Why are titanium alloy parts for humanoid robots difficult to process? In depth analysis of 5 technical difficulties

Titanium alloy's high specific strength, corrosion resistance, and good biocompatibility - these characteristics that make it a "god" in the aerospace and medical fields - are attracting the attention of the humanoid robot industry. The advantages of titanium alloy, such as lightweight skeleton, high-strength joints, fatigue resistant shell, etc., are almost tailor-made for the next generation of humanoid robots.

But those who have worked on machining titanium alloy parts for robots know that drawings can be drawn, but not necessarily cut. The poor thermal conductivity, high elastic rebound, high chemical activity, and severe work hardening of titanium alloys are all characteristics that make CNC machining difficult. However, humanoid robot parts have thin walls, complex structures, and high precision requirements, which adds a new level of processing difficulty.

This article breaks down the 5 major difficulties in titanium alloy processing for humanoid robot parts one by one, and provides solutions for each difficulty.

Difficulty 1: Cutting heat cannot be dissipated, and the tool life is directly folded in half

The thermal conductivity of titanium alloy is only 6.7W/m · K, which is about 1/5 of steel and 1/13 of aluminum. The heat generated by cutting cannot be transferred to the workpiece and cannot carry away chips, almost all of which are concentrated near the cutting edge. According to industry testing data, under the same cutting conditions, the cutting temperature of titanium alloy is nearly twice as high as that of 45 steel.

The direct consequence of this is rapid tool wear. The lifespan of ordinary hard alloy cutting tools on titanium alloys is often only 1/3 to 1/2 of that when processing aluminum alloys. If the cutting speed is even higher, the tool may experience crescent shaped wear or even chipping within a few tens of minutes.

This issue is particularly prominent in the machining scene of humanoid robot parts. The irregular surface of titanium alloy joints requires continuous cutting for a long time, and heat accumulates continuously. After tool wear, the dimensional deviation will gradually increase, and the accuracy of the last few pieces may be far from that of the first piece.

Response strategy:

Cutting speed control: rough machining 25-50m/min, precision machining 50-80m/min, it is better to be slow than too fast

High pressure internal cooling system: The coolant pressure is not less than 7MPa, directly flushing the interface between the blade tip and the chip to remove heat

Selection of tool coating: PVD coated tools with TiAlN or AlCrN have better heat resistance and can increase tool life by more than 30%

Difficulty 2: Rebound immediately upon release after processing, and the dimensions of thin-walled parts are completely off center

The elastic modulus of titanium alloy is about 114GPa, which is only about half of that of steel. This means that under the action of cutting force, titanium alloy will undergo greater elastic deformation - when the tool cuts over, the material is "compressed"; When the cutting force is withdrawn, the material bounces back.

For the machining of humanoid robot parts, this feature is simply a nightmare. The shell wall thickness of titanium alloy joints in humanoid robots is often only 1-2mm, and the joint seat cavity is deep and narrow. During processing, the cutting tool may cause incomplete cutting, and the elastic recovery after loosening the clamp may result in a smaller size - the nominal cutting path taken according to the tolerance may actually produce parts that exceed the tolerance.

Real case: When Shenzhen Huiwen Zhizao processed titanium alloy joint seats for a certain humanoid robot enterprise, the first batch of parts had smaller inner hole sizes. The reason is that the elastic rebound in the thin-walled area has not been compensated. Later, by calibrating the rebound amount through the first piece three coordinate measurement and correcting the tool path offset in reverse, the contour accuracy was stably controlled within the tolerance range.

Response strategy:

First piece calibration+reverse compensation: First, process the first piece according to the nominal size, measure the actual rebound, and then correct the precision machining tool path

Symmetric machining strategy: alternate machining of two opposing surfaces to cancel out residual stresses

Separation of rough and fine: Leave a 0.3-0.5mm margin after rough machining, release stress before fine machining

Difficulty 3: Titanium alloy "biting" knife, difficult to control surface quality

Titanium alloy has strong chemical activity at high temperatures and is prone to react with elements such as tungsten and cobalt in cutting tool materials, forming a low strength bonding layer. This is the common phenomenon of "sticking to the tool" or "chip accumulation" in titanium alloy processing - titanium alloy chips adhere to the front cutting surface of the tool, repeatedly scratching the processed surface as the tool rotates, resulting in a sharp deterioration of surface roughness.

Even more troublesome is that this bonding is not uniform. When one section of the tool is stuck and the other is not, the cutting force fluctuates, causing chatter. Flutter is particularly difficult to deal with in titanium alloy CNC machining because the low elastic modulus of titanium alloy itself makes vibration more easily amplified.

Humanoid robot parts have dual requirements for surface quality: the roughness Ra of the mating surface needs to be controlled within 1.6 μ m (the motion pair even requires 0.8 μ m or less), and non mating surfaces are not allowed to have microcracks - titanium alloy is extremely sensitive to stress concentration, and surface microcracks may become the starting point of fatigue fracture.

Real case: When Shenzhen Huiwen Zhizao processed titanium alloy joint shafts for a certain robot enterprise, vibration patterns frequently appeared on the outer surface of the humanoid robot. After investigation, it was found that the tool overhang was too long and the angle of the coolant nozzle was off center. After shortening the overhang to within 3 times the diameter of the tool and adjusting the nozzle to face the cutting area, the surface roughness decreased from Ra3.2 μ m to Ra1.2 μ m.

Response strategy:

Geometric parameters of the cutting tool: front angle of 5 ° -10 °, rear angle of 10 ° -15 °, maintaining sharp cutting instead of squeezing

Tool suspension control: not exceeding 3-4 times the diameter of the tool holder to reduce chatter

Coolant facing the cutting area: Multiple nozzles spray from different angles to ensure that the cutting edge is always covered with coolant

Difficulty 4: The harder the surface is cut, the slower the feed, which can actually cause problems

The work hardening tendency of titanium alloy is very obvious. During the cutting process, the hardness of the machined surface can be 20% -50% higher than that of the substrate. This means that if the cutting parameters are improper - especially if the feed rate is too small or the cutting depth is too shallow - the tool will "gnaw" in the hardened layer formed by the previous layer of cutting, not only accelerating wear, but also possibly causing the hardened layer to stack layer by layer.

This is a counterintuitive phenomenon in titanium alloy processing: the slower the better. If the feed rate is too small, the tool will rub in the hardened layer instead of cutting, which is more likely to cause problems. If the cutting depth is too shallow, the cutting edge cannot penetrate the hardened layer, and the surface quality will also deteriorate.

The titanium alloy parts of humanoid robots have many features and complex cutting paths, and some areas (such as rounded corners and narrow grooves) have to reduce feed and cutting depth. These positions are precisely the easiest places to get stuck in the hardened layer.

Response strategy:

Rough machining large cutting depth: 2-5mm, one size fits all through the hardened layer, without repeated grinding in the hardened layer

The feed rate cannot be too small: rough machining 0.1-0.3mm/r, precision machining 0.05-0.15mm/r, maintaining sufficient chip thickness

Avoid repeated cutting at the same position: plan the cutting path to reduce repeated cutting

Difficulty 5: The walls of humanoid robot parts are thin and complex, and deformation during clamping cannot be prevented

The first four difficulties are the "common problems" of titanium alloy itself, and the fifth difficulty is the superimposed challenge brought by the structure of humanoid robot parts.

The titanium alloy parts processing of humanoid robots has several typical characteristics: thin walls (shell 1-2mm), asymmetric structure (joint seat side opening), and dense features (dozens of mounting holes, threaded holes, and positioning surfaces on a single piece). These features make clamping the biggest uncertainty factor.

The clamping force is too strong, and the thin wall is directly deformed by clamping. After processing, it rebounds when released, and the size deviates completely. The clamping force is reduced, and the cutting force is much greater than that of aluminum alloy, which may cause slight displacement of the workpiece. The cutting force of titanium alloy is already high (tensile strength ≥ 900MPa), coupled with insufficient rigidity of thin-walled structures, the balance between clamping and cutting is very fragile.

Real case: When Shenzhen Huiwen Zhizao was processing a titanium alloy joint module for a humanoid robot for a technology enterprise, the thin-walled area of the shell showed a deformation of 0.15mm after the first process of clamping. After switching to a multi-point support specialized fixture and a low clamping force scheme, the deformation is controlled within 0.03mm.

Response strategy:

Special fixture design: multi-point support, low clamping force, uniform distribution, avoiding local deformation

Minimize clamping frequency as much as possible: Complete multiple processes in one clamping to reduce the cumulative deviation caused by repeated clamping

Arrange stress relief between processes: After rough machining of complex parts, arrange stress relief annealing before precision machining

Quality control: the "gatekeeper" of robot titanium alloy parts processing

Every step in the processing of titanium alloy parts for robots may introduce deviations, so quality control must be implemented throughout the entire process:

The surface integrity of titanium alloy parts for humanoid robots requires special attention. Surface microcracks, residual tensile stress, and depth of work hardening layer, these invisible "hidden dangers" may become the starting point of fatigue fracture. For key load-bearing components, it is recommended to add X-ray testing or fluorescent penetrant testing.

In conclusion

Choosing titanium alloy for humanoid robots is an inevitable trend driven by performance; However, the difficulty of processing titanium alloy parts has deterred many enterprises. From cutting heat management to elastic rebound compensation, from sticking tool control to titanium alloy CNC work hardening response, and to clamping deformation of thin-walled parts - each link requires dual support of process experience and equipment capabilities.

For example, Shenzhen Huiwen Zhizao, as a professional robot body hardware design and manufacturing service provider, intervenes from the drawing review stage when processing robot titanium alloy parts for robot enterprises: reviewing structural processability, marking key tolerances, predicting deformation risk points, and then developing a phased processing plan - rough machining to remove excess, stress relief annealing, semi precision machining to leave compensation, and precision machining in place. This approach of "design+manufacturing" linkage has a much higher success rate than directly starting work after receiving the drawings.

There are many service providers in Shenzhen who specialize in CNC machining, but if they can do titanium alloy CNC machining, some of them will be screened out. There are even fewer service providers who can do a good job in titanium alloy CNC machining. Service providers like Huiwen Zhizao are guaranteed in all aspects of service. The company relies on the technical background of the Chinese Academy of Sciences and has passed ISO9001 and IATF16949 quality management system certification. It has been recognized as a specialized and innovative enterprise with more than 20 related patents and has served many listed companies and industry-leading companies, such as Lenovo, Xinsong, Xiaomi Ecological Chain, Beijing Institute of Technology and other customers.

Huiwen Zhizao currently has a factory area of 20000 square meters and more than 370 processing equipment. It is skilled in five axis machining of robot core parts and has a stable batch delivery pass rate of over 98%. For humanoid robots, Huiwen Intelligent Manufacturing Innovation 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.

In addition to humanoid robots, the company can provide comprehensive OEM services for mechanical dogs, flexible robotic arms, bionic robots, medical robots, and more.

If you are evaluating a titanium alloy solution for machining humanoid robot parts, please feel free to provide drawings for consultation, process evaluation, and quotation.