Abrasive cutting processes are widely used for semi-finished products from metal bars, slabs, or tubes. Thus, the abrasive cutting processes are applied when requiring precision cutting and productivity at a moderate price. Cut-off tools are cutting discs composed of small abrasive particles embedded in a bonding material, called the binder.This work aims to compare the cutting performance of discs with different composition, in dry cutting of steel bars. Therefore, the method here presented allows identifying cutting discs with a superior abrasive-cutting capability, by combining profilometry and tomography to define micrometrical aspects, grit size, and binder matrix structure. Results show the conclusion that cutting discs with high grit sizes and protrusion, high grit retention by bond material, and closer mesh of fiberglass matrix binder were the optimal solution.
Abrasive discs are commonly used in metal working workshops to cut off slabs and rods of low and medium carbon steels. This type of tool allows cutting, trimming, deburring, welding blank preparation, and surface polishing. For machinists and technicians, abrasive disc’s cost and using life are important aspects in daily production. On the other hand, abrasive discs sizes and binder are the two most influencing facts. There are two abrasive disc types for manual machines, (a) use diamond as abrasive, typically used for cutting masonry, marble, and ceramics; (b) and those usually including aluminum oxide (Al2O3) or silicon carbide (Si3N4) as grit materials, commonly used for cutting metals.
Basically abrasive cutting is an operation producing a narrow and deep groove by abrasion till reaching the entire piece cut off. For instance, in the work presented here, abrasive discs are used in cutting rectangular rolled profiles of structural steel. This is an operation typically performed in dry conditions since the use of coolants using manual or semiautomatic machines is always complicated.
The present work aims at evaluating the cutting performance of abrasive discs in the dry-cutting of medium-carbon steel bars. The “stop-cutting” testing criterion was fixed based on the total surface of steel to cut, while other authors used cutting length. Likewise, disc wear was measured by the variation of wheel external diameter and the disc weight loss. Moreover, final abrasive disc profile and fine topography were evaluated to determine the abrasive particles bonding into the matrix binder.
- Materials and Methods
Tested material was hot-rolled rectangular profiles of a mild steel (steel AISI 1020, U.T.S. 424 MPa, hardness 120 HBN) cross-section of 32 mm × 8 mm. Steel AISI 1020 was (C 0.21 wt%, Mn 0.42 wt%), very common in structural applications and also used as profiles for light machinery construction. An ‘’in house’’ test bench was specially designed and adapted for testing in this project; it was very stiff to eliminate vibrations and geometrical inaccuracies. The testing bench (Figure 1) consisted in a table on which the rolled profiles were clamped by jaws.
Figure 1. Set up used for experimental tests: manual grinder on a stiff bridge structure. Working table includes two movements, X and Y onto rectangular profile is clamped.
3. Results and Discussion
Experimental results regarding different aspects are explained in the following sections, those are: macrogeometry wear, microgeometric wear, and final cut surface quality.
3.1. Macrogeometric Wear
Macrogeometric wear is defined as disc diameter loss or reduction. This wear type follows a typical wear curve, similar to several abrasion degradation processes. Results are gathered in Table 1, which shows average variation in diameter and weight of the four disc types analyzed. Average thickness of each disc type, material removed, and the calculated G-ratio are also indicated.
Table 1. Diametric wear, weight wear and G-Ratio results (average values).
Figure 2 shows wear measured per millimeter of disc width. The results show a linear correlation which gives an idea of the reliability of the results obtained.
Measurements performed by confocal microscope obtaining wear along disc cross section, which in some cases turned out to be non-homogeneous. Hence, Figure 3 shows four 3D topographies obtained from one from each of the four disc types. Regarding disc DE and TB, a more homogenous texture is observed without distinction between grits and bonding material since grain protrusions are lower. This is, if the length from 4 mm to 10 mm of the X axis is taken, the height varies approximately between 1100 mm and 1300 mm.
Figure 3. 3D topography of the worn surface of DE1, PE3, SP3, and TB4 discs.
Thus, from Figure 3, results seem to show that DE and TB discs had the flattest areas, while PE and SP discs show abrasive grains with spacing between them, which implies greater abrasive-cutting and spare room space for removed little chips. Some more specific measurements are in Figure 4.
Figure 4. Cross sections of worn discs: upper row PE and DE, lower row TB and SP.
In Figure 4, profiles that determine the discs form degradation are shown. Results indicate that DE disc is the one losing material uniformly over its entire width, with the flattest profile. In the rest of the discs, however, a more pronounced wear appears on the sides, with the SP disc presenting a lower radius in the profile (less uniform wear). This wear is closely related to the grade of the disc, and is essential to prevent damage to the bar cut surface. The profile flattening shape implies a larger contact wheel-part surface with higher force .
3.2. Microgeometric Wear
Firstly, microgeometric wear suffered by each disc were sorted out. Abott-Firestone curves of the 3D digitalized surface were analyzed, as they are one of the graphs that in roughness measurements do not use a mean value line, which is interesting for this type of non-homogeneous surface.
In Figure 5, the highest value for Spk corresponds to PE disc (Spk = 0.171 µm), as this is the disc with lower wear, and the lowest value (Spk = 0.092 µm) that corresponds to the TB as the most worn disc. As result, this 3D roughness parameter could be used as an indicator for the abrasive-cutting ability of the disc.
Figure 5. Abott-Firestone curves of worn discs.
The average peak height value in a disc section was measured to know the grain abrasive-cutting ability. The results are shown in Figure 6 where SP disc appears as the one with the highest cutting ability (average grain exposure = 0.23 mm), and has the highest grain exposure or protrusion. PE and TB discs have a medium height value of exposed grains. DE discs have the least exposure, which is consistent with a smaller size (average grain exposure = 0.088 mm).
Figure 6. Measurement of the average grain height of worn discs.
Finally, the discs were analyzed by means of X-rays (computed tomography) to observe their internal composition, and by optical stereo microscope (see Figure 7). The thin abrasive-cutting discs are reinforced with fiberglass, nylon, carbon, cotton, linen, wood, silk, or other materials . Reinforcement structures can follow various patterns. In the discs under this study, reinforcement is made of fiberglass with a flat pattern. However, there were great differences between them, because they come from different manufacturers. Figure 7 show the different structures of DE and PE discs using a Leica DMS300 microscope at 10×. This was measured to quantify the different surface patterns.
Figure 7. From top to bottom: (1st row) XR picture of the structure of DE disc, (2nd row) XR picture of the structure of PE disc, (3rd row) XR picture of the structure of SP disc, (4th row) XR picture of the TB disc structure.
From these images and cutting tests, it is concluded that the highest wear is in discs with an large “open mesh” with less fiberglass width, which gives it less resistance to radial wear (macro wear). The rest of the discs showed a similar relationship being the disc with lowest wear was the one with the densest mesh.
Figure 8 shows the relationship between G-ratio and open mesh size, concluding that mesh size is also another parameter to take into account for disc wear analysis, due to its influence on disc stiffness.
Figure 8. Relationship between G-ratio and mesh opening, average value, and deviation.
Regarding X-ray inspection, it indicates that all the discs had the same density in all their volume from the outside to the inside (each one), although there were differences in density between them. Figure 9 shows the difference. Thus, in DE discs there were larger areas of lower density than in PE discs, that is to say, the retention of the mesh is smaller in DE discs, which increases due to the smaller grain size. This is another reason why DE discs had greater wear than the other three types, which have a more homogeneous density. Some literature observations are related with failure in those reinforced by woven roving type glass fiber.
Figure 9. X-ray image of DE disc (shown top left corner) and PE disc.
3.3. Part Quality
To study whether the part was affected by disc degradation and quality, some surfaces of the trimmed steel profiles were analyzed, in the cases of discs with the greatest wear (i.e., DE—sample DE1) and the one with the lowest wear (PE—sample PE3).
For this purpose, roughness of cut surfaces was measured and images were obtained by means of SEM. Roughness results are shown in Table 2.
Table 2. Results of roughness in pieces cut with discs DE1 and PE3.
As shown in Table 2, roughness after abrasive-cutting with DE disc is 49% greater than with the PE disc.
On the one hand, the grain retained in a firmer way causes the grain to wear flat, thus producing highly negative rake angles at the microgeometry of cutting (abrasive grains produce a micro removal material cutting process, so rake angle can be defined as in the machining case). This type of wear increases power consumption and can damage the steel part in severe cases, but at the same time improve the surface finish by predominating sloughing and rubbing effects over shearing (shearing is the most important process in cutting). On the other hand, the softer disc, with higher wear, has a binder that retains abrasive grains with less force, thus releasing them instead of having heavy grain wear. Consequently, its abrasive-cutting ability is more aggressive, leaving higher roughness in cut surfaces. In addition, although this type of disc generates a higher consumption of wheel, it usually has lower power consumption.
In this work the wear analysis of discs used for abrasive-cutting of mild steel bars has been carried out.
Four disc types with different compositions have been studied. PE disc offers the highest cutting capacity with minimum wear (greater G-ratio).
Disc wear has been measured macro and micro geometrically. The decrease in diameter has been evaluated in two ways, by weigh control and by measuring the diameter with a caliper, while the loss of abrasive-cutting ability has been analyzed by 3D functional parameters of the Abott-Firestone curve (mainly Spk) and by 2D evaluation of the cut surface.
In relation with the internal mesh abrasive structure, is concluded that, discs with the greatest wear were those with a more “open mesh” structure. Also, a linear relationship has been established between open mesh quality and G-ratio for the studied discs.
By means of X-rays, internal density of discs binder has been evaluated, concluding that discs with the highest wear (DE) has low density, which increases its wear due to lack of resistance.
Abrasive disc wheels were defined by four parameters: disc-surface contact flatness, grain exposure, parameter Spk, and mesh size.
In summary, abrasive discs for mild steel parts should fulfill:
Higher grit size with high grit protrusion
Higher grit retention by bond material
Closer mesh of fiberglass matrix binder
In future work, a load cell will be installed so that the cutting forces will be recorded when dry-cutting with the aim of gathering the power consumption. Changes in grain forms will be tested as well. It is expected that, as the disc wears, the power consumption will increase. However, it should be very interesting to know how the power curve varies as a function of the wear stage so that the disc shelf-sharpening ability can be analyzed. Dissimilar materials cutting would be a next step, because tubes and metal profiles are regularly being cut-off by manual sawing machines.