Options in Dental Milling

In general, the type of material being milled determines whether a mill should be dry or wet. Certain ceramic materials such as lithium disilicate and feldspathic porcelains require wet milling, as do some metals like chrome cobalt. Other materials, such as zirconia and titanium, can be milled dry or wet. In general, wax and acrylic (for temporaries) are milled dry.

The liquid in a wet mill serves several functions. It continuously bathes the cutting tool and milling stock, keeping both the tool and material cool, and helps to remove material from the milling disk or block. Certain additives can also be included in the liquid to act as a cutting lubricant. Wet mills require periodic cleaning because the material milled away collects within the machine and eventually needs to be removed. Cutting liquid replacement also is generally required at regular intervals.

Dry milling does not use liquid to remove scrap material away from the cutting surface, but rather pressurized air, vacuum, or a combination of both. Maintenance cleaning is still required, as a considerable amount of dust and shavings are generated with dry milling. Vacuum size, noise, and operating expense should all be considered before making a purchase.

Most mills are configured to operate as either a wet mill or dry mill. However, some dry mills can be converted to operate as a wet mill. Others can operate as dry or wet depending on the material being milled.

Mills are available in three basic sizes: tabletop, bench-top, and standalone. Each size has characteristics that may make it more desirable, depending on milling volume and materials.

A tabletop mill is usually small enough that a single person can pick it up and move it from location to location. These mills typically use belt-drives to move the stock or milling spindle. Most tabletop mills are configured to mill single-unit blocks, but may have fixtures to accommodate small frames or even disks. While the most common dental mills have tool changers to swap burs (tools) during milling, some of the tabletop mills have fixed tooling that must be manually changed during the milling process. The price of tabletop mills ranges from an estimated $30,000 to $50,000. These smaller mills are, by and large, very capable; however, they may have tool usage and changing limitations. In addition, while smaller and lighter mill construction improves portability and keeps system cost low, they may also limit the lifetime and tolerances of certain key system components.

Bench-top mills tend to be considerably larger and heavier than the tabletop versions. Two or more people or mechanical equipment is necessary to lift or move such mills. They can weigh up to 500 pounds and may require special benches to support them. Bench-top mills generally use heavy-duty ball screws to move the stock or milling spindle. Bench-top mill spindles are also typically more robust than those found in tabletop mills. Automatic tool changers are standard, and in some cases, extra tool positions are available for additional milling detail or other materials. Some bench-top mills are also capable of milling metal and glass ceramics in certain configurations. The price range for bench-top mills is higher than their tabletop cousins, typically running from $50,000 to $150,000. This is because of their more durable construction, which makes them better suited for heavy usage.

Freestanding mills are typically the biggest, most robust, and most expensive mills available. They can mill the toughest materials, in larger stocks, and run longer with tighter tolerances than the smaller mills. Floors may need reinforcement to support the weight of these mills, as they can tip the scales at well over 1000 pounds. Prices for freestanding mills can range from $100,000 to $500,000. If a fixture is available to hold a material, these mills can mill anything that the smaller mills can. Typically, capability and durability are not issues with freestanding mills. With these massive machines, the question concerns which materials and products would generate enough revenue to justify the expense.

There is no correlation between the number of axes a mill has and the size of the mill. There are tabletop 5-axis mills and freestanding 3-axis mills; however, 3-axis mills are the most common dental mill configuration. They are capable of milling from the top or bottom of the stock material, but are unable to mill undercuts, which is adequate for routine crown and bridge work. Typically, mills with four axes can mill undercuts in only one direction, whereas 5-axis mills can mill undercuts in each direction.

Figure 1 through Figure 3 illustrate the limitations of 3-axis milling. Figure 1 shows a bridge design that has been placed in a standard height disc. The position of the bridge in the disc allows for 3-axis milling and no undercuts on the intaglio surface of the framework. However, this side view shows that the bridge is too tall to fit entirely into the disc. If the framework were milled in this configuration, the incisal edges and margin highlighted with the red arrows would be missing. In Figure 2, the framework was rotated to fit inside the disc. While the part now fits inside the disc, the machine can no longer mill using 3-axis strategies, as the framework does not have draw in respect to the spindle and cutting tools. Figure 3 shows the stock material rotated to an off-axis position for 5-axis milling.

5-axis milling is beneficial when milling custom implant abutments that may have undercut areas or for large-span bridges where the bridge can be rotated to fit in shorter stock material. Figure 4 shows another example of 5- versus 3-axis results. While the image on the left is actually a printed part rather than milled, the results would be the same with 5-axis milling. The part on the right is the same part milled in a 3-axis mill. The missing undercut region on the left side of the pattern is obvious on the 3-axis milled part.

CAM software determines how best to mill a part out of the stock material and generates a milling program for the mill to follow. Some CAM programs are integrated into the CAD or design software, others act as standalone separate programs. The CAM software must be configured with specific information about the mill, including the size and shape of the cutting tools, the material being milled, the spindle controller, and the motors that move or rotate the stock and spindle. Fortunately, this is usually taken care of by the manufacturer or the company supplying the CAM software for the mill. Laboratories looking to upgrade or add mills should always investigate whether or not their existing CAM software will work with the new equipment.

Most milling sequences are broken down into three basic routines: roughing, finishing, and detail. The routines always occur in this order. Each routine has its own set of step-over and step-down values. Step-over is the distance that the mill moves over, usually horizontally, before it makes the next milling pass; and step-down is the distance that the mill moves down, usually vertically, for the next pass.

Roughing is the step where bulk material is removed quickly. It usually is done with the largest-diameter cutting tool available. The spindle speed and cutting travel speed (feed-rate) are usually at the fastest settings. Step-over and step-down values are the highest, removing as much material as possible. Large step-overs and step-downs leave a considerable amount of material between passes (Figure 5).

Finishing is an intermediate step that removes the material left over from roughing. A smaller tool is used during the finishing sequence. Step-down and step-over values are much smaller, which results in a better surface finish (Figure 6). In the finishing step, because the tools are smaller and areas like margins are being milled, the feed-rate is usually slower. Sometimes there are multiple finishing sequences. The CAM software may use one strategy with certain step-overs and step-downs for axial walls, use a different one for margins, and yet another one for occlusal anatomy. The good news is that this process is automated.

The detail sequence is the final milling sequence. It is normally done with a smaller cutting tool than the finishing tool. This tool is used to cut in even finer occlusal detail and, sometimes, sharper line angles on intaglio surfaces. Because the tool diameter may be 0.6 mm or smaller, spindle speeds and feed-rates are the slowest of all of the sequences.

Milling strategies determine milling times. Step-down and step-over numbers can be reduced for optimum surface finish, but this extends per-unit milling times. Lengthen the step-downs and step-overs, and milling times will decrease, but surface finish is compromised. Finding the right balance between speed and finish takes a skilled CAM programmer, which is why milling strategies are usually protected and proprietary.

The company selling the milling machine generally decides which tools are supplied with the equipment. Some mills may only use two cutting tools, while others may use as many as four or five. Roughing tools may be 2 mm or 3 mm in diameter, while finishing tools are often 1 mm, and detail tools are only 0.4 mm to 0.6 mm in diameter. Some machines use diamond-encrusted tools, usually for milling glass ceramic materials. Traditional carbide steel is used for most other materials. Special coatings are often available for carbide tools. For example, gold-colored titanium nitride (TiN) coating facilitates material removal and can extend the life of the tool. Black-colored polycrystalline diamond (PCD) coating can significantly extend tool life. Pricing of coated tools can sometimes double or triple the cost of standard uncoated carbide tools.

Milling machines come in all shapes, sizes, and configurations. There is definitely more to these machines than meets the eye. Prospective buyers should question the anticipated volumes and materials they want to mill. They should evaluate the performance of mills before buying, and should understand the number of tools in the mill, what they cost, and how long they will last. They should consider the added cost of tooling each unit that they are milling. Most importantly, they should remember that machines that look nearly identical might perform completely differently depending on milling strategies and tooling.

Prospective buyers shouldn’t be afraid to ask questions and seek answers. Buying a mill is a costly capital investment and one that a laboratory owner likely has to live with for years to come.

Chris Brown, BSEE, is the business manager of Apex Dental Milling in Ann Arbor, Michigan.