Identifying Limiting Factors of a New Milling Process

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.

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?

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.

As indications become more complicated, more tool types are needed to address new part features. For standard crown-and-bridge applications, ball-nose cutters handle a majority of the organic shapes that need to be milled. Any sharp corners on the inside of copings are compensated for in the CAD software. However, with laboratories and milling centers producing more implant-related parts in their own facilities, a need exists for flat end mills and drills to manufacture these geometric shapes. Some milling strategies for screw-retained bridges in titanium and chrome cobalt utilize as many as 10 tools, so without the respective capacity in a machine, these parts cannot be milled as a single job.

Another way to ensure the machine is effective at milling new applications is by integrating proper tooling for the job. A popular new request for in-house milling is full dentures in wax and PMMA. Regarding an acceptable milling time, denture bases present new challenges for machines currently on the market because they require a significant amount of material removal before finishing the surface area and tooth pockets. To most effectively do this, a tool that is uncommon to the dental market should be used — a single flute, highly polished end mill. This tool properly manages the increased chip load while removing a larger amount of material, and it will cut very cleanly without generating as much heat due to its sharper cutting edge.

As dental laboratories continue to push their production equipment’s capacities, machine manufacturers are looking for more ways to accommodate the increased usage. Automation is a great way to optimize workflow and efficiency. Denture base milling is a perfect example of how automation can significantly increase a laboratory’s ability to justify the added expense of an automatic blank-changer when choosing a machine. They are one of the largest parts being milled in-house, and in most cases they’ll take up a whole blank. Milling times can range from 30 minutes to four hours depending on the machine, CAM software, and desired surface finish.

This creates a unique new problem for laboratories that want to add the ability to mill multiple denture bases at one time. If the machine has already reached its full capacity during the day, the most opportune time to mill parts with longer milling times is overnight. However, because only one denture base can fit per blank, unattended blank changes are necessary to produce multiple parts each night.

Popular milling indications being brought in-house by dental laboratories and milling centers include hybrid abutments with angulated screw channels, full-arch screw-retained hybrid bridges, denture bases, preform abutments, and titanium bars. Different machine features should be considered for each indication.

As previously noted, laboratories looking to add single hybrid abutments with angulated screw channels should look for either a 5-axis machine or the ability to use a fourth axis to mill the divergent occlusal direction.

Laboratories looking to add full-arch screw-retained hybrid bridges with titanium bases should look for a 5-axis machine that will address the potential for larger angulations; 25° or above is ideal.

To mill denture bases, investigate machines with slightly higher-powered spindles and faster movements to handle the increased amount of material removal and larger surface area finishing required for these types of parts. Another good consideration is an automation cell to enhance the unique workflow when the laboratory can only fit one part per 98.5-mm blank.

To mill preform abutments, look for a machine that effectively holds the preform blanks in quantities that make sense in a particular environment. For small laboratories that mill just a few abutments a day, a single blank holder is sufficient. However, some laboratories produce a large number of titanium custom abutments and would benefit from a six-blank holder and an automatic puck changer. In either case, a coolant or lubricant must be used. Industrial-grade features that add weight, power, and stability are also recommended.

To mill titanium bars, consider machines with similar attributes to preform milling equipment, but with an even higher level of accuracy and more tool positions. In these cases, additional weight and vibration dampening will also ensure that tool wear is acceptable and does not affect the fit.

Many factors must be considered when evaluating a new piece of CAD/CAM equipment. To ensure the most effective and impactful decision is being made, ask more questions. Don’t just ask which machine is the best fit. A laboratory needs to understand the hows and the whys of a new process before a purchasing decision can be made with confidence.

Jordan Greenberg is Managing Director of FOLLOW-ME! Technology North America.