Milling or Grinding Considerations in Digital Dental
Making an informed decision when purchasing a milling machine
Five or six years ago there were few options when it came to the CAM milling systems available to dental laboratories. Today, a walk down the aisle of any dental laboratory trade show reveals a variety of milling options. There are desktop systems, bench-top systems, and freestanding mills with attached robots. There are even manufacturers who are selling the same mill to multiple companies who then re-brand them and sell them as their own. However, while the inner hardware may be common between resellers, the tooling and milling strategies can be completely different. This is why mills that look alike can produce completely different results. So which features separate one mill from another?
The spindle is the heart of any milling machine. It consists of an encapsulated motor inside the mill that spins the cutting tool (bur). The tool is held in place by a collet. The milling program generated by CAM software tells the controller inside a milling machine how fast the spindle should spin. During a typical project, the spindle speed is adjusted numerous times based on the type of cutting tool being used, the size of the tool, the type of material being milled, and the amount of material being cut. Milling equipment manufacturers and CAM software programmers use industry-wide guidelines to select spindle speeds that optimize tool life, cutting speed, and cutting quality.
Wet VS Dry 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
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.
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. 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.
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.
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.
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 .
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.