinson, 2014).

Rockwell hardness test

The Rockwell test measures the hardness of a metal based on the extents of penetration of the indenter into the specimen. In this case, the indenting material may be a diamond cone or a hardened steel ball. To force the indenter inside the test material, a preliminary minor force (F0) of 10kgf is used (Figure 2 a.). An indicating device which monitors the motion of the indenter is applied when the equilibrium is arrived at to indicate the deviations in depth due to the penetration. To achieve this, the indenter is placed at a datum position. An additional major load is then applied to the preliminary minor load resulting to an increase in the depth of penetration (figure 2b). Upon reaching the equilibrium, the additional major load is removed while retaining the minor preliminary load to allow for partial recovery; a phenomenon that leads to a reduction in the penetration depth (figure 2c). The Rockwell hardness number is then calculated based on the permanent depth increment which emanates from offloading the major load.

Figure 2: Rockwell hardness test

In this case, the Rockwell hardness number is calculated using the formula

HR = E- e

Where:

e = the permanent depth increment resulting from the major load, F1

E = a constant of the indenter

HR = Rockwell hardness number

Vickers hardness test

This method indents the testing material with the aid of a right pyramidal diamond indenter with a square base whose opposite sides are at an angle of 136 degrees. The pyramid is then subjected to a load ranging from 1 and 100 kg.. In this case, the full load should be applied for a period of 15 to 20 seconds. A microscope is then used to measure the two diagonals of the indentation on the surface of the material after the load is removed. The average of the two diagonals (d1 and d2) and the area of the sloping surface are then calculated.

Figure 3: Vickers hardness test

In this formula: d is the mean of the diagonals, d1 and d2 and HV is Vickers hardness.

My Recommendation

While the three methods of testing hardness are effective, I would recommend that the company adopts the Rockwell hardness test. This is due to its ease of use and cost effectiveness. For instance, Rockwell hardness tests do not require the preparation of a specialized surfaces and indentation measurements like other tests. Further, this method has high accuracy levels in comparison to other tests since it employs a Rockwell indenter of a larger size; a phenomenon that allows for maximum sampling over a gross portion (Atkinson, 2014, p. 107). Rockwell tests are fast and take about 30 seconds with higher levels of precision since the measurements obtained are true representatives of the whole microstructure.

Other Metallic Properties

A part from hardness, strength, and toughness are two other metallic properties which need to be tested by the company. Strength is a metallic property that makes it resistant to deformation. Strength is multidimensional and looks into the extents to which a metal can withstand strain, stress, and load. Ultimate strength refers to the metal’s ability to withstand strain. Tensile strength looks into the extents to which a metal is resistant to pull forces when placed under a tension load. On the other hand, fatigue strength refers to a metal’s ability to remain resistant to different types of stresses expressed as the magnitude of alternating stress in a number of cycles. Impact strength refers to metal’s ability to remain resistant to sudden loads. The toughness of a metal refers to its ability to withstand deformation and shock without rapturing. The toughness of material is a combination of its plasticity and strength (Atkinson, 2014).

Part 2: Manufacturing Technique

Subtractive Machining

Subtractive manufacturing is a collection of processes which involve cutting a raw material into the desired shape using material removal techniques (Kalpajian and Schmid 2014, p. 913). Subtractive machining has undergone massive revolutions over the years and is currently done as computer numerical control (CNC) in which the movements of machines such as lathes, mills, and cutters are controlled by computerized systems. This technique embraces three key machining principles which include milling, turning and drilling with shaping, sawing, planning, broaching and boring being considered as miscellaneous processes.

Milling

This is a machining process in which tools with multiple cutting edges are moved relative to a material as a way of generating straightened or plane surfaces. In this case, the metal may be moved in linear or angular directions to get the desired surface. The direction of the force may also undergo variations as shown in the figure below.

Figure 4: Milling as a subtractive machining process

There are two forms of milling; face and peripheral. Face milling aims at creating flat or curved surfaces on the face of a metal as shown below.

Figure 5: Face milling

Figure 6: Milling processes

Turning

In this process, a single edged cutting tool is employed in removing some parts of the metal from a rotating work piece as a way of generating a cylindrical shape. The rotating work piece aids in providing the desired primary motion. To achieve a feed motion the metal is moved along the cutting tool in a slow way in a parallel direction to that of the work piece’s axis of rotation as shown below

Figure 7: Turning processes

Drilling

In this process, operations are made to create holes in a metallic material. The cutting tool (drill) is meant to rotate inside a metal on work pieces that are nonmoving. The figure below summarizes the process of drilling.

Figure 8: Drilling process

A part from subtractive machining, 3D printing is another technique of creating metallic objects. This is an additive process which makes use of digital designs to create physical objects. Here, a solid 3D object is created from a digital model by adding the desired mix of materials layer by layer (Henke and Treml, 2013). Objects formed in this process could take any geometry or shape produced through a digital data model like the additive Manufacturing8 File (AMF). Modeling, printing, and finishing are the three key steps involved in the three-dimensional printing process for metals as discussed below.

Modeling

This involves creation of 3D printable models using computer aided designs (CADs). Such models depict limited errors and may be corrected before printing is done. This allows for design verification before printing. The figure below is an example of a CAD model used in 3D printing.

Figure 9: A 3D model

Printing

3D models take the STL file format and must be checked to correct any errors. Most of these errors come in the form of holes, manifolds, and self-intersections (Henke and Treml, 2013 p. 140). Software known as the “slicer” is used in processing complete STL files which aids in converting the model to be printed in multiple thin layers to form a G-code file which harbors the data for the desired 3D printer. While traditional methods of 3D printing such as injection molding are known to be cheap in manufacturing of high-quality polymer products, additive manufacturing is faster, cheaper, and highly flexible in instances where small quantities need to be manufactured. The figure below shows the printing phase in 3D printing.

Figure 10: Fused filament fabrication 3D printing technique

The metallic material is fed through a heated moving top

The melt is extruded and is deposited layer after layer within the shape

A representation of the platform in motion

Supporting strips for hanging regions

This part lowers after the deposition of each layer

Finishing

While the resolution provided by the printer is effective for most applications, subtractive processes have been applied as ways of attaining higher levels of precision. This is particularly for oversized versions of the objects of desire in standard resolutions. Finishing could be achieved through smoothing and application of various chemical vapor processes.

Recommendations for Case Scenarios

Both 3D and subtractive machining processes find effective applicability in different scenarios. For instance, in the case of manufacturing 100 samples of two different metals, 3D printing would be recommended. This is attributed to the fact that 3D printing is less expensive for small production runs since it operates by customizing each item in the production process. This makes time-consuming less when larger batches are needed.

In manufacturing 10,000 samples of 10 different metals, subtractive machining would be recommended. This needs to be done with the integration of Computer Numerical Control (CNC). With the application of CNC, issues such as specific material requirements and explicit tolerance are put into consideration. In this case, the time of production is reduced since a larger number of parts are produced within a short period. As a matter of fact, CNC production is highly applicable when maximum precision tolerance and repeatability of operations is needed.

Conclusion

Hardness is a metallic property that must be put into consideration when selecting a metal for industrial use. There are three major methods of testing metal hardness; Brinell, Vickers and Rockwell tests. This paper recommends that the company should adopt the Rockwell hardness test because of its ease of use and cost effectiveness. On the other hand, 3D printing and subtractive machining are significant techniques of manufacturing metallic materials. This paper recommends that 3D printing should be adopted for small batches while subtractive machining needs to be embraced for larger batches.

References

Atkinson, H., 2014. Hardness tests for rock characterization. Comprehensive rock engineering. Principles, practice & projects, 3, pp.105-117.

Henke, K. and Treml, S., 2013. Wood based bulk material in 3D printing processes for applications in construction. European Journal of Wood and Wood Products, 71(1), pp.139-141.

Kalpakjian, S. and Schmid, S.R., 2014. Manufacturing engineering and technology (p. 913). Upper Saddle River, NJ, USA: Pearson.

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