Challenges and Solutions in Metal 3D Printing Parts Machining

Many 3D printed metal parts need to be machined to generate precise surfaces. However, 3D printed parts are often lightweight parts with complex geometric shapes, which brings challenges to subsequent machining. When machining 3D printing parts, it is necessary to consider whether the stiffness of 3D printing meets the requirements of machining, how to clamp these 3D printing parts with complex structures, and a series of problems. We discussed the challenges and solutions in the machining of 3D printed metal parts through a case shared by additive manufacturing experts.

3D printing is a flexible technology with few constraints on design. With the help of 3D printing technology, designers can realize some complex design schemes, such as lightweight structures and integrated structures with integrated functions. However, these advantages of additive manufacturing technology are sometimes weakened by taking into account the challenges arising from subsequent machining. If the challenges faced in subsequent machining are not fully taken into account in the initial design and manufacturing of additive manufacturing parts, losses may occur due to part processing failure.
3D printed parts usually need to be machined to achieve accurate round holes and smooth and flat surfaces, and then assembled with other parts. However, the complex lightweight structure of 3D printing parts sometimes cannot adapt to the processing process due to insufficient stiffness. In addition, the complex structure also increases the difficulty of safely clamping the workpiece.

Challenges of finishing
1. Is the rigidity of 3D printed parts sufficient to meet the load borne during machining? Does the part deviate from the tool and generate vibration, which makes the tool vibrate and leads to poor machining effect? If the stiffness of 3D printing parts is not enough to meet the requirements of machining, what solutions can be used to solve these problems?
2. If the problem of stiffness is solved, the next challenge is how to align the machine tool. 3D printed parts may have some deformation during printing, and lack of clear datum, which means that when machining 3D printed parts, it is necessary to first find the "good" part of the parts. It is very important to obtain the optimal 5-axis alignment of the part.
Renishaw explored the challenges and solutions faced in the finishing of 3D printed parts through a metal 3D printed microwave guide rod. From the preparation before machining to the final finishing of parts, there are a total of 9 steps.
The left figure shows the guide rod manufactured with traditional design ideas and manufacturing methods, which is assembled from several parts; The right figure shows the 3D printed guide rod, which is an integrated part. Compared with the original part, its weight is reduced by half. This is a part designed for telecommunications satellites. The main performance requirements for this part are lightweight, improving microwave transmission efficiency, and reducing the space requirements of this part for satellite payloads.

Solution
Step 1: Establish the desired cutting force
First, evaluate whether 3D printing parts have enough stiffness required by machining through experiments.
Dynamo Data shows the repeated load, and it can be seen that the peak force is about twice the middle value. You can also try cutting at different depths to see how it affects the load on the part.
Step 2: Simulate cutting force
Through the simulation process, it is found that the flange edge processing around the free end of the part causes obvious deflection (greater than 150 μ m), and the finite element analysis also shows obvious distortion, which may lead to uneven cutting.
Step 3: Initial cutting test
If machining is carried out under the above conditions, the parts will deviate from the tool and rebound, resulting in surface vibration, tool vibration and other problems. The result of these problems is poor surface finish.
The way to solve these problems is to improve the rigidity of the parts in the cutting process. There are two steps to improve the stiffness, one is to adjust the design of 3D printing parts, and the other is to change the clamping mode during machining. First, let's understand how to solve these problems by adjusting the design.

Step 4: Meet the challenge of machining by changing the design of 3D printing parts
The goal of changing the design of 3D printed parts is to make the parts more rigid. In this case, the designer added a support structure connecting the components at both ends of the parts to reduce the defects seen in the cutting test.
Or add a connected truss structure between two end components, which is more complex. The disadvantage of improving the stiffness by adjusting the design scheme is that it increases the volume occupied by the parts, which may affect the space occupied by other components and reduce the overall efficiency of the design. Another noteworthy problem is that in the conventional workpiece clamping mode, the parts after adjustment and design are often still unable to meet the machining requirements, so it is necessary to reconsider the clamping mode of the parts.

Step 5: Reconsider the clamping method of parts
In this case, the specific solution of the re clamping method is to design a customized fixture for the 3D printing part, and directly manufacture the customized fixture with the 3D printing equipment, reducing the risk of part deformation and surface damage, making the 3D printing part closer to the processing features, reducing deflection and vibration.
Step 6: Modeling customized fixture
During the finite element analysis of 3D printed parts in the fixture, the designer found that the stiffness could be further improved by better clamping the "straight" structure in the part.
Step 7: Machining preparation

After completing the design adjustment of 3D printing parts and the design and manufacture of customized fixtures, we can enter the preparation stage of machining.
The figure shows the topology optimized 3D printing part measured on the flexible gauge to generate 5-axis alignment for subsequent processing.
In this process, errors occur when the linear and rotational motion of the mechanical shaft exceeds the tolerances required to manufacture accurate parts. In this case, the engineer used Renishaw contact probe and metering software NC Checker to identify and monitor these problems.

Step 8: Part setup
In conventional machining, datum planes are often created first, and then these features are used to align and position parts for subsequent machining operations. However, for the 3D printing part in this case, the conventional method was not followed, because the precision datum must be added to the final machining operation after generating all other surfaces.
The challenge of 3D printing part setting is to set it according to the actual shape of the part, which involves understanding the material condition of the part in all areas where precision features are planned to be cut, taking into account the machining allowance, part deformation and other factors. In this case, the designer seeks to leave enough material at all these locations to allow consistent and efficient cutting. In this step, the probe and metering software can still be used to find the "best fit" setting of the finishing.
Another way to set up a 3D printed part for finishing is to use shop programmable specifications to measure the part and perform alignment. This method is more suitable for larger batch applications.

Step 9: Machining
Through the preparation of the above 8 steps, the obtained components have critical dimensions within the tolerance range and show good surface finish. Compared with the early cutting tests, the tool vibration and wear are greatly reduced.
Machining is usually a part of the metal 3D printing process chain, which is also a process with flight and risk. If the machining fails, a valuable 3D printing part will be scrapped. If the challenges faced in machining can be considered at the beginning of designing 3D printed parts, it will help reduce the risk of failure.