MACHINE TOOL BUYING GUIDES

Companies with more complex parts should start thinking about purchasing a five-axis machine, which is often considered the natural progression from a horizontal machining center (HMC). Likewise, companies heavily into vertical machining in which a fourth-axis table or trunnion may have been added to a three-axis vertical, should also think about buying a
five-axis machine.

Complex Surfaces, Complex Geometry

You may be surprised to learn that only about 5 percent of all machined parts require processing by full five-axis machines. Five-axis machining is most prevalent in the aerospace industry, where parts tend to be non-prismatic (not box-like), with complex surfaces, such as aerospace turbines, impellers or airfoil blisks. These complex designs require all five axes of the machine moving at the same time to create the shape, thus enabling the cutting tool to take a multidirectional approach to the part surface. Five-axis machining is also extensively used in moldmaking, where the need to produce complex geometry and surfaces is ever-present. Later in this article, we will cover some of the benefits of using a five-axis machine even if your parts aren’t that complex.

Processing Power – Consider the Controls

In full five-axis machining, all five axes can move and cut simultaneously. This type of machine requires highly responsive servodrives to move instantaneously while responding to thousands of move/position commands. High-accuracy rotary tables are needed to position the part precisely, despite the forces created by the cutting process.

To handle these computing tasks adequately, a machine control must have high processing power to calculate, keep track of and control the tool center point. (The tool center point is critical, because its position and orientation determine how material is removed to produce the desired shape.) Five-axis program code includes instructions for the X, Y and Z linear axes; the two rotational A and B axes working in unison to keep the tool normal (perpendicular) to the part surface when contouring; and I, J and K vectors for tool cutter offsets. This constant calculation includes all fixture offsets, tool offsets, tool cutter compensation and translation of all work planes, regardless of which axis or rotary table is moving.

To do this, the control must be able to respond instantly to the servodrives, process motion commands in blocks, and interpret large quantities of CAM data while “looking ahead” to where the tool is going next. All of this computing must be done accurately, so that no dwell marks are left on the surface of the part. Avoiding dwell marks is especially critical in mold work. This capability also enables probing to be used during the machining cycle to check the results. To support this processing power and ability, the cost of a full five-axis machine is substantially higher than three-axis machines.

Positional Machining with Five Axes

Although it may appear that the next step forward from three-axis machining is full five-axis, there’s a transitional step called 3+2 machining. This in-between mode gets the job done, but with a less-expensive machine. Just because a non-prismatic part has holes, angles or features that are not normal to the surface or machine axis does not mean that full five-axes capability is required.

To machine the necessary features with 3+2 machining, the machine tilts the tool or rotates the fourth and fifth axes to a fixed position, then executes a three-axis program, moving in X, Y and Z. This is sometimes called five-axis positional machining, rather than full five-axis machining in which the tool is continually being manipulated in all linear and rotary axes at once.

While the 3+2 control can track the initial origin of the part, it’s a somewhat slower, one-move-at-a-time process. Unlike with full five-axis control, the 3+2 control does not need to be capable of tracking the movement of all axes, translating and updating work offsets, tooling offsets and coordinate systems during the cut. With 3+2, only one work plane is computed at a time. That is, the control “thinks” of the tool moving in relationship to a flat, static surface correctly located on a certain spot on the part shape so that the tool can be perpendicular at that point. Move to the position, rotate, then cut.

Not all 3+2 machines are designed to move and cut at the same time. In this case, activities take place only in the static work plane established before cutting begins. No other machining or processing is going on during part rotation. Programming for 3+2 may require a kind of a multi-operation-by-workplane approach. The control on a 3+2 machine is not nearly as sophisticated as the control on a full five-axis machine, and the software need not be as powerful. Therefore, the initial price tag of the 3+2 machine is far lower than that of a full five-axis machine.

Increased Cost for Full Five Axis

There’s a big price difference between a full five-axis machine and a three-axis machine, and many factors contribute to this higher price. The construction and quality of the machine itself are costly. Five-axis machines are built for high precision. Ultra-precise, high-accuracy rotary tables and trunnions are essential, and these devices are expensive. With full five-axis machining, the machine’s movement is limited by the speed of the slowest rotary axis, so high-accuracy, high-speed rotary devices are installed. This is a big factor in the machine cost.

Likewise, high-performance, long-lasting spindles with high-speed capability often are used to produce very smooth surfaces, such as those required for mold work. Often, when machining complex surfaces, cut times can be quite lengthy. Over time, heat can build up inside the machine. To compensate for the effects of this heat (such as thermal expansion of key components), machines are likely to have cooling systems integrated into their casting, machine bed, ballscrews and spindles. These systems ensure and maintain machine accuracy, whether cutting the first part in the morning or the last part at night. Some five-axis machines are designed with a separate, isolated electrical cabinet to help prevent heat from power lines and circuits moving to the machine. Highly responsive servomotors and servo loops are also necessary to control the exact position of the tool center point at all times. These components also add to the cost of the five-axis machine, as do the highly sophisticated machine control, processor and software mentioned above.

Some Extras Required

Generally speaking, five-axis machining is complex and requires a lot of expertise and substantial investment, not only in the machine itself, but also in CAM and simulation software. Some machine builders also offer the required software, but others do not. With some machines, it is included as part of the standard package; with others, it is an option at an added cost. Many customers jump into full five-axis machining only to discover later that the CAD/CAM and simulation software were not included in the initial package. 

Options such as 3D cutter compensation are also available. This option enables the user to insert a tool offset into the control, say 0.002 inch, rather than change each tool-diameter setting. For years, you couldn’t use cutter compensation once the program rotated one of the axes. Today’s cutter-compensation software will keep the tool center point right on the surface, no matter which axis is moved or rotated.

Offline simulation software contained in a high-quality CAM software package can be extremely helpful to verify a five-axis machining process and avoid collisions, as it can be difficult for a programmer to mentally visualize all possible collision points among the tool, fixture and part when working in five axes.

Selecting the Right Machine

Not all five-axis machines are alike. Here’s where the application for which they will be used must be considered. You need to know what cutting speeds you’re going to run, for example. The type of spindle, the arrangement of rotary axes, rapid traverse rates, feed rates and available horsepower are other major considerations. Do you primarily intend to machine aluminum, stainless steel or titanium? How rigid does the machine need to be? What surface-finish quality do you require? What part accuracy are you trying to achieve? These are all questions you’ll need to answer in order to select the right machine for your application.

If you’re primarily machining aluminum, you may prefer a spindle capable of higher speed, such as 20,000 rpm, with higher rapid traverse rates, especially if you’re using smaller-diameter tools. Likewise, if you’re machining stainless or alloy steel for complex mold surfaces, you will likely be using small tools and high spindle speeds to achieve exceptionally smooth surface finishes.

Be aware that some machines are designed for cutting only aluminum. Others are suitable for steel and tough alloys, which require more rigidity, higher horsepower, lower spindle speeds, slower rotary speeds, higher torque and stronger box ways to make deep cuts with bigger tools. Machining different grades of steel, titanium alloys or even harder materials may require a heftier machine, however, this hefty machine would need to rotate the table excessively fast to achieve adequate surface speeds for cutting aluminum. The result might be disappointing.

When specifying out a five-axis machine, obtaining the expert advice of an experienced engineer is recommended.

• Horizontal or Vertical Five-Axis Machines

  • Horizontal five-axis machines are normally equipped with an automatic pallet changer (APC) ready to be installed on the shop floor. If you’re machining aerospace components that have deep pockets or waffling designed to reduce finished-part weight, the high volume of chips will naturally drop into the conveyor. In addition, horizontal five-axis machines tend to be heavier and more rigid, which helps when cutting steel and titanium.
  • In contrast, vertical five-axis machines tend to be more agile for processing smaller parts. VMCs tend to enable better operator access and can often take heavier cuts, but clearing chips can be inconvenient. High-pressure, through-the-spindle coolant delivery comes in handy to remedy chip accumulation.

• Swiveling-Head or Trunnion Style

  • There are pros and cons to different types of machine designs. If you’re loading heavy parts, the non-tilting table on a swiveling-head machine is often preferred, because this type of table offers greater rigidity for holding big, heavy parts. The swiveling head enables the use of shorter, standard tooling, because all tool rotations occur above the part. Swiveling-head machines tend to be more versatile, lending themselves to using multiple fixtures, vises or tombstones. This somewhat simulates the appeal of an HMC.
  • A trunnion-style machine is often preferred in moldmaking, because both rotary axes are contained in the trunnion table itself and the spindle head is stationary. This configuration is similar to that of the three- or four-axis machines most moldmakers are already used to. The spindle head reaches out over the tilting table, providing better undercut capabilities and some access to the underside of the part. As the spindle head itself does not rotate, trunnion-style machines tend to be more effective in heavy chip removal and can use full X, Y and Z travels to accommodate large parts. 

Advantages of Five-Axis Machining in Action

The “five” in five-axis machining refers to the number of directions in which the cutting tool can be oriented as it approaches the part surface. This maneuverability provides almost unlimited possibilities for the type and shape of parts one can effectively machine. A significant advantage of five-axis capability is being able to process five sides of a part in a single setup. All sides are accessible except the one resting on the table. For this reason, shops that don’t have full five-axis work involving complex shapes can still benefit greatly from the five-sided machining a five-axis unit allows.

If parts being produced on a three-axis machine must be flipped over or repositioned, producing them on a five-axis machining center in one setup may be more profitable. Most often, the process on a three-axis VMC requires flipping the part, or rolling it around from fixture to fixture, to access all sides the part. Unfortunately, whenever the operator must open the door of a VMC to flip or rotate the part for this purpose, or to load or unload the part, remove chips, or perform in-process quality checks, the spindle must be stopped. This means that a part requiring machining on six sides may have to be moved by the operator seven times (load, reposition five times, unload). Five-sided machining eliminates these extra stoppages.

With five-axis machining, you can grip the part, perform all roughing operations, then go back and finish machining—in essence, gripping the part only one time. This capability enables you to machine part features in the order that is most convenient and may make the most sense for optimum material removal. For example, a part may have neighboring features that chatter or vibrate. These features can be roughed from both sides to reduce the chatter. Being creative with the processing steps is likely to enable you to conquer more challenging features with the added flexibility.

Another advantage to five-sided machining must be mentioned here. If holes on a prismatic part must be located to key features with a tight tolerance, five-sided machining may enable the part to be positioned on the side that requires the least machining, leaving a high percentage of features accessible for processing in a single setup. On a highly accurate machine with five-sided capability, the position of these features will correlate. This is not usually the case when using multiple holding fixtures on a three-axis machine. Machining features in one operation reduces location errors resulting from moving the part.

From a mathematical or statistical process-control standpoint, process capability for, let’s say, 30 parts produced on a five-axis machine versus on a conventional machine will be higher on the five-axis machine by eliminating the human involvement required for part repositioning. Even on the most finely tuned conventional machine, slight error is introduced whenever parts are handled by the operator. Using five axes to complete a part in one cycle with little or no operator intervention avoids this source of error.

Having said all this, you might be surprised to learn that a five-axis machine is never quite as robust as a three-axis machine. In addition to X, Y and Z axes, the rotary tables or trunnion add more mechanical joints susceptible to flex and wear. Do not let this concern you. Most shops will use 3+2 positioning for the roughing cycle and then use full five-axis machining to finish. Position, then hog, position, then hog, then use five-axis machining to finish the part gracefully.

Machining molds on a three-axis machine has its own challenges and limitations. For deep-pocket molds or tall-core molds, the required tools tend to be longer and smaller in diameter. Feed rates must be slowed to minimize tool chatter and prevent breakage. In contrast, with full five-axis machining, you can use shorter, stouter tools; have better access to the surface without Z-axis interference; take heavier cuts; increase feed rates; remove more material faster; and achieve better surface finishes, all while requiring fewer setups and shorter machining times. The incremental increase in costs for a five-axis machine compared to a three-axis machine will generally be absorbed quickly through increased efficiency.

Spindle Probing–An Added Reason

Whereas spindle probing on a three- or four-axis horizontal machine is highly recommended, spindle probing on a five-axis machining center is compulsory. Although probing is used to automatically set fixtures and multiple work offsets; locate the part precisely without the need for expensive fixtures; and obtain other benefits, there’s yet another reason for a spindle probe on a five-axis machine:

A spindle probe is the quickest, most repeatable and most accurate method for establishing centers of rotation for the rotary axes and tables, trunnions, or programmable C axes relative to the center point of the cutter. To define the exact location of this point in space, appropriately called a pivot point, an offset value (like a tool offset or
work offset) must be applied to control parameters. A probe determines this offset precisely. The probe will also help compensate for any pointing error the machine spindle or table may have. Once this point is defined, all axes are then relative to the center point of the tool.

The pivot point is normally set using a known artifact and a calibration routine, a process that automatically maps the entire machine. This artifact data is then analyzed to derive a compensation value that is uploaded to the control. Periodically, the calibration routine can be quickly run and the values checked to see if movement in any of the five axes has occurred.

Today’s Five-Axis Technology

The current technology in today’s five-axis machines delivers greater capability. These machines always seem to get the job done faster by enabling you to do more work in a single operation. There are fewer operations to program and fewer fixtures to create, and there is less flipping of parts and less in-process movement of parts around the shop. This overall increase in shop efficiency means more money in the bank. Perhaps it’s time to start thinking seriously about five-axis machining.