Why is it so difficult to process robot joint parts and humanoid robot shoulders?

The shoulders of humanoid robots are equivalent to the shoulders of humans - they are the core hub connecting the torso and upper limbs, supporting the weight and movement of the entire arm while ensuring flexible rotation without jamming. Many people pay attention to the AI brain and actuator performance of humanoid robots, but few realize that the structural design of shoulder joints and the proper machining of parts directly determine whether the robot's upper limbs can be lifted steadily, moved flexibly, and rotated accurately.

From Tesla Optimus to Yushu H2, there are significant differences in shoulder design among different companies, but the core contradictions faced by robot parts processing are interconnected. Today we will dismantle the structure of the shoulder joints of humanoid robots, talk about the three most typical pairs of contradictions in processing, and discuss the solutions in actual processing.

After 8 years of working on shoulder machining for humanoid robots, the biggest fear is these three pairs of contradictions

The core of the shoulder joint of a humanoid robot consists of three types of parts, each with different functional positioning and processing requirements:

1. Shoulder connector: a "load-bearing bridge" between the torso and arms

The shoulder connector is fixed on the torso and is responsible for connecting the torso frame and shoulder joint actuators, bearing the weight and motion inertia of the entire arm. The characteristics of this component are: multi-faceted fit (with mounting holes on top, bottom, left, right), concentrated force (all actuator torque is transmitted here), and weight sensitivity (located near the upper end of the torso, excessive weight will lift the center of gravity).

Processing requirements: strict positional accuracy of multi-faceted holes, high flatness of installation surfaces, and overall lightweight.

2. Shoulder joint output end: the "axis of rotation" for torque transmission

The shoulder joint output end connects the actuator output shaft and the upper arm structure, and is a key component for achieving forward and backward swing, lateral lift, and rotation of the arm. The characteristics of this component are: high coaxiality requirements (the output shaft and mounting surface must be strictly concentric), high wear resistance requirements (frequent rotation causes wear), and compact structure (limited shoulder space, constrained part volume).

Processing requirements: coaxiality control of inner and outer circles, high precision of mating surfaces, and surface hardness treatment.

3. Shoulder shell: the "protective cover" of the actuator

The shoulder shell wraps around the shoulder joint to protect the actuator and cables, while also affecting the appearance of the robot. The characteristics of this component are: thin-walled structure (required for weight reduction, with a wall thickness of usually 2-4mm), biomimetic curved surface (fitting the shape of the human shoulder), and large differences in internal and external shapes (the inner cavity should avoid the actuator, and the outer surface should be adapted to the shape).

Processing requirements: thin-walled deformation control, surface forming accuracy, and synchronous processing of inner and outer shapes.

How to break the three contradictions of losing weight and strength, and losing precision when clamping?

The core challenge in doing shoulder processing for humanoid robots is not whether they can process it or not, but the balance of three conflicting pairs - each pair is a trade-off, one end is easy to lose the other.

Contradiction 1: Weight reduction slotting vs structural strength

The shoulder connector should be lightweight, and weight loss is essential - it should be positioned higher. For every 100g of weight, the load pressure on the lower limb actuator will increase by one point. The common practice is to make weight reduction slots in non stress areas, but the shoulder is precisely one of the most concentrated areas of stress. If the weight reduction slot is cut onto the stress path, the part may deform or even crack when subjected to arm swing inertia. Especially in the processing of aluminum alloy joints, the layout of weight reduction grooves and stress bars needs to be very precise, and a slight deviation may affect the strength.

Contradiction 2: Multi dimensional accuracy vs clamping error

Shoulder connectors usually have mounting holes on the front, back, and sides, and some designs even have matching features on all five sides. Traditional three-axis machining requires multiple flipping and clamping, which introduces positioning errors each time. Accumulated errors after processing multiple surfaces may lead to assembly interference - the arm cannot rotate smoothly when installed, and may even get stuck. What's more troublesome is that the shoulder parts are not large in size, the clamping space is limited, and the fixture design itself is very particular.

Contradiction 3: Surface accuracy vs machining efficiency

The biomimetic surface of the shoulder shell is the most time-consuming part of the entire shoulder joint processing. The shape should conform to the curvature of the human shoulder, and the inner cavity should avoid the actuator and cable channel. There is a large difference in shape between the inside and outside, so only five axis linkage processing can be used. However, five axis programming is complex and time-consuming, which puts pressure on the R&D team in the iterative stage in terms of processing costs and delivery cycles. Some teams dismantle the shell into multiple pieces for cost saving, but splicing introduces fitting errors and overall strength is not as good as integrated structures.

Topology optimization+partition processing, five axis one-time clamping breaking error

In recent years of processing robot joint parts, we have explored some practical ideas to address these three contradictions:

1. Balance between weight loss and strength: topology optimization approach+partition processing strategy

The weight reduction groove does not rely on a "head tapping" layout, but combines the force analysis of the parts to groove in the truly unstressed area, and retains sufficient material on the force path. When we process aluminum alloy joints, the process engineer will participate in the force path analysis during the drawing review stage to confirm in advance whether the layout of the weight reduction groove affects the structural strength. Some customers' designs may appear to have good weight reduction effects, but in reality, they cut into the main force path. We will point out and provide optimization suggestions during the review stage to avoid discovering insufficient strength only after processing. During processing, different cutting parameters are used for the stress area and the weight reduction area - conservative parameters are used for the stress area to ensure surface quality and dimensional accuracy, while the weight reduction area can appropriately improve feed efficiency. At the same time, the processing of thin-walled weight reduction grooves adopts layered milling to avoid cutting force accumulation

2. Balance between multi-faceted precision and clamping: Five axis one-time clamping+tooling customization

The most effective solution for the requirement of multi-faceted coordination of shoulder parts is to use five axis linkage to complete all surface processing in one clamping, avoiding cumulative errors from the root. When we process shoulder joint seats, we design specialized fixtures based on the structure of the parts to ensure the positioning accuracy of one clamping, and then complete the machining of each surface through the rotation of a five axis turntable. For holes with strict positional requirements, the process of "drilling the bottom hole first and then reaming the hole" is adopted, which is more stable than directly boring in small batch scenarios.

The most effective solution for the requirement of multi-faceted coordination of shoulder parts is to use five axis linkage to complete all surface processing in one clamping, avoiding cumulative errors from the root. We have multiple five axis machining centers dedicated to producing core structural components for humanoid robots. When making shoulder connectors, we design specialized fixtures based on the part structure to ensure the positioning accuracy of a single clamping, and then complete the machining of each surface through the rotation of a five axis turntable. For holes with strict positional requirements, the process of "drilling the bottom hole first and then reaming the hole" is adopted, which is more stable than directly boring in small batch scenarios.

3. Balance between surface accuracy and efficiency: Five axis programming optimization+process combination

The biomimetic surface of the shoulder shell requires five axis machining, but it can be optimized through programming to shorten machining time - by planning the tool path reasonably to reduce idle stroke, using large-diameter tools for rough machining to quickly remove materials, and using small-diameter tools for precision machining to ensure surface quality. For shells that do require disassembly (such as considering maintenance convenience), we at Huiwen will provide comparative recommendations between disassembly and integration solutions, allowing customers to choose according to their actual needs.

The process review step is saved, and we will be making up for each subsequent step

The difficulty in machining robot parts for shoulder joints lies not only in the machining itself, but also in whether the process can be linked with the design end - many machining problems can be discovered and avoided in advance during the drawing review stage, which can save a lot of costs and cycles.

We at Huiwen specialize in robot parts processing and insist on intervening from drawing review. Based on the functional requirements and processing feasibility of the parts, we provide optimization suggestions: which rounded corners can be increased to reduce processing difficulty, which wall thickness can be fine tuned to improve process stability, and which structures can be simplified to reduce clamping times. Simultaneously possessing the full process capability from structural design optimization to CNC precision machining and component assembly, a team can run the entire chain.

As a high-end robot full industry chain service provider specializing in the field of robotics in Shenzhen, Huiwen Zhizao relies on the technical background of the Chinese Academy of Sciences and has passed ISO9001 and IATF16949 quality management system certifications. It has been recognized as a national high-tech enterprise and a specialized new enterprise, and has served many listed companies and industry-leading companies, such as Lenovo, Xinsong, Xiaomi Ecological Chain, Beijing Institute of Technology and other clients. At present, our factory covers an area of 10000 square meters and has more than 200 processing equipment. We are skilled in five axis machining of robot core parts and have a stable batch delivery qualification rate of over 98%.

For robot intelligent manufacturing services, in addition to humanoid robots, we can provide comprehensive OEM services for mechanical dogs, flexible robotic arms, bionic robots, medical robots, and more.

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 have machining needs for robot shoulder joints or other core components, please feel free to send drawings for consultation on process plans and quotations.