## 1 Introduction
Additive processes, though, have moved into real manufacturing with moulds (rapid tooling) and, as processes improved, for manufacture of real parts, repaired parts, and re-furbished parts. Even though the use case has broadened, mass production is still a challenge to AM processes because of lack of accuracy and the physical realities of additive processes. Most additive parts with curved or sloping surfaces have a characteristic step-wise surface roughness because models are sliced to produce layer shapes that are extruded in a single direction. Post-processing operations, such as grinding processes, solvent immersion, and EDM (Electrical Discharge Machining) subtraction, improve the surface accuracy, but only when the parts are slightly larger than the required shape. This is not guaranteed in the normal chain because facet corners are on surfaces but edges may lie outside or inside the real shape. In addition, while the faceting is done relative to the surface, slicing is done in the build direction that does not take the surface normal vectors into consideration. This can lead to the realised object having missing volume in places, which creates problems for subsequent manufacturing. Finally, the data format used for AM processes is a tessellated model, which means increasing the size of the data, if accuracy needs to be high. A further complication of using a facetted representation for additive processes is that finishing processes use the exact shape for control, which means that it is necessary to have two models of the same part and that these models will not match. [PERSON] performed an examination of BRep (boundary representation) based additive process at the Ecole Centrale de Nantes [1]. His work led to the development of the STEP-NC standard for additive processes, which defines a consistent format for additive processes as part of a manufacturing chain. The seamless representation of the consistent data format shows the advantages in term of advanced data model [8], the environmental impact analysis [8], hierarchical manufacturing structure [9]. The philosophy of STEP-NC is oriented to keep the identical quality of manufactured products even though the machine tool is changed to a different one until the end-of-life stage of product life cycle. As a demonstration of this concept, [PERSON] et al. [10] proved the advantage of using STEP-NC in AM processes as part of remanufacturing. Through the whole product life cycle, additive manufacturing is widely utilised for more complex applications. This means that additive manufacturing is considered as one of a number of production processes in the whole supply chain. Hybrid manufacturing, which combines subtractive processes and additive processes, has appeared in commercial machine tools and requirements for mixed process planning are discussed by [PERSON] et al. [11]. Various application industries, as well as typical manufacturing industry, utilise AM, as mentioned by [PERSON] [12]. Advanced production technology, such as 4D printing, has also been launched [13]. These new applications also present the need for STEP-NC in additive manufacturing. Medical products, aircraft, racing cars and dental work are recent applications of additive manufacturing, but STEP-NC does not cover all additive manufacturing specialised operations and features [14; 15]. This paper proposes a novel data representation based on STEP-NC and a new geometric procedure for producing slice geometry. This data representation also follows Part 17 which address the additive manufacturing standard of ISO 14649. In Section 2 the representation method of STEP-NC Part 17 is introduced. Section 3 describes new features and representation of AM features based on the STEP-NC representation. Slicing operations and a way to ensure that volume is added for post-processing by machine tools follows in Section 4. Section 5 explains the result of experiments with several shapes.