What are the difficulties in CNC machining of robot motor shells?

CNC machining of robot motor housing is widely recognized as one of the high difficulty processes in precision parts manufacturing. What's the difficulty? It's not that the precision of the machine tool is insufficient, nor is it that the cutting tool is not sharp enough - it's that the thin-walled shell almost "never changes shape" during the machining process: deformation during clamping, deformation during cutting, and even deformation after machining and placement.

This article breaks down the entire processing flow: Where does deformation come from? How to match the fixture? How to coordinate the process? By understanding these three things, the yield of CNC machining of thin-walled motor housings can be improved to a higher level.

1、 Why does it deform: Disassemble according to the processing stage

Many processing plants tend to attribute deformation to improper clamping, but in reality, deformation runs through the entire processing flow of robot shells, occurs at different stages, and has different causes.

1. Clamping process: Radial concentrated force causes elastic deformation

The clamping force of the traditional three jaw chuck is concentrated at three points, causing the circular shell to undergo "angular" elastic deformation. During processing, the cutting tool cuts the deformed shape. After releasing the fixture, the shell rebounds and the cylindricity of the inner hole immediately exceeds the tolerance.

The physical root of the common phenomenon of "qualified clamping and out of tolerance loosening" in production is not due to insufficient precision of the machine tool, but rather due to the elastic deformation constrained by the fixture. After loosening, the dimensions of the parts change when they return to a free state.

2. Cutting process: the superposition effect of heat and force

The friction between the cutting tool and the workpiece during the cutting process generates a large amount of cutting heat. The coefficient of linear expansion of aluminum alloy is about twice that of steel, and the heat distribution in thin-walled areas is extremely uneven. Local temperature differences directly lead to size drift. At the same time, cutting force can cause a "tool yielding" phenomenon in thin-walled areas, resulting in uneven wall thickness.

An intuitive quantitative relationship: According to the theory of thin plate deformation, under the same cutting force, when the wall thickness decreases from 4mm to 2mm, the deformation amount will increase by about 8 times (the deformation amount is inversely proportional to the cube of the wall thickness). Small changes in wall thickness can significantly increase the difficulty of CNC machining.

3. Intermittent period of process: slow release of residual stress

There are residual stresses inside the blank itself. After removing a large amount of material through rough machining, the original stress balance is disrupted, and the parts slowly warp within hours or even days. This kind of deformation does not occur on the machine tool, but gradually becomes apparent during the process flow and is more easily overlooked.

2、 Core idea of fixture design: matching according to processing stages

The core idea of fixture design for CNC machining of robot motor shell lifting is to change the clamping force from concentrated to uniform, and to gradually decrease the cutting force from large to small, in response to the deformation mechanism of the three stages mentioned above.

1. Rough machining: fan-shaped soft claws (fully enclosed)

Abandon standard hard claws and use fan-shaped soft claws made of aluminum or nylon. Use them after precision boring according to the outer diameter of the shell. The contact area is close to 360 ° full circumference containment, transforming point contact into surface contact, significantly reducing the pressure per unit area, and effectively avoiding angular deformation caused by radial concentrated forces.

2. Precision machining: vacuum adsorption and elastic expansion/hydraulic expansion core

For the processing of robot shells with large end faces and extremely thin wall thickness, vacuum suction fixtures are used. Generally, when the precision machining allowance is small and the cutting force is small, using atmospheric pressure to uniformly act on the surface of the part and switching to vacuum adsorption during the precision machining stage is an effective means of controlling accuracy. But the clamping force is limited and not suitable for scenarios with large cutting volumes.

For the processing of inner holes, elastic expansion plugs or hydraulic expansion cores are used to evenly apply force from the inside out, which can better ensure the coaxiality of the inner and outer circles. This is crucial for the assembly of the bearing position of the motor housing.

3. For extremely thin areas: temporary filling support

For extremely thin walls of 1.5-2mm, low melting point alloys or industrial hard wax can be used to fill the thin-walled cavities to provide temporary rigid support, which can be removed by heating after processing. This supplementary method can effectively solve the problem of insufficient local rigidity of robot joint shells with ultra-high precision.

3、 Process coordination: key measures beyond fixtures

Good fixtures require scientific process coordination, otherwise it is difficult to completely solve the deformation problem in CNC machining of thin-walled motor shells with fixtures alone.

1. Rough and fine separation+symmetrical machining: After rough machining, release the fixture to release stress, and then lightly clamp for fine machining. Further, a symmetrical strategy of alternating inner and outer circles can be adopted to reduce stress offset caused by one-sided material removal.

2. Layered feeding+decreasing clamping force: Fine machining uses a small amount of knife and multiple passes, reducing the clamping force by 30% -50% compared to rough machining, avoiding the clamping force itself from becoming a source of deformation.

3. Aging treatment: Adding aging treatment (natural aging for 2-4 hours or artificial aging) between rough and fine machining accelerates the release of residual stress and significantly reduces dimensional drift after finishing. Taking the processing flow of Huiwen Zhizao as an example, for robot joint shells with high precision requirements, manual aging treatment will be arranged after rough machining to ensure the dimensional stability of subsequent precision machining.

4. Adequate cooling: Ensure that the coolant continuously covers the cutting area, reducing thermal deformation caused by local temperature rise.

It is also necessary to pay attention to distinguishing the detection status. The dimensions of the shell are different between the clamped state and the loosened state. If the drawing tolerance is based on the free state, the detection must also be carried out in the free state to avoid "qualified on the machine tool but exceeding the tolerance after loosening".

4、 Comparison of deformation prevention schemes for thin-walled components

5、 Summary

The difficulty of CNC machining of robot motor shells lies in the deformation control of thin-walled parts. Understanding the manifestation of deformation in different processing stages, selecting matching fixture schemes, and combining with process measures such as coarse and fine separation, symmetrical processing, and aging treatment, can systematically solve this problem. In actual production, CNC machining enterprises such as Huiwen Zhizao, which focus on robot components, use this systematic process control approach to maintain a stable high yield rate for thin-walled shells.

As a service provider specializing in CNC machining of robot motor shells, Huiwen Zhizao has a factory area of 20000 square meters and more than 370 processing equipment, which can meet the full stage of OEM services for parts such as humanoid robots, mechanical dogs, flexible robotic arms, bionic robots, medical robots, etc. from single piece sampling to medium to large-scale production. At present, Huiwen Zhizao has passed ISO9001 and IATF16949 quality management system certification, providing robot shell processing services to many technology innovation enterprises and research institutions such as Huawei, Xinsong, Xiaomi ecological chain enterprises, Beijing Institute of Technology, and Chinese Academy of Sciences. The accuracy and stability have been unanimously recognized by customers.

Especially for humanoid robots, Huiwen Intelligent Manufacturing innovatively adopts a combination process of "new materials+new molds+precision machining", relying on integrated design, intelligent manufacturing, and assembly full chain services to complete the entire process from research and development to batch delivery in one stop.

If you need CNC machining solutions or quotations for thin-walled shells of robot motors, please feel free to provide drawings for consultation.

Frequently Asked Questions

Q: Why did the dimensions exceed the tolerance during assembly despite passing the inspection on the machine tool?

A: During the inspection, the parts were still in the clamped state of the fixture, and the shell was forcibly fixed in the deformed position. After releasing the fixture, the elastic recovery will cause a change in size. The correct approach is to detect key dimensions (such as bearing inner diameter and installation surface flatness) in a free state, and clarify the requirements for the detection state with the design party in advance to avoid "qualified on the machine and out of tolerance on the machine".

Q: What can be done in drawing design to reduce processing difficulty?

A: Make the wall thickness as uniform as possible to avoid uneven stress distribution caused by significant differences in thickness; Add process reinforcement ribs (to be removed later) to enhance rigidity during the processing stage; Avoid slender cantilever structures. Reasonable design of drawings can fundamentally reduce the difficulty of processing.

Q: Which manufacturer in Shenzhen can undertake high-precision robot thin-walled motor shell CNC machining?

A: It is recommended to find manufacturers with non-standard fixture development capabilities, such as Shenzhen Huiwen Zhizao. They specialize in providing anti deformation processing solutions for thin-walled parts such as robot motor housings and reducer housings, with a stable batch delivery pass rate of over 98%.