Computer Numerical Control CNC Machining

Machining generally includes conventional processes such as milling, turning, grinding, and unconventional processes such as Electric Discharge Machining (EDM), wire-EDM, laser cutting, and abrasive jet machining. With the aid of computers and digital technology, CNC machining is one of the most efficient and reliable fabrication processes.

With the introduction of newer and tougher materials in the dental prostheses market, any fabrication process needs to provide crowns, bridges, and implant restorations economically with no compromise on quality. The evolution of CAM has enabled generating the NC tool path with less operator intervention. It also provides higher quality output at a faster rate. Most commercial CAM packages provide different options of tool path generation and a facility to simulate the tool path before actually cutting the physical part. In this way, the NC programs are ensured to be safe from gouging, material left out, heavy cut, or idle run.

Commercial systems like CERECĀ®, CerconĀ®, and LavaTM use a milling station to fabricate the dental restorations. In these systems, the NC codes are automatically generated by the built-in software. As examples, Fig. 8.29(a) shows a single crown machined with a CNC milling machine, and Fig. 8.29(b) illustrates four views for milling a 3-unit bridge with two abutment retainers (crowns) at the two ends supporting the replaced missing tooth (pontic).

Besides these dedicated dental milling units, general-purpose CAM packages and selections of machineries are also available. Software packages like Uni-graphics, PRO/E, Surf-CAM, DelCAM, and MasterCAM offer high-level NC programming modules. DelCAM offers separate modules, DentMILL and PowerMILL, to mill dental restorations. PowerMILL is a CAM system that produces NC toolpaths from CAD models for machining with 2, 3,4 and 5-axis milling machines. It can import data from any CAD system, through standard transfer files in IGES, STEP, VDA, STL or a variety of direct interfaces. PowerMILL generating NC paths and simulation of NC machining for a single crown is shown in Fig. 8.30.

Machining crowns, onlays, and bridges is simpler, as their basic features are very similar although the size and scale vary. Though these variations are critical in design, they are insignificant in generating NC programs. There are other dental components like dental bars for implant-supported restorations, which are more difficult to design and plan the machining process because they are functions of several variables (e.g. the number of implants, positions and orientations of the implant abutments). Nevertheless, they can still be fabricated by CNC machining with cutter paths generated by CAM packages like Uni-graphics, PRO/E, DelCAM. As an example, a 3-implant dental bar machined from a titanium block from its digital design model is shown in Fig. 8.31.

A key advantage of CNC machining is the tight tolerance and high accuracy achieved by this process. Further, CNC machined parts require little or no secondary finishing work. A main limitation of CNC milling is the geometric

Fig. 8.29 CNC milling: (a) single crown machined with a milling unit; (b) four views for machining a bridge (Chang et al., 2003)

Fig. 8.30 PowerMILL:

(a) tool path generated;

(b) simulation of NC machining (Ref: DelCAM website)

Fig. 8.29 CNC milling: (a) single crown machined with a milling unit; (b) four views for machining a bridge (Chang et al., 2003)

Fig. 8.31 The 3-implant dental bar: (a) digital design model; (b) machined physical part in titanium (Delli, 2006)

complexity of machined part that can be achieved. Narrow or deep cavity preparations or features like undercuts or sharp corners cannot be easily replicated by milling operations. In these situations, special techniques like EDM and ultrasonic machining may be of assistance.

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