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A new DFM approach to combine machining and additive manufacturing

Olivier Kerbrat, Pascal Mognol, Jean-Yves Hascoët

arXiv:1106.3176v1cs.OH

TL;DR

Most DFM systems focus on one manufacturing process, leaving limited guidance for choosing how hybrid modular products should be made. This paper develops a CAD-based method to evaluate manufacturability for additive and subtractive processes; its examples report reduced machining-difficulty indexes and indicate that the processes can be combined.

  • Problem

    Most DFM approaches consider only one manufacturing process, while designers lack quantitative guidance on manufacturing complexity for choosing hybrid modular rather than one-piece designs.

    Source: §1 · p. 2

  • Method

    The method calculates global and local manufacturability indexes from a one-piece CAD model, using octree decomposition to map difficult areas and guide hybrid modular design choices.

    Source: §2.2 · p. 4, §5 · p. 16

  • Results

    60% in C(c)-, 45% in C(f)max, and 53% in C(f)mean reductions were reported for modular design; analysis also indicates combining subtractive and additive processes.

    Source: §4.1.5 · p. 12, §4.2.3 · p. 14

  • Takeaways & Limitations

    The analysis indicates that areas hardest to machine may be easier to make additively, supporting a hybrid choice of process by module.

    Source: §4.2.3 · p. 14

    The indexes are based on geometric parameters and exclude manufacturing-preparation factors such as cutting-tool path strategy.

    Source: §5 · p. 16

Abstract

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Design For Manufacturing (DFM) approaches aim to integrate manufacturability aspects during the design stage. Most of DFM approaches usually consider only one manufacturing process, but products competitiveness may be improved by designing hybrid modular products, in which products are seen as 3-D puzzles with modules realized aside by the best manufacturing process and further gathered. A new DFM system is created in order to give quantitative information during the product design stage of which modules will benefit in being machined and which ones will advantageously be realized by an additive process (such as Selective Laser Sintering or laser deposition). A methodology for a manufacturability evaluation in case of a subtractive or an additive manufacturing process is developed and implemented in a CAD software. Tests are carried out on industrial products from automotive industry.

1. Introduction

Most DFM systems focus on a single manufacturing process, leaving designers without quantitative guidance for choosing between machining and additive manufacturing. The paper proposes a hybrid modular DFM approach and tests it on two industrial tooling parts.

1. Introduction

  • Most DFM systems focus on one process, while this work aims to support multi-process manufacturing decisions during design.DFM considers design goals and manufacturing constraints to identify problems while parts are being designed.Source: §1 · p. 2
  • The proposed hybrid modular view treats parts as 3-D puzzles whose modules are made separately and then assembled.Modules can be produced independently and simultaneously, and alternatives can be created by changing one module rather than the whole product.Source: §1 · p. 2
  • Each module can be assigned to the manufacturing process best suited to its time, cost, or quality requirements.The approach compares additive processes such as SLS or powder projection with high-speed machining, and smaller modules may reduce manufacturing difficulties.Source: §1 · p. 2
  • The approach addresses the difficulty of choosing hybrid modular rather than traditional one-piece designs when process-specific manufacturing complexity lacks quantitative DFM guidance.Careful assembly remains necessary to achieve the quality of a one-piece part.Source: §1 · p. 2
  • The methodology was tested on two industrial parts from tooling.Source: §1 · p. 3

2. Hybrid modular design methodology

The methodology evaluates additive and subtractive manufacturability from CAD design parameters, then compares one-piece and hybrid modular designs using global and local indexes. It combines hybrid and modular options to address manufacturing difficulties.

2.1. Manufacturability evaluation

  • Manufacturability indexes use CAD-derived design parameters affecting manufacturing difficulty, but exclude factors requiring complete manufacturing preparation, such as toolpath strategy.Indexes cover geometry, dimensions, material information, and specifications.Source: §2.1 · p. 3

2.2. Manufacturability indexes

  • Local indexes use octree decomposition because manufacturing-relevant details may be missed by global measures, while feature decomposition is limited for additive processes and free-form surfaces.Octree subdivision adapts cell size to local geometric complexity and is process-neutral.Source: §2.2 · p. 4, Figure 1 · p. 4
  • Local indexes are calculated for grey octants and displayed as a color map, with blue indicating easiest-to-manufacture areas and red the most difficult.The map visualizes the distribution of local manufacturing difficulty across the part.Source: §2.2 · p. 4, Figure 2 · p. 5
  • Indexes range from 0 to 1 and provide global or local, process-specific information about manufacturing complexity; additional surface-orientation, material, and tolerance indexes are not discussed.The indexes are based on design parameters influencing manufacturing time, cost, and quality.Source: §2.2 · p. 5, Table 1 · p. 5

2.3. Hybrid and modular approaches to improve the manufacturability

  • A hybrid design can machine one module while additively manufacturing zones that are difficult or impossible to machine, whereas modularity can reduce difficulty in other cases.The choice depends on the parameters involved in the difficult areas; separately machining modules can reduce chip generation in the cited example.Source: §2.3 · pp. 6–7, Figure 3 · p. 6, Figure 4 · p. 7
  • Changing design parameters could also reduce manufacturing difficulty, but this option is excluded because the paper focuses on hybrid and modular approaches.The example considered is changing the radius of a concave shape.Source: §2.3 · p. 7

2.4. Schematic view of the methodology

  • The workflow evaluates the one-piece CAD model, creates a hybrid modular design, evaluates it again, and compares both analyses to quantify its advantages.The approach is implemented in six stages.Source: §2.4 · pp. 7–8, Figure 5 · p. 7

3. Development in CAD software

The CAD implementation calculates manufacturability indexes, maps local difficulty, and compares alternative designs; it is applied to two industrial tooling parts.

3. Development in CAD software

  • The SolidWorks system calculates global index values and local index values across octree cells, displaying local difficulty as a color map that can be refined with further decomposition.For local indexes, it also calculates maximum and mean values; the interface lets users select the index to calculate.Source: §3 · p. 8
  • The interface presents manufacturability analysis for additive manufacturing and lets users select which index to calculate.Results are exported as Excel files for designers.Source: §3 · p. 8
  • The six-stage methodology is tested on two industrial tooling parts, selected because additive fabrication is an alternative for complex dies and moulds.The examples are dies and moulds.Source: §3 · p. 9

4.1. Industrial modular injection mould

For an industrial injection-mould core, manufacturability analysis guides a two-module design and comparison with the one-piece model. The modular design reduces several machining-difficulty indexes, although assembly constraints are not quantified.

4.1.1. One-piece CAD model (stage 1)

  • The initial model is a one-piece CAD model of an industrial injection-mould core for investment casting.It represents stage 1 of the methodology.Source: §4.1.1 · p. 9, Figure 7 · p. 9

4.1.2. Manufacturability analysis for machining (stage 2)

  • The core’s long required cutting tool and low rigidity motivate splitting it into two machined modules; local C(f)- reaches its maximum value of 1 in difficult-to-machine areas.The difficulty map identifies the areas that inform the modular design.Source: §4.1.2 · pp. 9–10, Figure 9 · p. 10

4.1.4. Manufacturability analyses for the modules (stage 5)

  • Module index values are combined into totals using module weights; global indexes and mean local indexes are aggregated, while a local maximum uses the highest module maximum.Table 2 presents results for the two-module core, but the supplied passage does not give its cell values.Source: §4.1.4 · p. 11, Table 2 · p. 12

4.1.5. Comparison between the one-piece design and the two-modules one (stage 6)

  • In this example, the modular design considerably reduces chip quantity by 60% in C(c)- and allows less flexible cutting-tools, reflected in decreases of 45% in C(f)max and 53% in C(f)mean.All index values decrease; the time, cost, and quality impact is not quantified.Source: §4.1.5 · p. 12

4.2. Industrial automotive die

An automotive stamping die illustrates comparison of machining and additive manufacturing for one CAD model. The resulting hybrid design assigns difficult-to-machine areas to additive fabrication and easier areas to machining, improving manufacturability.

4.2.1. Presentation of the die

  • At C(f)max = 0.986, the die’s pocket corner radii are difficult to machine; the example compares additive and subtractive processes for a 630 x 182 x 100 mm stamping die.The authors state that a modular approach alone would not significantly reduce the index in these areas.Source: §4.2.1 · pp. 13–14, Figure 13 · p. 13

4.2.3. Hybrid point of view

  • The hybrid design machines the easier areas and uses additive fabrication for the hardest-to-machine zones, improving manufacturability without adding new manufacturing difficulties for the additive module.Module 2 is powder-projected directly onto the machined module 1 support.Source: §4.2.3 · pp. 14, 16, Figure 16 · p. 15, Figure 17 · p. 16

5. Conclusions and objectives for further researches

The paper presents a CAD-based hybrid modular DFM methodology that uses global and local manufacturability indexes to help designers compare one-piece and hybrid modular designs. The indexes are geometry-based, and future work must address richer indexes and assembly constraints.

Conclusions

  • Global and local manufacturability indexes, with local analysis based on octree decomposition, identify difficult regions and help compare one-piece and hybrid modular designs.The CAD-based method combines additive and subtractive processes and is illustrated with two tooling examples.Source: §5 · p. 16

Objectives for further research

  • Current indexes use geometric parameters, excluding manufacturing-preparation factors such as toolpath strategy; material and technical specifications are proposed for future indexes.Further work includes optimizing the methodology and comparing different indexes.Source: §5 · p. 16
  • Assembly constraints still need integration into the methodology because modules must be gathered to achieve quality comparable to a one-piece part.Source: §5 · p. 17
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