The CAMtionary: A Guide to the Essentials

4-Axis Machine

[fohr ak-sis muh-sheen]

A milling machine with 3 linear axes and 1 rotary axis (Figure 1). This machine type is typically used to produce crowns, smaller bridges, and screw-retained implant parts (singles only). The rotary axis can be utilized in three ways: to flip the workpiece 180° and mill from each side of the blank; to rotate the workpiece to an intermediate position that can effectively mill undercuts and angled screw channels, or optimize tool life; and to activate a simultaneous movement (swivel or continuous 360°) that primarily works in conjunction with the machine's Z-axis.

Although 4-axis machines place a restriction on the types of restorations a laboratory can produce, there are also many benefits. In some cases, the fixture can be supported with more stability. Also, the initial investment is lower, and they cost less to service and maintain.

5-Axis Machine

[fahyv ak-sis muh-sheen]

A milling machine with 3 linear axes and 2 rotary axes. This machine type places almost no restrictions on the type of dental restorations that can be produced.

A 5-axis machine uses some of the same techniques (see Toolpath) as a 4-axis machine for many of its milling processes. However, the addition of a second rotary axis is what differentiates this class of machine, offering complete versatility utilizing 3+2 milling and 5X simultaneous milling (Figure 2).

For related terms, see 3+2 milling and 5X simultaneous milling.

Toolpath

[tool pahth]

The route on which a cutter travels to accomplish its purpose—ie, roughing stock material, rest-machining leftover material, or finishing part features and surfaces. Created in the CAM software and output as numerical code for the machine controller to process, a toolpath is primarily composed of a list of X, Y, Z, and/or B-axis coordinates that the tip (or center) of the tool must travel between—like a very dense 3-dimensional game of "connect-the-dots."

In most cases, a toolpath can be visualized within the simulation features of the CAM software. This makes it possible to evaluate the result before any milling or grinding has begun.

3+2 Milling

[three pluhs too mil-ing]

A type of 5X toolpath that locks both rotary axes at an angle to engage an angled or undercut element (Figure 3 through Figure 5). It's an extremely efficient way to mill complex parts effectively because the rotary axes aren't actively moving; they're locked in the correct position throughout the whole toolpath.

A popular use for 3+2 milling toolpaths is with multi-unit implant-based restorations. Each implant site is typically at an angle divergent from the others, so while milling each screw channel and implant base, the machine will position its rotary axes parallel to the implant and use 3D milling strategies to create the part features (Figure 3). 3+2 milling toolpaths are also used to unlock the potential of machines with C-shaped fixtures. This type of fixture holds the material disk on only one side, which allows for unrestricted access to the open end. There are high-end CAM software solutions that allow the user to extend the part boundary over the edge of the puck and choose an extreme angle to mill the undercut details. Using this method on a full-arch bridge, especially with facial cutbacks or tissue reduction in the design, the technician can save a significant amount of time hand finishing at the bench (Figure 4 and Figure 5).

5X Simultaneous Milling

[fahyv eks sahy-muhl-tey-nee-uhs mil-ing]

A type of 5X toolpath that activates the movement of all axes at the same time to mill complex undercuts on part surfaces or to optimize which portion of the tool makes contact with the material.

For this toolpath to be successful, the machine controller and CAM software must both support the complex milling operation. While its versatility is unrivalled, it can often be less efficient than other toolpaths due to inverse time-the reduction in speed of a linear toolpath to accommodate the completion of a rotary axis movement.

Grinding

[grahynd-ing]

A process of removing material with an abrasive diamond-coated tool that's thermally controlled by a liquid coolant (Figure 6). Unlike tools used for milling, grinding tools do not have flutes that pull the chips away from the workpiece. Instead, the toolpaths primarily use the side of the tool at full depth to shave away the material.

A milling machine must be equipped with a flood coolant system and an extremely high-RPM spindle to accommodate this operation. A high-pressure, multi-nozzle system is recommended to assure the glass chips are completely removed from the workpiece and not re-cut.

Spindle

[spin-dl]

The heart of a milling machine, a spindle's primary purpose is to hold the tool and rotate at its prescribed speed (measured in RPMs, or rotations per minute) to remove material.

For a light-duty benchtop mill that's suited for soft materials and glass ceramics, spindles with high RPM ratings are used, but it's also necessary to ensure the spindle's torque rating is sufficient at these high speeds. With the popularization of hard-metal milling for preform abutments and implant bars, machines that have spindles with high torque ratings at slower RPMs are also becoming more prevalent in the dental industry.

The spindle also determines the shank size of the tools (burs). Popular shank sizes in dental machines are 3 mm, 4 mm, and 6 mm. Smaller shank tools are less expensive and suitable for lighter-duty applications. Larger shank tools are necessary when higher loads and forces are being placed on the tool-like for aggressive soft material milling or the machining of hard metals.

For related terms, see kilowatt (kW), Newton centimeter (Ncm), and runout.

Kilowatt (kW)

[kil-uh-wot]

1000 watts. A measure of power that a spindle can generate.

An important consideration when evaluating the kW rating of a spindle is what the number actually represents. Unfortunately, spindle manufacturers use nomenclature that can differ from that of other manufacturers. In some cases, the continuous rating of a spindle (IEC 60034-1 S1 rating) is used as its primary identifier while others use the spindle's max power rating. It is important to identify which power rating is being represented by the manufacturer. Once the rating is clear, what does it mean? Throughout the milling process, loads are placed on the spindle motor as the tool removes material. Whether or not the available power output can withstand these loads is based on the relationship between its power rating and the amount of torque the spindle is able to produce as a result.

Newton Centimeter (N•cm)

[nyootn sen-te-meder]

1/100 newton meter. A measure of rotational force (torque) a spindle can deliver to the cutting tool. More torque makes milling and grinding easier, which allows for heavier cuts or harder materials without the motor stalling.

For example, imagine a father and son on their bicycles. Due to the strength of their legs, the adult has a higher power output potential. On a flat road with the occasional bump, both father and son can manage the the power and torque required. This is the equivalent of milling soft materials like wax, PMMA, or zirconia with conservative toolpaths. However, when a higher load is placed on a spindle by a more aggressive toolpath that removes more material, it's the equivalent of riding up a steep hill. More torque is required to maintain the speed, and in this case, the father is the only one with enough power to provide it. The son's bicycle stalls because the necessary power output to deliver the required torque is above his maximum capacity. However, it is important to note that if the gearing of both bicycles cannot provide enough torque to maintain speed, they will stall.

Runout

[ruhn-out]

The concentricity of a rotating tool, or lack thereof (Figure 7). With runout, the cutting tool or shaft does not rotate in perfect alignment with the spindle's main axis. The amount of error from zero is runout, and just a small amount (5 μm or 0.005 mm) can affect part quality and tool life.

Imagine runout as the tip of the tool wobbling. This way, it's easier to envision the vibration that runout can introduce into the process, which leads to a less-than-ideal surface finish, chipped margins, and decreased tool life. Also, the fit is affected due to more material being removed. Theoretically, the tool diameter has been increased by the runout value, so adjustments need to be made in the CAM software to accommodate this error.

Bottom Line

[bot-uhm lahyn]

The most important point or result. Choosing a milling machine depends on the specific needs of each dental laboratory or milling center. Considering these needs vary from case to case, and there's no such thing as a universal solution, it's important to discuss these terms and ideas with every manufacturer and distributor to ensure the appropriate purchase decisions are being made.

About the Author

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