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PAPERS: Part Design Methods and Specification Challenges in AM

Investigating the Role of Geometric Dimensioning and Tolerancing in Additive Manufacturing

[+] Author and Article Information
Gaurav Ameta

School of Mechanical and Materials Engineering,
Washington State University,
Pullman, WA 99164-2920

Robert Lipman, Shawn Moylan, Paul Witherell

National Institute of Standards and Technology,
Gaithersburg, MD 20877

Contributed by the Design for Manufacturing Committee of ASME for publication in the JOURNAL OF MECHANICAL DESIGN. Manuscript received February 15, 2015; final manuscript received August 4, 2015; published online October 12, 2015. Assoc. Editor: Christopher Williams. This work is in part a work of the U.S. Government. ASME disclaims all interest in the U.S. Government's contributions.

J. Mech. Des 137(11), 111401 (Oct 12, 2015) (10 pages) Paper No: MD-15-1127; doi: 10.1115/1.4031296 History: Received February 15, 2015; Revised August 04, 2015

Additive manufacturing (AM) has increasingly gained attention in the last decade as a versatile manufacturing process for customized products. AM processes can create complex, freeform shapes while also introducing features, such as internal cavities and lattices. These complex geometries are either not feasible or very costly with traditional manufacturing processes. The geometric freedoms associated with AM create new challenges in maintaining and communicating dimensional and geometric accuracy of parts produced. This paper reviews the implications of AM processes on current geometric dimensioning and tolerancing (GD&T) practices, including specification standards, such as ASME Y14.5 and ISO 1101, and discusses challenges and possible solutions that lie ahead. Various issues highlighted in this paper are classified as (a) AM-driven specification issues and (b) specification issues highlighted by the capabilities of AM processes. AM-driven specification issues may include build direction, layer thickness, support structure related specification, and scan/track direction. Specification issues highlighted by the capabilities of AM processes may include region-based tolerances for complex freeform surfaces, tolerancing internal functional features, and tolerancing lattice and infills. We introduce methods to address these potential specification issues. Finally, we summarize potential impacts to upstream and downstream tolerancing steps, including tolerance analysis, tolerance transfer, and tolerance evaluation.

FIGURES IN THIS ARTICLE
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Copyright © 2015 by ASME
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References

Figures

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Fig. 1

Modified figure from Ref. [42], showing the ubiquitous role of tolerances in product life cycle

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Fig. 2

(a) A simple part with GD&T and (b) support structures when the part is built along different build directions

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Fig. 3

Schematic depicting the effect of discrete layer thickness on the geometry of a freeform part based on a given build direction. Discretizations 1 and 2 are generated with build directions 1 and 2, respectively, for the same profile.

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Fig. 4

Adopted figure from ASME Y14.5 [6] showing the use of coordinate system indicators (x, y, and z axes explicitly shown) in a drawing

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Fig. 5

Application of an area indicator showing the percentage of surface (10%) that can be covered by support structures. This will aid designers in limiting support structures at certain functionally critical locations.

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Fig. 6

Graded material distribution shown as grayscale color of surfaces and volumes in a part from Ref. [62]

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Fig. 7

Example of grain direction specification from ASME Y14.8 [43] standard for casting and forgings and proposed table to use grain direction specification as track specification for multiple layers. The angle in each layer is measured from the direction shown in the figure.

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Fig. 8

Modified part from the GE bracket design competition [64] winner [42] with GD&T tolerancing

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Fig. 9

A freeform target area indicator with subscript F. This area indicator can be coupled with feature control frame for profile tolerancing to specify tighter control of profile in this area for functional purposes.

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Fig. 10

Figure adopted from ASME Y14.5 [6] showing application of tolerancing a pattern of holes

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Fig. 11

Examples of lattice unit cells that are used to create lattices in AM

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Fig. 12

Part from Fig. 2(a) being built. Smaller hole will be completed before the larger hole (a datum for smaller hole).

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