A 5-axis machining center rough cuts vanes on an aerospace blisk.

Five-axis machining isn’t simple in any industry, but it gets especially interesting in aerospace. With subtractive manufacturing, you must consider everything from material and workholding, to tooling and the toolpaths, to get to a finished part.

Adding more axes complicates not only the selection process for toolpaths and feeds-and-speeds, but also the machine itself. There are a lot of considerations when looking for a 5-axis CNC machine for aerospace milling, and the options might not be as intuitive as you think.

Why 5-axis?

One of the biggest reasons 5-axis machining is so prevalent in aerospace is machine setup. The parts often have complex geometries that don’t easily fit to 3- or 4-axis operations. While many of the parts might be possible without 5-axis machining, you’d have to continuously refixture your part to get the geometry correct.

A perfect example of this is NASA’s Orion bulkhead, which is domed near the heat shield. It’s a big forged piece of aluminum and all the pockets are normal to the surface, so you wouldn’t be able to get those angles with any other operation.

Another key consideration is weight reduction – you’re building a machine that flies, after all. More often than not, the goal is to find the best way to get the most strength with the least amount of weight. This is a major factor in defining all the weird geometries in aerospace manufacturing, and to that end, the need for 5-axis machining.

Work envelope

The aerospace industry has a broad array of parts, components, and structures that require varying levels of machining, molding, and additive manufacturing. Defining your work envelope before buying a machine is vital to making the right choice. Some machined parts are massive, such as fuselage sections. Machining on five (or six or seven) axes is necessary for this type of machining, but the catch is that the machine itself ends up being the size of a building.

In situations like this, every milling machine is custom-built for a specific part, and the machine is rarely re-used once production ends. Those machines are typically in operation for 5-to-10 years, and once the part is no longer made, the technology has advanced so much that it’s more efficient to upgrade to a new machine. Even if it costs $10 million for one new machine, you’re going to make it up in time savings and production.

With smaller work envelopes, 5-axis machines that aren’t made for one-off production are usually tool-room style machines typically used in aerospace organizations’ research and development (R&D) centers, including Lockheed Martin’s SkunkWorks or Boeing’s Phantom Works. That’s where they’ll reconfigure the machines to do different parts. These off-the-shelf machine tools can also be used to produce smaller parts, such as landing gear – which is still a large component – but the principles stay the same.

Another key element to defining your work envelope is considering workholding and the actual cutting process. Aerospace lends to very odd, or uniquely-shaped parts, which can be hard to hold and machine. Once you’ve considered a part’s size, only then can you plan how it will be held and the tools needed for machining. If you’re dealing with cumbersome workholding or long tools, these elements need to be considered when determining the machine’s size.

Once you know your work envelope and the tolerances you need to meet, the number of suitable machines will reduce, but there are usually still plenty of choices.

The logical next step would be to look at the price. Since, at this point, you’re only looking at machines that fit your tolerances and your work envelope, comparing prices is the intuitive thing to do.

But that’s wrong.

Determining which machine has the best uptime can be as important as metal removal rates. If your machine is down, you’re not making parts, which is less than if you had a slow (yet reliable) machine. Having spares to repair the machine without a service tech can certainly tip the decision. So, if several machine tools can all meet your criteria, then it comes down to which machine is least likely to break down and/or if it does break down, which can be repaired the quickest.

Autodesk PowerMill software simulates blisk machining on a 5-axis machine.

Cheapest is rarely best

When the machine isn’t producing anything – when it’s going from one toolpath to another, and not actually touching your part at all (rapids) – that time is huge. When you’re talking only 10 seconds of off-part motion and you have another machine that can do it in 2-to-3 seconds, adding that up for a machining operation that takes 4-to-5 days for one part can result in up to 12 hours of lost time (depending on the operations and the part).

When dealing with five different axes of movement, the machine can get wrapped around itself trying to get into various nooks and crannies of those odd-shaped parts. Determining what kinds of angular limits the machine has is therefore key to making rapids as fast as possible – if the machine needs to unravel itself from a weird angle on a part, that is all off-part motion.

Once you’ve determined machines that can handle your work envelope and tolerances, the next step to picking a machine comes down to speed. That’s why machine rapids are such a bragging point. Not just that companies can make parts faster, but that they are making more money. Determining which machines have better removal rates for the materials you’re planning on machining should also weigh in your decision. Price is of little concern when value is achieved elsewhere.

In the end, every application is going to be unique, but consider these key factors before deciding on which 5-axis machine is right for your aerospace part.

Autodesk
www.autodesk.com
IMTS 2018 Booth #133222

About the author: Clinton Perry is product marketing manager at Autodesk. He can be reached at clinton.perry@autodesk.com.