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Identifying Limiting Factors of a New Milling Process
CAD/CAM milling technology is adding to in-house capabilities, but integration can be complicated
By Jordan Greenberg
“Can MY machine do that?” It’s a common question in the world of CAD/CAM dentistry. However, it’s not always the right question.
After years of resistance and uncertainty, dental laboratories have fully embraced milling technology as an efficient and high-quality alternative or supplement to traditional laboratory techniques. This growing confidence has caused new applications to move from the laboratory bench to the milling room. However, in order for a traditional laboratory process to make this digital transition, a large number of limiting factors must be considered. The showcase following this article details milling equipment available in today’s market, and while it’s an extremely helpful tool in determining which machines can handle certain indications, it’s the final step to an in-depth line of questioning for evaluating a new CAD/CAM process.
Milling New Indications
When a laboratory is considering new equipment or evaluating its existing technology to add new in-house milling capabilities, it must consider a number of factors. As an example, consider the recent implantology trend of angulated screw access holes.
The process can begin with an examination of the CAM software, which translates the CAD file into something the machine can understand. In this case, it utilizes part features such as the abutment base margins, insertion directions, emergence profiles, and screw channels to generate tool paths that will effectively mill these elements. If the laboratory or milling center is already milling hybrid screw-retained structures without angulated screw channels, a majority of these features should populate in a similar fashion. However, unless a CAM software with the ability to identify angled screw channels is being utilized, the resulting milled restoration would have excess material in the screw channel above the bend and below the part that is accessible from the occlusal milling direction.
To address this, no changes are made to the machine. Once the CAM software has integrated the ability to identify the angled screw channel and apply a tool path that is parallel to the occlusal direction of the access hole, the part will now be milled correctly and completely.
For multi-unit cases, like the example above, 5-axis machines are necessary to achieve all the appropriate occlusal screw channel angles. Some would argue a 4-axis machine would limit the ability to add an angulated screw channel to single hybrid abutments; however, that is only the case if the machine is incapable of indexing. A machine with indexing utilizes the fourth axis in two ways: 1. It flips the blank 180° in order to mill from each side of the blank; and 2. When the part has undercuts or a milling direction that is not perpendicular to the blank surface, it rotates the axis to an intermediate angle, locks it in place, and then uses 3-axis milling to finish the undercut. The tooling is unchanged, and there are not any special fixtures or workholding that are necessary to access the problematic area of the part. The only change will be addressed in the CAM software, which needs to align the angulated screw channel with the machine’s rotary axis using a 3+1 optimization function.
In either of these situations, three questions require attention before incorporating the new screw channel features:
1. Can my CAD software design a part with an angled screw channel?
2. Can my CAM software identify the angled screw channel and apply the appropriate toolpath?
3. Is my machine either 5-axis, or 4-axis with indexing capabilities for single units?
Other Factors Material
Some milled materials are not available in standard blank sizes — such as glass ceramic ingots. In order to accommodate these blanks, machine manufacturers have created different ways to hold these types of materials. In some cases, there is an additional holder shaped like standard 98.5-mm pucks that holds multiple individual ingots. In other cases, the whole blank holding mechanism is removable and replaced with a fixture that holds the ingots instead. In both of these cases, the machine manufacturer usually works with the CAM companies directly to integrate these features into their programs for the laboratory or milling center. On the CAD side, there are no major differences from the standard crown-and-bridge designs for other materials.
Even if a material is produced in a standard 98.5-mm disc, it can still have properties that are unfit for a particular machine. For example, milling titanium requires a machine’s construction to be much more rigid and have a higher-powered spindle than a typical benchtop milling unit. The lighter-weight benchtop machines are more geared for softer materials such as zirconia, wax, and PMMA. Without industrial-like attributes that reduce vibrations in the tool and material clamping, the machine’s components will wear significantly faster or become damaged almost immediately. Also, due to titanium’s heat-generating thermal properties during milling, it requires either flood coolant or lubricating oil to be used in order to keep the material and tool from overheating and wearing prematurely. On a machine that is unfit for a certain material, tool wear can be so significant that a laboratory’s costs to mill them in-house exceed the cost of outsourcing.