Non-Fungible Blockchain Tokens for Traceable Online-Quality Assurance of Milled Workpieces

This work presents a concept and implementation for the secure storage and transfer of quality-relevant data of milled workpieces from online-quality assurance processes enabled by real-time simulation models. It utilises Non-Fungible Tokens (NFT) to securely and interoperably store quality data in the form of an Asset Administration Shell (AAS) on a public Ethereum blockchain. Minted by a custom smart contract, the NFTs reference the metadata saved in the Interplanetary File System (IPFS), allowing new data from additional processing steps to be added in a flexible yet secure manner. The concept enables automated traceability throughout the value chain, minimising the need for time-consuming and costly repetitive manual quality checks.

💡 Research Summary

The paper presents an end‑to‑end framework that combines real‑time simulation‑based online quality assurance with blockchain tokenisation to create immutable, verifiable proof of quality for milled workpieces. First, a digital twin of the CNC milling process is built by linking the robot controller to a real‑time simulation consisting of a visualisation model, a behaviour model and a material‑removal model. As the machine operates, actual and target position data are streamed into the simulation, which continuously generates a virtual workpiece. Feature‑based inspection algorithms evaluate geometric attributes (e.g., hole roundness, edge straightness) against standards such as DIN 4760 and BS 7172, producing a set of deviation values and pass/fail flags.
These inspection results are mapped to the Asset Administration Shell (AAS) – specifically the “Quality Control for Machining” sub‑model – and serialised as a JSON document. To protect confidential manufacturing data, the JSON is encrypted with a symmetric key that is shared only with authorised partners. The encrypted file is then uploaded to the Interplanetary File System (IPFS); the resulting content identifier (CID) serves as a permanent, content‑addressable reference.
On the blockchain side, the authors implement an ERC‑721 (ERC‑721) smart contract using the OpenZeppelin library. The contract stores a mapping from token ID to an array of IPFS URIs. During minting, the contract receives the owner address, a sequential token ID (or serial number), and the initial IPFS URI containing the first quality record. The contract exposes an addUri function that allows the current token holder to append additional URIs after subsequent manufacturing steps, thereby creating a chronological chain of quality records linked to a single NFT. Ownership transfer of the token automatically transfers the right to invoke addUri and to read the associated quality data, enabling seamless hand‑over across supply‑chain participants.
The prototype is validated on an industrial robot performing a benchmark part (the “Diamond Circle Square”). Real‑time simulation produces a 3‑D model of the finished part; a manual comparison with the nominal geometry yields deviation values that are stored in the AAS. After encryption and IPFS upload, the CID is embedded in the minted NFT on an Ethereum testnet. The NFT is viewable on a public marketplace (OpenSea), displaying the token ID, image link, description, owner address, and a list of history entries. The metadata JSON contains the IPFS hash of the encrypted AAS file; when the hash is resolved in a browser, the encrypted payload appears, and only parties possessing the symmetric key can decrypt it to retrieve the full quality dataset.
Key findings include: (1) the NFT acts as a single source of truth for the entire lifecycle of a workpiece, linking all quality snapshots without storing large data on‑chain; (2) the approach reduces redundant manual inspections because downstream partners can verify quality by reading the immutable record; (3) using IPFS mitigates on‑chain storage costs while preserving immutability through the hash; (4) the system maintains data privacy via encryption, yet provides transparency of provenance through the public blockchain. Limitations discussed are Ethereum’s scalability and gas price volatility, reliance on IPFS node availability, and the need for robust symmetric‑key distribution mechanisms. The authors suggest future work on layer‑2 scaling solutions, decentralized key management, and integration with permissioned blockchains for enterprise‑grade deployments. In conclusion, the study demonstrates that coupling real‑time digital twins with NFT‑based tokenisation can automate quality assurance, enhance traceability, and foster trust across modern, flexible manufacturing supply chains.