1. Introduction
Remanufacturing refers to a process of restoring performance and re-commercialization through a series of processes such as disassembly, cleaning, inspection, repair, adjustment, and reassembly of high value-added products or parts that have expired. It is an industry that can reduce the energy consumed in the manufacturing process and recycle products with high added value. The remanufacturing industry is mainly applied to large and expensive machine tools to obtain a large effect of reuse [1-3]. If a high value-added product is damaged due to aging, significantly high manufacturing costs are needed to manufacture just one part. Therefore, research on restoring expensive damaged parts by applying an additive manufacturing technology is being actively conducted.
A directed energy deposition (DED) and powder bed fusion (PBF) are the most representative additive manufacturing technologies. In the DED method, powder is melted with a high energy source and the melt pool is fused to the part. PBF is a method of selectively melting the powder layer with a high energy source and depositing the layers according to the design plan [4-10]. Among the two methods, PBF is advantageous in implementing the degree of freedom of shape. The DED method is more suitable for restoring damaged parts by way of deposition.
This study relates to the drive gear of planner miller, a large and expensive machine tool. In order to restore the planner miller, which cannot be used due to aging, additive manufacturing technology was applied. The drive gear, which can be the most representative part of planner miller, cannot be operated to many defects in the teeth. Defective drive gears due to aging were treated with deposition by the DED method and then original tooth shape was restored through post-processing.
2. Experimental Method
2.1 Analysis of damaged drive gear in planner miller
The drive gear was dismantled from a planer miller that had been used for over 30 years. The gears were full of oil and impurities from long-term use. Gears were dismantled and cleaned. Fig. 1 shows the drive gear of the planner miller. Rust was generated at various parts of the gear, and in particular, a severe defect was generated in the teeth. Except teeth, there were no other defects other than rust.
Fig. 1 Damaged drive spur gear in planer miller
A gear defect usually occurs in its tooth. The defects that occur from teeth are pitting, spalling, chipping, scuffing, wear, crack, breakage, deformation, and melting. [11-13].
The drive gear of the planner miller has severely damaged top land and face. Wear, chipping, and splitting might have occurred mainly due to long-term use. In order to analyze the damaged tooth in more detail, tooth part was enlarged and measured using a digital microscope. Fig. 2 shows the shape of a single tooth measured with a digital microscope. Most of the surface was rusted. The top land was seriously damaged, and damages were also found from the face and flank. Each tooth had a different degree of defect, and the type and location of the defect were also different. Its surface was measured at a higher magnification with a digital microscope. Fig. 3 shows the macrostructure of the teeth surface. Rust covered more than half of the teeth surface, and a relatively thick layer of rust was formed in some places.
Fig. 2 Damaged tooth of drive spur gear
Fig. 3 Surface on damaged tooth by digital microscope
2.2 Reverse engineering and structure analysis
The drive gear was scanned using a non-contact 3D scanner, and the dimensions of the gear teeth were precisely measured using a 3D measuring machine. Reverse engineering was performed from the measured data. Shape of the gear teeth were irregular due to defects, and all had different shapes because each damaged shape was different. Therefore, the design of the damaged drive gear was performed by surface modeling. Fig. 4 shows the shape modeling of a defective drive gear that has been reverse engineered. The number of tooth of the gear was 31, and the pitch diameter was Ø93 mm. The module was 3, and the pressure angle was a full depth spur gear of 20°. By applying the values of the standard tooth systems used in spur gears, the detailed dimensional information of the gear tooth section can be calculated. Table 1 shows the shape dimensions of spur gears calculated by applying a general standard tooth shape. The dimension of the addendum was 3.0 mm, dedendum was 3.75 mm, clearance was 0.75 mm, working depth was 6.0 mm, whole depth was 6.75 mm, tooth thickness was 4.713 mm.
Fig. 4 3D design of damaged drive spur gear
Table 1. Dimension of tooth of spur gear in planner miller
From the calculated information in Table 1, a drive gear of the original shape without any defects in the tooth was designed. Fig. 5 is the contrasting image showing the shape of the defective teeth and the shape of the original spur gear. An enlarged image of the teeth was also added for detailed description. The fault tooth profile is expressed in blue, and the original tooth profile is expressed in transparent red. Fig. 5 clearly shows the extent of teeth defects.
Fig. 5 The comparison for shape between fault tooth and original tooth
A defect-free original spur gear was designed using the 3D scan data and the data in Table 1. Fig. 6 shows the shape of the drive spur gear of the planner miller. The body length is 104.0 mm, and the face width is 34.2 mm. Dedendum circle was Ø85.5 mm, clearance circle was Ø87.0 mm,
Fig. 6 Dimension and shape of original drive spur gear
and addendum circle was Ø99.0 mm. And the bore connected to the shaft is Ø55.0 mm. The shape and dimension data for the drive spur gear of planner miller in Fig. 6 are arranged in Tables 1 and 2.
Table 2. Dimension of body of spur gear in planner miller
The planner miller rotates with two spur gears of the same size meshing with each other.
When the two gears are meshed, the flank of the drive gear (pinion) teeth and the face of the driven gear teeth come into contact with each other. The driven gear is subjected to vertical and bending loads. The driving gear is also subjected to the same load due to the reaction. Due to the maximum compressive stress according to the size of the bending moment and the size of the moment of inertia, the position of the gap diameter of the gear becomes the most vulnerable position for breakage [11-13].
The stress and strain generated when two gears meshed with each other and rotated were analyzed using the ANSYS program. Fig. 7 shows the boundary condition. Two gears of the same size mesh with each other, and the inner surfaces of the two gears are constrained by frictionless support. The driving gear was subjected to a load condition in which a torque of 3 kNm was applied in the axial direction. The material of the gear is structure steel.
Fig. 7 Boundary condition for two drive spur gears
Fig. 8 shows the stress and strain analysis results of the drive gear of the planner miller derived by ANSYS. To confirm the analysis result, the driven gear was hidden. In Fig. 8(a), the location where the equivalent stress is greatly generated is the bottom land and around the root of the tooth. In addition, the stress value is relatively large even at the location of the keyway. The yield strength of the structure steel is 250 MPa, and all positions of the gear are perfectly elastic. The equivalent elastic strain result in Fig. 8(b) is the same as the equivalent stress result, and the strain is also large at the location where the stress value is large. In particular, the strain is large at the boundary between face and flank.
Fig. 8 Results of structure analysis: (a) Equivalent stress and (b) Equivalent elastic strain
2.3 DED method of damaged drive gear
The DED-type 3D Printer used in the experiment was MX-450 from INSSTEK. It is suitable for additive manufacturing and maintenance with Numerical Control (NC) control method. With Directed Metal Tooling (DMT) mode, the height of the deposition can be analyzed using two vision cameras, and the height of the volume can be optimized by adjusting the laser power in real time. The DED-type deposition process proceeds in the order of review of basic information such as shape and material, detailed review of work area shape, design of deposition shape, creation of tool path, and execution of printing operation.
The material of planner miller drive gear is SCM 415. Therefore, SCM 415 powder was used for the DED deposition experiments.
SCM 415 powder is composed of spherical powder particles with a size of 45-150 μm, and the chemical composition is shown in Table 3.
Table 3. Chemical compositions of SCM 415 powder
The magnitude of defect in the 31 teeth of the aged drive gear was different each other, and the types and locations of the defects were also different. If the deposition shape modeling is specified separately for 31 teeth with different shapes and deposited, the working time would be longer. So, in order to implement an efficient deposition process, its shape was cut to the same size. The maximum defect size of the gear tooth analyzed by the 3D shape measuring instrument was from the top land (addendum circle) of the defect-free circular gear tooth to a depth of 2.5 mm. Therefore, the remaining parts except for the radius of 46.25 mm from the center of the gear were machined off. Fig. 9 is the contrasting image of a gear with a tooth defect and a gear cut from a radius of 46.25mm to the addendum circle. From the detailed image of the gear teeth, the cutting part of the teeth can be confirmed.
Fig. 9 The comparison for shape between fault tooth and machined tooth for deposition
Fig. 10 shows the deposition shape modeling for deposition the machined gear teeth in the DED method. With the DED deposition method, the shape of the gear teeth can be reproduced and restored to original. However, since the powder is melted by the laser and deposited, numerous fragments are generated around the deposition area. Since these fragments are not easily removed, a post-processing, which is a cutting process, must be performed. Therefore, the deposited shape was not modeled in this shape of the original gear tooth, but was modeled in a round shape with a larger area than the tooth shape of the original gear. After deposition, a cutting process which was a post-processing was performed to restore the shape of the original gear tooth. The surface of the gear teeth cut from the radius of 46.25 mm to the addendum circle became the substrate, and the round-shaped modeling became the deposition part.
Fig. 10 The comparison for shape between machined tooth and deposition shape
A CAM process is required to create the tool path required for the additive process. Parameters that affect the quality of the DED deposition method are laser power, powder feed rate, coaxial gas, powder gas, and shield gas. The optimal parameters for deposition width, deposition depth, and dilution were found through preliminary experiments on SCM 415 powder [14]. Table 4 shows the optimal parameters for the DED deposition method of SCM 415 powder. The parameter values in Table 4 were applied to the deposition process for the gear tooth. After setting the material RPM, the powder was calibrated. After fixing the gear to perform the deposition work on the chuck, the deposition was started by setting the origin.
Table 4 Optimal deposition condition of SCM 415 by DED
3. Experimental Results
Fig. 11 is an image of the DED equipment depositing the teeth of the drive spur gear. When the land area is deposited, the chuck rotates at an interval of 10° and at the same time, the deposition head moves in a zigzag motion while deposition is performed. When depositing the flank area and the face area, the chuck does not move, and the deposition head moves in a linear reciprocating motion while deposition is performed. Fig. 11 shows the molten powder scattered around the melt pool during deposition. This scattering occurs when the distance between the deposition head and the substrate increases. When depositing the flank area, the laser contacts not only the flank area but also the bottom land. That is why the molten powder in contact with the bottom land is scattered in various directions. It took 3 hours to deposit the gears of Fig. 9 into the shape of Fig. 10.
Fig. 11 Image of deposition process by DED
Fig. 12 shows the drive spur gear with teeth deposited from a DED equipment. The surface around the gear teeth turned to red due to heat source of the laser. Among the 31 deposited teeth, about 2/3 were deposited as shown in the deposited shape modeling in Fig. 10. The remaining 1/3 of the teeth were deposited in an inclined cone shape. Those on the opposite side of the keyway have a round shape like deposition shape modeling, and those near the keyway have an inclined cone shape. This phenomenon is considered to be due to the location where the origin was set. The origin was set at a tooth located 90 degrees counterclockwise from the keyway. The deposition of the teeth is performed in the order of the tooth while rotating the tooth counterclockwise from the origin. Therefore, as the position of the origin and the position of the deposition head became farther, a little error occurred between the NC data and the actual distance. As the circumference increases clockwise from the origin, the error increases, and the shape of the deposited teeth becomes an inclined cone. In order to prevent this phenomenon, if the deposited shape differs from that of the modeling, the origin must be reset. Since the deposition shape modeling was designed to be larger than the shape of the original gear teeth, there won’t any problem in restoring it to the tooth shape
Fig. 12 Drive spur gear with teeth deposited by DED
Fig. 13 is an enlarged image of the shape of the teeth of the deposited drive spur gear with a digital microscope. Due to the scattering of molten powder, numerous fragments of various sizes adhere to the tooth surface. This makes the tooth surface look quite rough. The substrate appeared red by the heat source of the laser, and the deposition part is a dark basalt color. In the deposited area, there are several layers on the surface as well. Fig. 13(a) shows the same shape as the deposition shape modeling in Fig. 10, confirming perfect deposition achieved. The deposited shape can also be confirmed in the side view.
Fig. 13 Tooth deposited by DED
The surface of the deposited gear teeth was further enlarged and measured with a digital microscope. Fig. 14 shows that enlarged image of the deposited surface of the drive spur gear teeth. The measurement locations are the substrate (①), the interface between the substrate and the deposition part (②), and the deposition part (③). The substrate (①) has straight lines of uniform intervals in the horizontal direction, round projections of various sizes, and irregular layers. The straight line is the cutting surface of the original gear teeth, and the round projections are remnants of scattered molten powder. Irregular layers were formed by diffusion of the melt pool due to heat transfer. The boundary surface (②) of the substrate and the deposition part has a large depth difference between the deposited layers, causing irregular surface. The deposition part (③) has round projections of various sizes, and the depth difference between layers is smaller than that of position ②. That is, the position ③ has a relatively uniform surface. The boundary area between the substrate and the deposition part and the area around it have irregular surfaces, and the surface becomes more uniform as the distance from the substrate increases, which might be due to the temperature difference. The initial temperature of the bottom part is room temperature. When the molten powder is fused to the substrate, great heat transfer occurs due to the temperature difference. Therefore, not all molten powder is fused to the substrate and a some portion is scattered to the surroundings. As the layers are stacked, heat is transferred between the molten powder and the substrate, and the temperature of the substrate increases. That is, as the layers are stacked, the temperature difference between the substrate and the molten powder decreases, thereby the molten powder is completely fused to the melt pool and the surface becomes uniform.
Fig. 14 Surface of tooth deposited by DED
Fig. 15 shows a drive spur gear that has been cut from the deposited tooth through the post-processing. The post-processing was performed by wire cutting and CNC milling. The shape of the gear teeth was restored to the same shape as the shape of the original gear in Fig. 6. The teeth deposited in the shape of an inclined cone located around the keyway in Fig. 12 were also restored to this shape of the original gear teeth.
Fig. 15 Post-processed drive spur gear after deposition process
Fig. 16 shows the enlarged measurement image of the shape of the gear tooth with a digital microscope after the post-processing. The gear tooth has been perfectly restored to the shape of the original. However, the side view shows that the bottom land remained rough. This was because the post-processing was not performed for the bottom surface.
Fig. 16 Post-processed tooth after deposition process
The surface of the post-processed drive spur gear teeth was further enlarged and measured with a digital microscope. Fig. 17 is an enlarged measurement image of the surface after cutting the deposited teeth. The measurement locations are the substrate (①), the interface between the substrate and the deposition part (②), and the deposition part (③). A clear straight line can be seen on the boundary surface (②) of the substrate and the deposition part. The upper part of the straight line is the deposition part, and the lower part is the substrate. There is a clear difference between image ① and image ③ measured at a higher microscopic magnification than the image ②. In both the substrate (①) and the deposition part (③), fine straight lines are arranged layer by layer. A straight line is a flow line, and the shape of the flow line varies depending on the manufacturing process. It can be seen that the arrangement of the flow lines of the layered part is narrower than the flow lines of the substrate. It is known that the narrower the arrangement interval of flow lines, the better the mechanical properties [15,16].
Fig. 17 Surface on post-processed tooth after deposition process
Vickers hardness was measured with a micro Vickers hardness tester. Five hardness measurements were taken for each location. The boundary surface (②) between the substrate and the deposition part was punched with an indenter along a straight line. The maximum, minimum and average values were calculated and displayed on the graph. The hardness of the base part (①) was 172 HV, and the hardness of the interface (②) of the substrate and the deposition part was 268 HV. The hardness of the deposited portion (③)was 345 HV. The highest hardness value was obtained in the deposition part, and the lowest was at the substrate. The deposition part was 173 HV higher than the substrate and 77 HV higher than the interface.
4. Conclusion
In this study, the shape of the defective drive gear of planner miller was restored by deposition with the DED method. The conclusions drawn from the experiment are as follows.
1) As the location of the set origin and the location of the deposition head became distant, an error occurred between the NC data and the actual distance, resulting in the deposition shape different from the deposition modeling.
2) The substrate had round projections of various sizes due to scattering of molten powder and irregular layers due to diffusion of melt pool. At the interface between the substrate and the deposited portion, the depth difference between the deposited layers was large. The deposition part had round projections of various sizes, but the surface was relatively uniform. The surface became uniform as distanced from the substrate.
3) After the gear teeth were deposited, post-processing was performed to restore them to the same shape as the teeth of the original gear teeth. The arrangement interval of the flow lines of the deposition part was narrower than that of the substrate. The hardness of the substrate was 172 HV, and the deposition part was 345 HV, which was twice as high as that of the substrate.
Acknowledgement
This work was supported by the Energy technology development program (20206310200010, Advanced Remanufacturing of industrial machinery based on domestic CNC and building infrastructure for remanufacturing industry) funded By the Ministry of Trade, Industry & Energy(MOTIE, Korea)
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