Published 02/19/2018

Buying a Five-Axis: Selecting the Right Machine

Horizontal or vertical? Trunnion or swivel head? What’s the effective difference between different configurations of five-axis machining centers?

Barry Rogers, President, Alpha Strategies

This blog post is adapted from an article by Barry Rogers that appeared in the November 2017 edition of the Machine/Shop print supplement to Modern Machine Shop.

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 intend to machine primarily 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

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.

This is part three of a four-part series about buying five-axis machine tools.