CAM Milling: A Cut Above

Milling machines can serve various purposes, however, and determining your own priorities is key when making purchasing decisions. The large majority of laboratories utilize milling machines only for zirconia, but some also use them to mill metal and hybrid bars, a small percentage mill digital dentures, etc. The latter options offer the most potential for profitability in the near future.

The first question the author asks any laboratory seeking advice on purchasing a milling machine is, "What are your goals for the next 1, 2, 3, 5, and 10 years?" This question involves identifying what type of work the laboratory handles currently and what capabilities they would like to add in order to accentuate their portfolio. Some laboratories might be best served focusing on zirconia and outsourcing the milling of materials such as chrome cobalt or gold to third parties. Others may be able to grow their business effectively by becoming more diverse in their in-house capabilities. Each laboratory should narrow down its goals and develop a plan.

That plan should consider the fact that the effectiveness of a milling system depends heavily on the quality of the STL files loaded into it. Various CAD systems and modules on the market offer different capabilities and levels of complexity. Before shopping for a milling machine, a laboratory should examine its CAD system in terms of what type of work they want to mill, how actively they are taking scans and combining data, and how advanced the skill level of their operators is.

Once those factors have been considered, the laboratory can compare the many milling machines available on the market. Every mill is different; some are very basic, some are standard, others are of a higher caliber, and still more are top-of-the-line, Rolls Royce-type machines. While some of those high-end mills are large, production-type machines, size and cost do not always directly correlate with quality; some small desktop machines are very robust. However, often a more expensive mill can be worth the cost due to the wide range of materials it can handle and better durability. A variety of considerations factor into what milling machine is best for each laboratory.

Milling machines vary in size and weight—anywhere from approximately 120 lbs. to well over 800 lbs. Some fit easily on a desktop or countertop, while others take up a significant amount of space in a room.

Different machine sizes warrant different electrical needs and compressed air pressure. Most machines on the market today run on a standard 110v electrical supply. However, some larger machines may need a direct 220v line or electrical transformer installed by an electrician. Compressed air pressure requirements also vary per manufacturer; 6 bar usually is enough, but checking manufacturer recommendations and air requirements is almost as important as meeting electrical needs.

Milling machines typically are classified as wet, dry, or wet/dry. Wet milling involves lubricating the tools in order to avoid damaging them when working on very hard materials.

Wet milling typically is necessary for PMMA, lithium disilicate, and titanium. Some laboratories mill PMMA dry, but the author recommends wet-milling it instead, because the plastic particles can melt due to the friction of the bur, which clogs the burs and shreds the tools; recent innovations, however, are intended to combat that (more on that later).

Dry milling should be utilized only for zirconia, wax, and chrome cobalt. Zirconia theoretically can be milled wet, but needing to dry it afterward is not ideal. Chrome cobalt can be milled dry because it is not as hard as titanium, which requires a lubricant to help mill and preserve tool life and sharpness. The green state or powdered version of chrome cobalt found in certain products must be placed in an argon matrix sintering furnace afterward to achieve complete hardness.

Denture materials can be milled wet or dry, depending on the system being utilized. Most machines and templates mill denture base material dry. The speed of the milling process and the templates used control the results in the material being milled. Milling the denture base usually causes the material to powder or chip. Tools will not clog or have plastic adhere to their cutting surfaces.

Wet/dry combination machines must be used carefully. Using them requires planning work according to the type of restorations that are needed at specific times, to prevent cross-contamination of zirconia from materials that need to be milled wet. A thorough cleaning when switching from wet to dry is necessary to maintain an optimal operational level.

Most dental milling machines on the market today are described as either 4- or 5-axis mills. It is important to note, however, that most 5-axis mills do not actually use all five axes simultaneously. Approximately 90% of all CAM software strategies are written for "3+2," utilizing XYZ and AB axes separately. As such, 4-axis mills often are sufficient for milling simple crowns and simple bridges with zirconia because they use three axes and add a fourth for some of the more challenging spots. However, complex restorations such as implants and digital dentures can require five axes in order to mill specific areas as precisely as possible.

The speed of a milling machine is based on the size of the machine and the spindle; the spindle size and speed control the cutting rate of the tools that are turning and being controlled by the CAM strategies. Typically, larger spindles that can accommodate larger tools are capable of achieving faster speeds and feeds with more stability and accuracy for the cut that is required. Benchtop mills generally can produce a single-unit, full-contour crown in 12 to 20 minutes.

Most spindles can last from 1 to 6 years, depending on the conditions in which they are being used. Factors influencing the durability and effectiveness of the spindle include type of material (hard or soft), type of tool changer, direct tool clamping or a tool clamping system (eg, pressure collettes or heat-fitted collettes, tool sharpness and pressure put on the spindle when cutting), cleanliness, and maintenance. Most desktop units use direct spindle clamping with a built-in tool collette.

The ADA's marginal integrity certification is approximately 30 μm, and most of today's milling machines offer accuracy well within that standard. Milling accuracy is based on the inner workings of the machine—how tight and dialed-in everything is.

Stability is very important. Benchtop mills are at a disadvantage compared with larger, heavier machines and must be secured and protected from any movement. If the mill vibrates even slightly, the accuracy can suffer due to tools skipping or moving, so even the location of the mill and any furniture underneath it need to be stable.

Different mills offer different tool sizes, from 3-mm shafts to 6 mm and even larger. These tools go in different styles of holders, which also affect the quality of the cut. For some smaller machines, tools insert into the spindles. Larger machines have tool holders that are separate and heavy, and they are held in by friction or pressure; the machine picks up the tool from the holder, loads it into the spindle, and then cuts. The size of tools and the tool holders contribute to the high level of accuracy of many larger mills.

Some of today's milling machines offer automated tool changers and blank changers. Tool changers are primarily used so that production does not need to stop in the event that a tool breaks. Blank changers are particularly useful for high-production laboratories that need to mill overnight. If their CAM software offers batch calculation, they can take one puck, nest projects for it, calculate, and then start working on the next nesting to continuously feed the machine so it is loaded for an entire night.

In most milling environments, carbides, diamond-coated carbides, and different-coated carbides can be used for metal milling, and straight diamonds can be used for lithium disilicate grinding. Most zirconia is cut with chemical-vapor diamond-deposited carbides for accuracy and longevity. Non-coated carbides are recommended for PMMA and wax, and straight diamond-coated for all-ceramic wet milling. There is a wide variety of pricing for tools, but cheaper ones may not be best. Consistency, accuracy, and control of results in many situations depends on the properly designed tool. Therefore, always do your research and use good tools to achieve the proper cut.

CAM software is the brain that controls the machine. All of the tools and components that have been discussed in this article are controlled by CAM software strategies.

Simple CAM software that is packaged with the mill is often sufficient for crowns, bridges, and copings. However, trying to mill complex restorations such as dentures or bars with simple software often is tedious and results in substandard quality. If a laboratory plans to expand its milling capabilities in the future, one option is to purchase a basic version of a more robust software at a reasonable cost, and then add additional features or modules as needed. In the future, CAM software likely will be available that can control both milling and additive technology together, potentially in a cloud-based manner that does not require dongles.

Another factor to consider is expandability. If you eventually purchase another mill, will you need to add another software license for it, or can one license be used for multiple machines?

Most new developments in subtractive technologies involve milling plastics, wet dentures, pre-mills for abutments—the extra-coronal portion, not the interface-bars with multi-unit attachments, hybrids, all-on-X, and more. Whatever innovations the industry develops, today's milling technology is up to the task.

Some new ancillary features have gained traction as well. Ionizers, for example, pull particles down to where they can be vacuumed out very easily, instead of allowing them to free-float in the machine and coat the material that is being milled. C-clamps, meanwhile, are used in some machines to mill more of the disc than when it is placed into a fully contained cassette.

Attachments also are available for milling pre-mill abutments; the implant interfaces are produced by manufacturers, but the user can design the extra-coronal portion of the implant abutment and wet-mill this pre-mill. Lithium disilicate and all-ceramic units are also milled in extra holders other than 98.5-mm round discs.

Customer service is very important. If your machine is not working correctly, you need instant service. Some companies offer to send loaner machines to keep your system operational during repairs. Others offer backup milling services at an outsourcing facility during that time.

Some machines are costlier to repair than others. Most companies offer a maintenance service package, so that is one consideration when purchasing.

Most companies cover under warranty any necessary repairs in the first year; beyond that, proper maintenance, cleaning, and calibration should keep most mills operating well for many years. However, machines do break unexpectedly; spindles, sensors, air valves, drive motors, and milling axes are all vulnerable elements, so parts and labor should be factored into your budget.

In the future, mills will continue to get sleeker and more precise. Even as additive technology continues to improve, milling likely will still have a role, and the two will be used in conjunction. For example, a laser-sintered implant case may require a mill to finish the components perfectly, or a 3D printer could be used to retouch a milled restoration. The latter could be seen as soon as this year's International Dental Show (IDS) this month in Cologne, Germany.

As the technology continues to evolve, the key will be how laboratories utilize and optimize it. The author hopes that laboratories will increase their levels of communication and collaboration with dentists to involve them in the digital process, and hire young people who are more computer-savvy, train them in dentistry, and push the envelope because there is no limit to what today's milling technology can accomplish.

Ron Rosenthal, CDT, is the Business Development Manager North America for WorkNC Dental Hexagon Manufacturing Intelligence