(A Comparative Study Of Printing A Complete Assembly And Individual Parts Using FDM Printer)
ABSTRACT
Additive Manufacturing (AM) is used to describe the technologies that build 3D objects by adding layer-upon-layer of material, whether the material is plastic, metal, or concrete. The issue of successfully printing an assembly through additive manufacturing is the need of the hour. Therefore, through this project, we intended to study the effects of printing the parts separately and the entire assembly at the same time using FDM.
One of the features of the additive manufacturing is to create interlocking parts without the need for the assembly, which can be used for the manufacturing of rotational and translational parts in functional 3D models. Through this project, we focused on printing 3D models through two different process –
1) Printing parts individually and then assembling them manually
2) Printing the complete assembly without the need of printing the parts.
The project also focuses on analyzing the process parameters which needed to be considered when fabricating parts and the complete assembly. Several aspects such as tolerances, clearance, surface finish, dimensions, precision, trueness to the CAD model, and surface finish are taken into consideration. Other important factors such as part orientation are also taken into consideration.
The Project also focuses on the comparative study of both the process and analyzing them.
INTRODUCTION
1 RAPID MANUFACTURING
Rapid manufacturing also goes by the names of direct manufacturing, digital manufacturing, and direct fabrication. Another popularly accepted definition is “the use of an additive manufacturing process to construct parts that are used directly as finished products or components”.
2 ADDITIVE MANUFACTURING METHOD USED FOR THE PROJECT
Fused Deposition Modelling (FDM): It is an inexpensive, convenient process that can be found in most non-industrial desktop additive manufacturing printers of which VIT is a great example, with one of the largest 3D printing labs in any college campus in the country. This process uses a spool of thermoplastic filament which is melted inside a printing extruder right before the liquid plastic is laid down layer-by-layer according to a computer deposition program.
Fig 1: FDM CAD Assembly made using Solidworks
3 OBJECTIVE OF THE PROJECT
To make a CAD of the mecanum wheel using Solidworks for separate part print and complete assembly printing.
Create a Mecanum Wheel by separately manufacturing each sub-component and then putting all the components together manually to form a single assembly. Each sub-component will be 3D printed using FDM.
Create a Complete Mecanum Wheel Assembly by using the 3D printing process FDM. The Assembly itself will be completely 3D printed and should function on its own. Exploring the effects of tolerancing and dimensioning on printing complete assemblies by FDM.
Conducting a comparative study for analyzing the differences in methods 1 and 2 for 3D printing a Mecanum Wheel.
Fig 2: CAD Assembly of Mecanum wheel
4 PROJECT DESCRIPTION
Mecanum wheel: It is also known as Omni or illion wheels. These wheels are basically conventional wheels with rollers attached to it at 45° to the plane. The perk of using this wheel comes from the fact that it can move the vehicle in any direction including side to side and forward-backward direction.
Applications-
Mecanum wheels can be used at several places where movement in all directions is required. This capability of mecanum wheels makes it versatile for many applications.
They can be used in rovers for movement in all directions which any rotation of the complete body. Though it can be used to rotate the body of the rover as well.
Mecanum wheels can be used in robots for it’s better navigation and movement in a 2D plane without the need for rotating the body.
Mecanum wheels if used in skateboards can serve for better functionality.
Mecanum wheels can be used in forklifts which are used in warehouses. It can increase the functionality of the forklift as it can enable it to move in constrained spaces.
Moving all four wheels of anybody (robots, rovers, forklifts) in the same direction causes forward or backward movement, running the wheels on one side in the opposite direction to those on the other side causes rotation of the vehicle, and running the wheels on one diagonal in the opposite direction to those on the other diagonal causes sideways movement. Combinations of these wheel motions allow for vehicle motion in any direction with any vehicle rotation (including no rotation at all).
For the project, we decided to use the FDM process of additive manufacturing to print different parts which we plan to assemble manually and to print the entire assembly at one go. We chose to use FDM for this project because of its easy accessibility in VIT.
The filament cost is cheaper than the raw material cost of SLA or SLS.
Fig 3: Ender 3 printer
NozzleBrass 1.75mm x 0.4mm (can be swapped out for 0.3mm or 0.2mm)
Printer weight 7.5 kg
Power input 24V / 270W
This printer was chosen due to its accessibility, its low price, and its high-performance capabilities.
2 METHODOLOGY AND EXPERIMENTAL WORK
2.1 METHODOLOGY:
For this project, an iterative trial and error based system has been followed to print parts separately and to print the entire assembly at once. In a trial and error based approach, a process is repeated with varied attempts and slight modifications with respect to the results obtained in various trials until a satisfactory result is obtained. The parameters for printing are changed in consecutive attempts.
Different parameters which were taken into consideration while printing:
Infill – A huge advantage of 3 additive manufacturing comes from the fact that the part can have varying degrees of hollow.
Tolerance – Tolerances are assigned to parts for manufacturing purposes, as boundaries for the acceptable build. The three types of fit are:
Clearance: The hole is larger than the shaft, enabling the two parts to slide and / or rotate when assembled.
Location/transition: The hole is fractionally smaller than the shaft and mild force is required to assemble/disassemble
Interference: The hole is smaller than the shaft and high force and / or heat is required to assemble/disassemble.
Support structure alignment – They are used to support parts of the model during printing like overhanging beams. After the printing is over, there is an additional task of removing the structures before the model is ready-to-go. The density of the support structure was also taken into consideration.
Fig 4: Image showing support structure generation in a component
Bed temperature and nozzle temperature – Different material has different requirements for printing temperature of nozzle and bed. For a PLA temperature even the color of the spool affects the print. The temperature of the ambiance, for example, the temperature of the cold breeze coming from the window may also affect the building temperature. The ideal range of temperature for black PLA is 200-220°C and this can be decided with the help of hit and trial method too.
2.2 MODELLING AND CURA SLICING PARAMETERS
CAD model of the mecanum wheel was created using Solidworks 2018.
Fig 5: Front view of Mecanum wheel
Fig 6: Side view of Mecanum wheel
Fig 7: Drafting of assembly for Mecanum wheel
Fig 8: Drafting of frame
SLICING OF THE STRUCTURE AND PRINT PARAMETER USING CURA
Fig 12: Assembly slicing using CURA
Table 3: Table for parameter description of the assembly
Run time for the entire assembly is – 12 hrs 23 mins
Print dimension – 74.7 x 74.7 x 35.3 mm
Infill – 100 %
Fig 13: Frame slicing using Cura
Table 4: Description of parameters used for part
Trial 3 – Infill percent 100%
Print time – 5 hours 7 mins
Dimension – 66.4 x 66.4 x 32.0 mm
2.3 EXPERIMENTAL PROCEDURE
TRIAL 1 (Tolerance of through-hole for roller and the rod):
Step 1 – Radial tolerance taken was 0.3 mm
The result – The rod could not fit the roller and thus this part was rejected due to dimensional tolerance inaccuracies and interference fit was observed.
Fig 14: Run time picture of 3D printer
Step 2 – Radial tolerance taken was 0.4 mm, this increment in the size was considered due to the failure observed in the fit.
Result – The rod could not fit the roller and thus this part was rejected due to inaccuracies in dimensional tolerancing.
Step 3 – Radial tolerance was increased to be 0.5mm
Result- The rod could fit the roller perfectly.
TRIAL 2 (Change in the infill of the side frames):
Step 1: Infill of the side frame was taken as 30%
Result – 3D printed model rejected due to lack of accuracy and breakage at many points mainly due to sizing problems, low support, and infill error.
Step 2: Infill of the side frame was taken as 40%
Result – 3D printed model rejected due to lack of accuracy and breakage at many points mainly due to sizing problems, low support, and infill error.
Steo 3 – Infill of the side was taken as 100%
Result – Holes were too big in size thus this part was rejected as well.
Fig 15: Component with 100% infill
From our previous experience of working with Ender 3 FDM for additive manufacturing, we have observed that there is a change in prescribed dimensions of the object with respect to CAD upon printing. The change observed has been of 1 mm, for example, if the prescribed dimension is 40 mm the print obtained is 39 mm. Thus, this had been kept in mind while printing the mecanum wheel (parts as well as the assembly) and therefore an external tolerance of 1 mm has been kept while designing the CAD of the mecanum wheel.
TRIAL 3
Step – Direct printing the assembly
Fig 16: Printed wheel assembly
Fig 17: Printed wheel assembly 2
RESULTS AND DISCUSSION
Accuracy of the printer Ender 3:
We followed a trial and error based approach for this project and after working with Ender 3 for a number of times we have observed that there is a change in prescribed dimensions of the object with respect to CAD upon printing. The change observed has been of 1 mm, for example, if the prescribed dimension is 40 mm the print obtained is 39 mm. So, while printing from Ender 3 this point has to be kept in mind for the accuracy of the obtained product.
Fig 18: Ender 3 print time picture
The print approach followed for the frame:
The sidewalls of the frame has a thickness of 2 mm, due to such less thickness significant damage was obtained during post-processing while removing the support structure. Different percentage of infill gave different results.
At first, with 30 % infill, significant damage was observed. In the second attempt, a 40% infill was used to minimize the damage and significant damage was observed again.
Fig 19: Printed frame attempt 19
Fig 20: Printed frame number 2
Result: We used 100% infill to omit the damage in the sidewalls due to support structure post-processing. We ended up with the following result as shown in the picture:
Hole radius
From the product obtained at 100% infill, we observed that the obtained diameters for the hole is larger than expected from the CAD design. This error may have occurred due to the size of the nozzle and its in capabilities of making small holes. To omit this problem we made some amendments in the design and increased the size of the overhanging structures of the frame which carry the holes.
Roller and rods
Rods with a 4 mm diameter were successfully printed in the very first attempt.
Fig 21: Ender 3 print time picture of rod printing
Radial tolerance of the holes of rollers were changed thrice to obtain the perfect tolerance. At first, it was taken 0.3 mm and rod couldn’t fit the rollers, the radial tolerance was increased from 0.3 to 0.4 mm to omit this problem but it couldn’t be rectified and interference fit was observed. The radial tolerance was then made 0.5 mm and the desired results could be achieved.
Fig 22: Printed rollers, rods, and grid support structure
Complete assembly printing
The complete assembly was printed with a printer run time of 12 hours 27 mins. The desired result was obtained. The support structure is yet to be removed for the rollers to move and function.
Printed wheel assembly
4.1 CONCLUSION
Based on the 3D printing done by two methods we conclude the following-
Printing Assembly in One Transaction-
This is the preferred method if you don’t need to use different materials/colors in a moving model.
This method is also a time saver.
Further curing and material removal have to be done which can take more time.
Tolerances of 0.3 mm between touching surfaces need to be given to allow the support structure to fill in the gaps between the parts so that the moving components are prevented from bonding together.
Printing individual components to be assembled later-
This method should be used when a multilayer printer is not available.
It should be used when different colors and materials need to be used to generate the parts.
Due to the complexity of the assembly, the support removal is a challenging process (intricate channels or hard to reach inner gaps).
A tolerance of 0.3 mm between touching surfaces will let you assemble the components once they are finished and allow the parts to move and interact freely.
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