Consistency and Collaboration in CAM Milling

There are many reasons why milling is a preferable fabrication method over analog means. Once the appropriate pieces of hardware and software are in place, milling can offer a higher level of consistency than analog processes. One of the main reasons that dentists change laboratories is inconsistent work. This can be the result of the human variable and the effect it has in the analog (or traditional) process. Each technician is unique, whereas one CAD library of tooth anatomy is uniform despite multiple designers. While adjustments of a library pattern for tooth No. 30 may be required across different patient cases, once a CAD library is involved, those tooth No. 30 restorations will have a more consistent look no matter who designs them. With milling, this stability can be maintained across multiple materials.

Stability and repeatability are extremely important because they determine the effort and time needed to produce a high-quality product. Without these key components, mill operators lose the benefits of a digital approach to fabrication. Great milling machines use sophisticated systems to control temperature (±0.1°C) and every aspect of the milling process to guarantee that each part is consistent and precise. The following are some of the major aspects of milling systems.

Spindle capability

The spindle is very important because it is the primary interface between the milling machine and the workpiece. The milling spindle must be very rigid and have low vibration. Vibration can cause very high tool wear and negatively affect surface finish, whereas a rigid spindle design maintains high surface finishes and low tool wear. Vibration negatively affects spindle life, which leads to higher maintenance costs. Another key aspect of spindle capability is rotations per minute (rpm). The rpm of a milling spindle correlates directly to the programmable feed rate of the machine. If higher feed rates can be achieved, cycle times per part can be reduced.

Onboard self-diagnostics

Self-diagnostics in the mill software manage manufacturing better and lead to better product consistency (Figure 2 and Figure 3). For example, onboard software that is working properly does not forget to perform tasks. One self-diagnosis feature is the ability of the mill to clean and measure tools with a laser after each use, discarding them once they display a predetermined amount of wear. This factor can be programmed to apply to any range of tool wear by the user. Other diagnostic elements—such as automatic calibration of mill breaks, axis alignment, and coolant levels, and notification of tool breakage—are also very beneficial.

Run time

It is important to discuss run time—not just with your mill manufacturer but also with the CAM software developer and tool vendor. The answers will help eliminate surprises regarding production or profitability and assist you in deciding how refined a part needs to be. How long will it take to mill the part at that quality level? What is the production capacity of the mill at that run time per part? At that speed and feed, what will be the tool costs and other costs associated with this production level over a day, week, or month? These and other questions help the laboratory owner visualize the overall picture of what this new digital tool can produce and at what cost. From there, it is important to check those answers against the business plan to ensure that the profitability/production theory is realistic.

Tool capacity/management

Increased tool capacity equates to increased strategy capacity. The 80/20 rule comes to mind; most of the work will likely fall into just a few milling strategies that run a common set of tools, while the minority of the work will be more specialized. The demand for more varied parts such as implant-borne structures, threaded holes in bars, 5-axis milling, or deep internal spaces can demand special tools. Some are drills; others may be long-reaching, end-cutting, or other specialty cutting tools for specific attributes of a part. The more tools the mill can hold, the more streamlined the production process (Figure 4 through Figure 6).

Remote access

Remote access allows the user to maximize the ROI of the mill. Adding files remotely is a remarkable feature in this technological time, allowing parts to be nested 24/7. Machine malfunctions, broken tools, break tests, and software glitches holding up production are just a few issues that can be managed from anywhere—even from a moving car or even sometimes from an airplane (albeit slowly). Mills can also send a message to a remote operator when they encounter an error, speeding up its resolution. For items requiring vendor access, the milling machine manufacturer can diagnose and determine machine errors remotely, which can also reduce maintenance costs and machine downtime.

Recovery of milling debris

Having a process for recovering and removing milling debris is an important issue, no matter what its composition (Figure 7). If the debris has value, developing a process for collection is imperative to maintain fiscal efficiencies. If the debris has no value, it too must be properly removed to keep the mill axes free and in motion. Mills can go into error if the software recognizes the buildup of debris that impedes the movement of an axis, leading to interruptions in milling.

Calibration of the mill's universe is critical. The mill needs to know the center of each axis and the combined center of them all. If the mill is not able to do this automatically, a trained technician must address this all-important issue.

Mill movement during cutting is bad. Therefore, weight is a positive attribute to a mill. If the mill is not heavy enough to be stable, it must be bolted or tied to the floor. A mill's overall weight is a feature that is designed into the machine for stability, which is why some mills weigh tons. Setting smaller mills on freestanding tables or structures that are not stable can lead to inaccuracies such as chipped margins on materials like zirconia or lithium disilicate.

All mills need ongoing attention; certain elements must be monitored and adjusted. A mill manufacturer can define what will wear out and how many cycles those specific parts will last before needing replacement. Slides, gears, valves, pressure sensors, relays, scales, drive motors, gaskets, hard drives, pumps, etc, are all parts that are susceptible to wear. Maintaining these items in accordance with the manufacturer's recommendations is key to precise running and cutting over time. Better and typically more expensive milling platforms have features like magnetic levitation, pneumatic drive, and oil floats, which eliminate the wear of mechanical parts engaging with each other. Preventive maintenance is also a key component to accurate, consistent production.

Each mill operator must have a conversation with the tool manufacturer regarding which tools are best for certain materials, applications, etc. Cultivate a strong relationship with the vendor's representative. They will assist in evaluating the finished part and make suggestions and recommendations regarding the tools themselves—new coatings, different flute geometry, feed, speed, etc. Having a good relationship with the vendor's or manufacturer's representative is invaluable, especially to answer questions or concerns a mill operator may have during the setup phase of the mill and over time as tool companies develop new tool geometry and coatings. Of course, recommendations will vary among different manufacturers and vendors; tool developers are competitive as they develop proprietary geometry and coatings.

Mill manufacturers work to improve control software constantly in order to keep their systems current. The most recent dramatic software development provides a view of what is coming in the mill's tool path, not just what the mill is doing at any moment. This allows the mill to perform at significantly higher dynamics (acceleration/deceleration) while maintaining the same level of quality. This also yields shorter cycle times and higher productivity with a relatively inexpensive upgrade. In some applications, it can reduce cycle times by up to 25%.

Knowing and being able to discuss the different components and capabilities of CAM milling machines can only help laboratory managers and owners make the best investments for their business as the technology progresses. This is just the beginning for digital production in the laboratory. Soon enough, as with photography and typing, we will wonder how we ever did it any other way.

Scott A. Mappin is CEO of Strategy Milling, LLC, in Leetsdale, Pennsylvania.