Blank

Chapter 3

CHAPTER 3

FINITE ELEMENT MODEL

Overview

The following section describes the process used to create the FEM, finite element model, of the guitar. Due to the complex shape and features of the guitar, the model creation was a daunting task, with the documentation proving to be even more difficult. A blueprint of the Martin D-28 folk guitar was purchased and scanned into the computer. A construction model was created initially using Autodesk Mechanical Desktop 6.0 and implemented the scanned blueprint as a template. Points were obtained from the construction model and then used in I-DEAS 10 to create the finite element model, the FEM.

Construction Model in Autodesk Mechanical Desktop 6.0

First, a blueprint of the Martin D-28 guitar was purchased through an online source [5]. The blueprint was scanned on a drum scanner and saved as 2 jpeg images. The scanned images were imported into Autodesk Mechanical Desktop 6.0. A construction model was created in Mechanical Desktop 6.0 using the scanned images as a template. The measured points from the construction model were recorded in a Microsoft Excel spreadsheet. A picture of the complete construction model of the guitar is shown below in Figure 1. A complete step by step account of the construction model creation is listed in Appendix B.

FIGURE 1. Complete construction model in Mechanical Desktop 6.0.

Geometry in I-DEAS 10

Top Plate Geometry

Points were then created in EDS IDEAS 10 using the points created above. Splines and lines were created from the top plate points to form the top plate contour, brace lines, bridge lines, and fingerboard lines of the top plate of the guitar. Surfaces were then created from the intersecting splines and lines. A complete step by step account of the top plate geometry creation is listed in Appendix B. Completed top plate geometry is shown below in Figure 2.

FIGURE 2. Top plate geometry in I-DEAS.

Back Plate, Ribs, Headblock, and Tailblock Geometry

An angled workplane was created to represent the location of the back plate. A rectangular surface was created on the workplane that enveloped the size of the back plate. The top plate contour spline along with the neckblock and tailblock lines was extruded through the rectangular plane. The surfaces were then trimmed leaving the top plate, back plate, rib, and headblock and tailblock surfaces. The brace lines were created and used to trim the back plate surface into sections. Enclosed volumes were created from the surfaces making up the headblock and tailblock. A complete step by step account of the geometry creation is listed in Appendix B. Completed top, back plate, ribs and headblock and tailblock geometry, the effective makeup of the body of the guitar, is shown below in Figure 3.

FIGURE 3. Complete body geometry in I-DEAS.

Neck Geometry

Points were created first for the heel of the neck, obtained from the construction model. A spline, and then a surface were created for the bottom of the heel. The contour spline of the heel was extruded to the top plate workplane to create the sides of the heel. Points were created, obtained from the construction model, for the mating surface between the fingerboard and neck. Lines were created from these points and surfaces from the lines. Points were created at either end of the neck to outline the cross-section of the 2 ends of the neck. Lines were created between the points and surfaces from the lines. The upper neck surfaces were trimmed with and used to trim the heel. A complete step by step account of the neck geometry creation is listed in Appendix B. Completed body and neck geometry is shown below in Figure 4.

FIGURE 4. Body and neck geometry in I-DEAS.

Fingerboard Geometry

Points were created for the cross-section of the fingerboard at both ends of the fingerboard. Lines, then surfaces, were created. An enclosed volume was then created from the surfaces of the fingerboard. A complete step by step account of the fingerboard geometry creation is listed in Appendix B. Completed body, neck, and fingerboard geometry is shown below in Figure 5.