Marine equipment CNC processing: What are the practical misconceptions in material selection technology for sealing components of underwater robots?

I have conducted research procurement for ROV AUV marine equipment projects, most of them have encountered such troublesome situations. The complete machine plan material grades have been finalized, the prototype water pressure test is also normal. Once it enters small-scale trial production, there will be problems with water leakage salt spray point corrosion of the components. Many people initially suspect that the material is good, repeatedly changing the grade struggling for a long time before realizing that the root cause of the problem is the material selection itself, but the outsourcing process ignoring the coupling relationship between CNC precision machining, machining dimension compensation, surface treatment. For marine equipment projects, understanding the practical constraints of component processing can save a lot of rework costs avoid delaying project testing milestones.

Different water depth conditions, material selection does necessarily solve all component risks

The working environment of underwater robots is very demanding on parts, the selection logic of parts is completely different for different water depths.

1. Shallow water teaching, observation type ROV (0-50m): 6061-T6 aluminum alloy POM、ABS， High cost-effectiveness low processing threshold;

2. Offshore industrial ROV (50-500m): 5083 aluminum alloy, 316L stainless steel, balancing weight offshore salt spray resistance;

3. Deep sea AUV (500-1500m): 2205 duplex stainless steel, 7075-T6 aluminum alloy, emphasizing structural strength;

4. Kilometer level deep-sea submersible (>1500m): TC4 titanium alloy, PEE special plastic, with a focus on pressure resistance seawater corrosion resistance.

Many researchers developers have a common misconception: as long as they choose the right corrosion-resistant materials, putting the parts in seawater will make everything fine. I have seen many negative cases in actual projects: the substrate grade is a problem, but sharp knife marks are left after processing, the sealing grooves are compensated for after processing, resulting in pitting corrosion sealing failure in the future. Materials are only basic conditions, processing techniques, surface treatment, groove surface quality can also affect the actual performance of components underwater.

Frontline selection reference (only for mechanical component selection processing reference)

1、 Sealing groove: Machining must be considered in conjunction with surface treatment

The sealing groove is the most prone location for underwater cabin component failures. Aluminum alloy undergoes hard anodizing, with the film layer growing in both directions inward outward; Anodizing of titanium alloys also results in an increase in film thickness. If the drawings directly indicate the finished dimensions of the grooves after oxidation, without pre compensation in the CNC machining process, the actual effective width depth of the grooves will be compressed after oxidation treatment. When installing the O-ring at the end, sometimes the pressure is too tight sometimes the gap is too large.

Some processing factories on the market have a relatively crude approach: CNC milling is completed directly outsourced for surface treatment, only focusing on machining the finished dimensions without accounting for the size changes caused by oxidation, leaving all risks to assembly hydrostatic testing.

During the execution of CNC precision machining projects, when encountering underwater cabin parts, we will first distinguish between the dimensions of machined blanks marked on the drawings the dimensions of finished products after oxidation during the DFM review stage. Prioritize the use of groove pre compensation scheme, enlarging the grooves in advance according to the target film thickness to offset the size reduction caused by the inward growth of the film layer. If the tolerance margin of the sealing position is very tight, local masking technology will also be evaluated as an alternative. In addition, the roughness of the groove sidewall must be controlled in place during the milling stage. If the blade pattern is too deep, even if the substrate is corrosion-resistant, seawater ions will accumulate in the gaps. Under high pressure environment, leakage corrosion hazards will be buried, surface treatment cannot be relied on to repair defects in the processing stage.

2、 Special materials for marine engineering should pay attention to incoming material verification matching with cutting processes

The materials used in underwater robots have a wide range: 6061/5083 aluminum alloy, 316L stainless steel, TC4 titanium alloy, PEEK engineering plastic, each material has significant differences in cutting characteristics.

TC4 titanium alloy has poor thermal conductivity, fast tool wear, unreasonable parameters can easily cause tool deformation micro damage to the surface of the parts; 316L stainless steel processing is prone to work hardening; PEEK belongs to special plastics, it is prone to deformation when the temperature is too high; 5083 marine aluminum alloy has differences in cutting oxidation processes compared to ordinary 6061.

Many general machining factories are accustomed to using the cutting parameters of ordinary shells. Even if a single sample is barely qualified, it is difficult to maintain the surface quality dimensional dispersion of the parts in small batches.

When Huiwen Zhizao undertakes CNC parts processing orders for marine equipment, the incoming materials will be checked against the original factory material report, special materials such as titanium alloy, stainless steel, PEEK will undergo factory re inspection; Match corresponding cutting tools, coolant, cutting parameters for different materials; Separate rough fine machining, increase the aging process to release residual stress in the blank, reduce deformation hazards caused by subsequent temperature water pressure changes.

3、 Key screening: Can the prototype process be replicated in small batches

Most marine equipment research development projects involve multi version iterations small-scale trial production. Some outsourcing manufacturers rely on experienced technicians to manually adjust the machine use simple fixtures during the prototype stage, manually polish the sealing groove, pass the sample testing. But this set of temporary construction techniques for sampling, with fixtures, programs, cutting parameters solidified, cannot be replicated for small-scale production. A very tricky phenomenon will occur: after all the prototype water pressure passes, the consistency of the sealing groove will decrease after batch delivery, the problem of water leakage will repeatedly occur after assembly.

In the actual project docking of CNC machining in Shenzhen, when dealing with the requirements of underwater robot components, we synchronize the solidification of fixtures, NC programs, cutting parameters, testing standards during the sampling stage to avoid "sampling one set mass producing another set". The roughness film thickness compensation requirements of the sealing groove are directly written the process documents, without relying on manual remedies by workers on site.

4、 DFM evaluation should be based on the actual working conditions underwater

Many procurement structural engineers regard the processing plant as a purely "cutting according to the drawing" processing party. But it is difficult to clearly depict all the dimensional changes caused by seawater pressure, salt spray corrosion, surface treatment in the drawings.

We cannot directly schedule production after obtaining drawings for pressure chambers acoustic chambers. It is necessary to identify the risks of sealing grooves, thin-walled deformation, galvanic corrosion hazards caused by dissimilar metal assembly based on actual diving depth, seawater conditions, sealing ring specifications, surface treatment types.

The DFM review conducted by Huiwen Zhizao will only verify the basic geometric dimensions, but also evaluate the groove compensation amount key assembly surface roughness based on the underwater assembly usage scenario. At the same time, it will alert the corrosion risk of dissimilar metal assembly. Before starting the processing, optimization suggestions will be output in advance to reduce the problems exposed in the subsequent testing stage.

5、 Huiwen Intelligent Manufacturing Marine Equipment Machinery Parts Matching Processing Ideas

There are many CNC factories on the market that can produce ordinary hardware parts, but there are many manufacturers that truly consider the working conditions of underwater robots by linking machining, surface treatment, assembly risks. Most processing plants only look at the dimensions on the drawings ignore the chain effects brought by the marine environment.

Huiwen Zhizao can undertake precision machining of mechanical structure components for underwater robots ranging shallow water to mid to deep sea levels, including pressure cabin components, acoustic communication cabins, underwater connection flanges, sensor bases, other parts; Support marine materials such as 6061/5083 aluminum alloy, 316L stainless steel, TC4 titanium alloy, PEEK, etc., fully covering prototype sampling, multi version iteration, small-scale trial production.

The factory is equipped with multiple five axis machining centers, which can complete the one-time clamping forming of irregular cabin bodies multi angle sealing grooves; Equipped with Zeiss CMM, roughness meter other testing equipment, implement first piece confirmation, process sampling, finished product review. Key features such as sealing grooves are verified for size roughness within the process, then transferred to the surface treatment process. At the same time, with the synergistic ability of surface treatment such as anodizing, passivation, micro arc oxidation, film thickness compensation is taken consideration in the DFM stage to adapt to high salt spray ocean conditions.

VI. Summary

When selecting in practice, do use quotation material grade as the sole criteria for judgment. The good performance of prototype testing does mean that small batches can be stably replicated. Taking account the process sedimentation, process quality control, iterative delivery capability, DFM evaluation for offshore engineering conditions, there will be fewer pitfalls to overcome. If you are outsourcing underwater robot components have non-standard part drawings such as pressure chambers acoustic chambers, please feel free to send them for DFM process evaluation itemized quotation.

7、 Frequently Asked Questions

Q1: Will using TC4 titanium alloy 316L stainless steel prevent seawater corrosion of the parts?

A: really. Corrosion resistant substrates only reduce the probability of corrosion. If there are sharp knife marks surface scratches during processing, slight corrosion may still occur. Direct contact between dissimilar metals also poses a risk of galvanic corrosion, insulation isolation needs to be added during assembly. When conducting DFM reviews for offshore components, we will raise concerns about the risk of assembly corrosion control the surface quality of the processed parts.

Q2: Can the sealing groove be CNC machined directly according to the size of the oxidized finished product?

A: recommended. The bidirectional growth of hard anodized film will compress the effective size of the groove. Prioritize pre compensation of grooves during the CNC stage; If the tolerance window is extremely small, local masking of the sealed area can be evaluated. Without any compensation, small batch assembly can easily result in unstable compression of 0 turns.

Q3: The prototype water pressure test is fine, but the batch parts repeatedly leak water. What is the reason for this?

A: Most of them are two sets of processes: prototype mass production. The prototype relies on manual adjustment polishing to conceal defects, the fixtures programs are solidified, making it impossible to replicate the process to small batches. The selection needs to be confirmed, the sampling stage should be carried out according to the mass production standards. The roughness size of the sealing groove should be included in the process inspection.

Q4: What points should EEK pay attention to when processing deep-sea components?

A: PEEK is prone to deformation when heated, the cutting temperature must be controlled during processing to avoid overheating softening; suitable for one-time cutting with large margins; At the same time, deburring should be done well. Sharp edges corners are prone to stress concentration in high-pressure seawater environments. Huiwen Zhizao has experience in CNC machining PEEK special engineering plastics can undertake parts such as deep-sea insulation components sealing pressure rings.

Q5: What documents can be submitted to the processing plant to reduce communication disputes?

A: ① Clear distinction between 2D drawings: machined blank size/finished product size after surface treatment; ② Indicate the actual diving depth seawater medium; ③ Sealing ring model, groove design compression ratio; ④ Complete material grade surface treatment technology requirements; ⑤ Identify key testing items. Drawings can be submitted to Huiwen Zhizao for free DFM working condition review.