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The Difference CAM Software Makes in Dental Machining
Optimizing today’s mills with effective software
By Mark Ferguson
A typical CAD/CAM system in a dental laboratory consists of six distinct parts. These are the scanner, software to run the scanner, a case management system, design software, CAM or nesting software, and a milling machine. Some of these parts get lost or forgotten when we as dental laboratories or clinicians look into purchasing a system. In the beginning, for the most part, people wanting to go digital felt they were buying a couple items (scanner, PC, and mill) because all the parts came packaged together. Just a few short years ago, the debate in digital dentistry was open vs. closed systems, and now most would agree open systems are preferable. Open systems are more desirable because of the interest in customization for each laboratory’s use. Users of CAD/CAM started to want to change parameters in their system, or upgrade/replace different parts. This could be an upgraded PC, adding a second mill, or the need to adjust parameters for better fit of the restoration, to reduce milling times, or to get a different finish out of the machine.
As CAD/CAM has become mainstream in the dental workflow, more and more people have looked deeper into how each of the six individual parts work and can be optimized. The early adopters have been tweaking design parameters for years. It has not been until more recently that milling parameters have started to be examined. This is where the CAM software comes into play. Initially, the motives were to add smaller tools, thereby enabling a finer detail of the designed anatomy. Shortly after that, however, other milling parameters started to be examined. How fast should the spindle speed be set? How fast should the tool be moving across the material? How deep should the tool plunge into the material? All of these questions have bearing on parameters within the CAM software. While often overlooked, the CAM software has more of an impact on the milled result than the machine itself. The machine is exactly that—a machine; it does only what it’s told. The CAM software is what tells the machine how and where to move.
Early versions of CAM software would be laughed at today. Most early laboratory machines were 3+1-axis milling. This means the spindle would move in three directions (x, y, and z), with an added fourth axis (a) in the puck holder. Early CAM software would mill with the disc in 0° and 180° positions. This would allow for milling the top and bottom of a restoration. Later the CAM software allowed for milling at different a-axis angles, always 180° off each other. This little advancement in the CAM software enabled laboratories to mill taller bridges in thinner discs. Obviously, this isn’t a problem in today’s reality of machining, where most machines have some sort of 5-axis capabilities, be it full 5-axis or 3+2. Meanwhile, other advancements are being made that were unheard of previously. For example, when machining glass ceramic materials, the software is required to have grinding capabilities. This is different from milling in that the tools that are used change. Grinding uses an abrasive machining process, meaning diamond burs, whereas milling is cutting the material, using a fluted bur. Up until recently, when machining a glass ceramic material, the entire block had to be ground away. Now the CAM technology will carve away the end of the block in one piece to make the process considerably faster.
This is a great example of how CAM software and machines need to work together. There have been machines purchased with capabilities that the CAM software does not support. A 5-axis machine being controlled by 3+1-axis CAM software is not going to get the same results as the same machine run with a 3+2- or full 5-axis CAM. Along those same lines, different CAM software will calculate tool paths differently. Some CAM software will calculate a step-over tool path with movement along only one plane, while other software will follow the contour of the restoration. This will affect the surface finish of the finished product. When a linear step-over is used, the end result will show more distinct mill lines on steeper surfaces than one that steps over based on a measurement calculated on distance on the milled restoration. This is just one way the same machine can give totally different outcomes from the same design file. Another advantage of more advanced CAM software is the ability to read additional files that designate particular parts of a restoration. Often a .pts file will indicate the margin line. Then the CAM software can protect at the margin and a preset distance from the margin line, and also create a different (finer) surface finish.
As the dental machining world has evolved, most laboratories have moved to 5-axis machines. This has given the versatility to mill screw-retained prostheses, as the screw access holes can be off axis in any different direction. In order for the tools to have access to completely and precisely mill those holes, the milled material must be able to tilt in each direction. Tilting the angle of the material in relation to the spindle is generally done with 3+2 milling, as there is little added benefit to full 5-axis machining for this. Full simultaneous 5-axis milling requires a machine to be in perfect alignment and CAM software that is far more powerful. When executed properly, this would allow machining for a finer surface finish and more complex geometries. However, if one axis motor is not perfectly calibrated with the rest for this type of machining, the results are far more catastrophic than with 3+2 milling, as the movement would put the spindle farther and farther away from the desired position.
The marriage of CAM software and machines has typically been the best argument for closed systems in dental CAD/CAM. The idea of the closed system has always been that it is more plug-and-play ready. Parameters in the design software are preset to work with the milling tools to create a restoration that fits the indication. One of the most important factors in these parameters is the drill compensation. Drill compensation is a parameter in the design software. It creates space around the prep in order to ensure that the smallest tool used in milling is able to reach all areas of the intaglio surface of a crown. Drill compensation is necessary because CAM software is designed to never mill away material that exists in the design part. Therefore, if the intaglio surface of a crown is designed with areas that a bur can’t reach due to its diameter, those areas will remain unmilled. The software will always choose to leave material on the part rather than remove part of the crown.
The best possible result for a CAD/CAM system is if each individual component is configured to work within the entire system. Like CAM software controlling a machine, scanner software must move the scanner and the part being scanned properly to achieve the best possible scan, while calculating the data properly. These two elements working together can show how a software update can help the physical scanner achieve faster or more accurate results, without any hardware updates. Once this information is captured, the design software must have tested and proper parameters aligned with the tools and machining strategy that will be used later. The case management software should be set up to give the CAM software the proper files, in the proper format. With dental restorations, certain areas should be treated differently than others. A margin line, for example, can be protected from chipping if the CAM software can read the file and take consideration of the data. Lastly, the CAM software needs to be set up to consider the different parts of a restoration, be it a screw access hole, a margin, or occlusal anatomy, and create tool paths to get the most out of a machine. With all parts set up as a single system, the flow from one area to the next can be seamless and produce the high-quality restorations every patient deserves.
Mark Ferguson is General Manager at Vulcan Custom Dental in Birmingham, Alabama.