Machining Natural Shell (Mother of Pearl, Abalone, etc)
Mother-of-pearl, abalone, and other shell products (nacre) are naturally occurring composites of inorganic platelet/crystals (calcium carbonate) imbedded in a witches brew of organic binders. The platelets are stacked together in an offset pattern like microscopic bricks, an arrangement that gives natural shell its incredible strength and resistance to the propagation of transverse cracks. The thickness of each platelet is comparable to the wavelength of visible light (500 nanometers) so that the stack diffracts incoming light into the incredible rainbow patterns that make this material so desirable for inlay.
Because the composite is laid down by the epithelial cells of the resident mollusk, the consistency if successive layers can vary dramatically. The marine environment in which the shell is formed also contributes material to the composite, a fact that increases the difficulty of machining and shaping natural shell into useable pieces. Adding to the difficulty is the tendency of the cutting debris to pack into the kerf being formed as well as into the flutes of any rotary cutter.
Although much progress has been made in the form of the laminated shell products (invented by Larry Sifel and Chuck Erikson), it is still VERY hard, if not impossible to arrive at a chart of reliable feeds and speeds for cutting natural shell products. Nonetheless, the stunning beauty that can be wrought using these rather pathological materials, inspires luthiers, pool cue makers, and furniture makers to continue developing new techniques for shaping and forming natural shell.
sample courtesy of Bryan England, Custom Inlay
With the application of CNC (Computer Numerical Control) and the development of a new generation of solid carbide cutters, shapes of great precision and elegance can be fashioned from virtually any shell material. Bryan England of Custom Inlay (Caneyville, Ky) supplies inlay parts and components to many luthiers, custom knife makers, and cabinet makers. The microphotograph at left is part of a white pearl logo for the Gibson Guitar Co.
Larry Sifel and his crew at Pearlworks (Charlotte Hall, MD) set the bar almost impossibly high with this floral burst created for the C.F. Martin & Co. signature guitar series.
It is exactly what it looks like. Abalone inlaid in a spider-work of white mother-of-pearl. Although it appears too delicate to touch, it is remarkably tough. Composed of a series of intertwined "florets", the burst winds its way up the entire fingerboard of the instrument before merging with the stylized company logo on the tune head. Larry developed the sequential inlay techniques, material fixturing and a host of other innovations that make the production of this, and other shell masterpieces look almost easy.
Of course, mastering these techniques is quite another matter. As you might imagine, the first step is figuring out how to cut shell without breaking a bit every 5 minutes. Although a comprehensive speeds and feeds table does not yet exist, the experiences of some accomplished inlay artists can serve as guidelines for getting started with this amazing family of materials.
Santa Cruz Guitars, the brainchild (and labor of love) of Richard Hoover has been the birthing place of some of the finest acoustic guitars to be had on this planet*. In addition to their incredible instruments, one of the distinguishing characteristics of Santa Cruz Guitars is their willingness to embrace new technologies to extend and enhance the capabilities of the artists and luthiers working there.
* If you don't believe me, check out their player list.
Reith Guitars / Custom Luthier produces some of the most innovative, dynamic and smooth playing electric guitars on the market. Originally a software engineer, Todd has incorporated 3D modeling and precision 3-axis CNC machining into virtually all aspects of guitar fabrication. Featuring custom solid aluminum Sonic-Block T bridges, headless tuners, and graphite headless necks, each Reith guitar is a one-of-a-kind instrument with unbelievable clarity and seemingly bottomless sustain.
Optimized Shell Cutters Feeds and Speeds
|Diameter||Depth / Pass||Speed
|Infeed / Plunge
|0.0100" (0.25mm)||0.005" (0.125mm)
|0.0120" (0.30mm)||0.006" (0.15mm)
|0.0156" (0.40mm)||0.008" (0.25mm)
|0.0200" (0.51mm)||0.010" (0.33mm)
|0.0230" (0.58mm)||0.012" (0.30mm)
|0.0250" (0.64mm)||0.014" (0.41mm)
|0.0313" (0.80mm)||0.016" (0.51mm)||24,000||12||5.0|
|0.0625" (1.59mm)||0.033" (1.00mm)||24,000||28||5.0|